Why You Probably Need an ERV, Not an HRV
One of the most frequently asked questions regarding the type of balanced ventilation devices known as the energy recovery ventilator (ERV) and the heat recovery ventilator (HRV) is: “Can I get that in red?” OK, not really. Everyone knows that most people prefer their ventilation systems in blue. The real question that gets asked a lot, however, is whether to get an ERV or an HRV. As you know already from the title, my take is that you probably need an ERV, not an HRV.
Climate and occupancy
Deciding between an ERV and an HRV should land on ERV for most people in most places. In a warm humid climate, an ERV brings in less outdoor humidity than an HRV. (Note: an ERV is not a dehumidifier. It does still add to the latent load in the house.) In a hot dry climate, an HRV will make your already dry air even drier, sending your precious water vapor out into that desert air. In a cold climate, bringing in outdoor air without moisture exchange can result in extremely low humidity in winter because cold air is dry air. On the basis of climate, it’s only in mild climates where it doesn’t get too cold, too humid, or too dry where HRVs make sense…sometimes. That’s why they’re popular in the Pacific Northwest.
Occupancy, though, is another important factor to consider. The higher the density of people in a space, the more you might need to dry out the air with an HRV. A small, airtight apartment or condo with two or three people in it, for example, may be too humid indoors with an ERV. In a dry or cold climate, this works year round. In a humid climate, it works in the winter, but an ERV would work better for the warm, humid days.
History, efficiency, and core swapping
Some people think HRVs are the way to go because older ERVs didn’t have good control over frost forming on the core. That’s not the case anymore. ERVs work fine in really cold places now. Another reason people choose HRVs is that they’re more efficient at transferring heat than are ERVs. What good is it to have high efficiency ventilation, though, if you end up growing mold or going through 50 liters of skin lotion each year?
Another possibility is to do both. If you’re in a humid climate and don’t want to bring in a lot of water vapor with your summer ventilation, you need an ERV. But if your house is very airtight, you may get too humid indoors in winter with an ERV. In that case, you could have an ERV core for the summer and then swap it out with an HRV core for the winter. (Not every manufacturer makes models with swappable cores, so check the one you’re buying if you think you may want to do this.)
The primary way to choose between an ERV and an HRV is to understand the moisture control needs of the space being ventilated. In general, when the outdoor air is significantly drier or more humid than indoor air, you should go with an ERV.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
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Greetings,
Are there considerations for seasonal routing of a dryer vent (exhaust) in a home to be built in coastal Georgia?
You’ll want to route dryer exhaust outdoors 365 / 24 / 7 via the shortest, smoothest and most easily maintained duct path and outdoor terminal fitting.
If the home is to be small and particularly airtight, you might want to consider the how the dryer exhaust outflow (straight to the exterior during all 4 seasons) is made up / replaced.
Figure 150 – 200 CFM for 30-45 minutes per laundry load.
Curt, I agree with what you are saying about dryer exhaust, but I have to admit that when I lived in upstate NY in a house with a heat pump, I used to vent my electric dryer indoors during the cold winter months. In addition to saving energy, it helped bring the humidity up to more comfortable levels. But I must warn anyone who does this to make sure that they watch it closely and switch back to the outdoor vent when the weather is milder. Keep the laundry room door open at all times too.
In case you are wondering, we did put an additional lint catcher on the exhaust when venting indoors.
Roy, are you going to get the ASHRAE 62.2 standard to allow venting a dryer to the indoor air?
“6.3 Clothes Dryers. Clothes dryers shall be exhausted directly to the outdoors.”
:~)
Not until after I get 62.2 to require that toilet rooms be mechancially vented to the outdoors 😉
There is an exception to this venting requirement for condensing clothes dryers which typically use a heat pump and thus recover some of the heat and humidity. With the trend towards electrification, it will be interesting to see if it will force people to electric-resistance clothes dryers or if more efficient electric clothes dryers will become more common. But either way, we should probably consider when clothes dryers should or shouldn’t be vented outdoors.
Gas clothes dryers should always be vented outdoors!!!!!!
No. ERV & HRV installation manuals all say to never connect a clothes dryer to the unit. This would void the warranty and might cause a risk of fire. Also on the banned list is range top, stove top fan, and central vacuum system.
Re: history…unless someone can show me otherwise, the first ERV in North America was patented as a “Heat Recovery Fan” by Johannes Kirchmeier in 1979…we frequently specified them up until the late ’80s…before switching to heat-pipe HRV’s then plate type HRV’s then back to plate type ERV’s. The early ERV’s had terrible efficiency’s and cross contamination, but they never froze up – even in our Canadian arctic conditions…here we are 40 years later and the world of HVAC still treats them like “new”…I’ll be dead before these devices become the “kleenex” of ventilation…our industry operates at the viscosity of frozen maple syrup…
Mr Bean, I actually installed a couple of the heat pipe HRV’s in a new home, back in 1982. The homeowner was intent on using a concept called “The Arkansas House”. It mirrored some of the Passive Haus attributes with double 2×4 walls, double glazed windows with storms, R-50 overhead, yada yada. We also installed two (2) geothermal units, one for each level. He had read these tight houses also needed fresh air. So we installed an air to air heat exchanger for each floor. The geo units were replaced in 2015 with air to air heat pumps. And since bigger is better, I understand both units were enlarged.
Sweet! Hope all is well with you and yours sir.
Would a ventilating dehumidifier be a good option in the Southeast (high country not coastal) so that you can control the actual humidity level while still bringing in outside air?
Allison, as usual I always appreciate your expertise & knowledge sharing. Ventilation is an essential part of the strategy for a HEALTHY HOUSE…as the 3rd essential strategy to complement : (1) elimination of emitting air contaminants, (2) hi level filtration of indoor air for those emitting sources you can’t eliminate. But installing ventilation in a residence that was not designed & built with it, can be very challenging. I want to share my 25 yr experience with ERV vs HRV practical problems & challenges. I live in the mid-Atlantic region, where our hot humid months have been getting longer, hotter and more humid as result of climate change. 25 years ago, I started out with a top rated brand ERV retrofitted into our 1960s house. Some major unexpected problems. PROBLEM #1: finding an HVAC contractor who even knew what a ERV was, let alone knew how to install and BALANCE the air flows. Now-a-days lots more know what a ERV/HRV is…but I still find it’s a rare HVAC contractor who knows how to properly install and balance air flows! PROBLEM #2 after finding the ONE HVAC contractor who was ERV literate: during very humid months & at ‘low’ ventilation rates, the ERV increased indoor humidity to over 60%. Consult with the manufacturer’s techs: “oh, an ERV removes only insignificant amount of humidity from incoming air. That’s a problem in humid climates! So you need to run the ERV incoming air thru a dehumidifier before dumping the air into your duct system.” So I added a dehumidifier. That solved that problem…for a while [more about that later]. PROBLEM #3: adding the dehumidifier made it difficult to ‘balance’ the ERV air flows! Consult with ERV manufacturer techs: “yep, that can be a problem bcuz ERV has its own fan, dehumidifier has own fan, and they can’t be synched with variable speed HVAC air handler. And when you add use of bathroom or kitchen exhaust fans, that creates negative air pressure that disrupts your ERV air balance.” “Solution: that’s a tough one; you just have to try different settings under different conditions and get as close as you can to a slightly positive pressure when ERV is running.” LOTS of hrs of HVAC contractor measuring pressures under different conditions & different ERV air speeds,etc. Result: nearly impossible to keep ERV air flows balanced let alone a ‘slight positive pressure’. But, acceptable in some situations for at least short periods of time. NEW PROBLEMS in more recent years as climate change has changed our local weather to more months of high outdoor humidity. PROBLEM #4: ERV treated ‘paper’ cores get moldy. Solution from manufacturer techs: “change to using HRV aluminum CORES…since ERV cores really don’t help much with dehumidifying the outdoor air.” YES–aluminum cores = NO MOLD! PROBLEM #5 as climate got more humid for more months: 80 pint capacity dehumidifier NOT sufficient to keep HRV incoming air dehumidified! Solution: replace it with the maximum sized residential dehumidifier possible = 120 pints. YES–it does much better. REALITY: now with so many more months of high humidity, there are still numerous days when the outdoor heat/humidity is so high for so many hours that we cannot run the HRV at all. And, it’s still very challenging to find HVAC contractors who have any staff expert on ERV/HRV install, duct design, air balancing. I had to resort to educating myself as much as possible, so we could end up with effective ventilation & dehumidification, after spending way too many $$ on HVAC contractors who screwed up the duct design, air balancing, etc. I’m NOT an expert; but it’s so sad that I know more than the vast majority of the HVAC contractors in our large metro region.
Hi Terry, excellent comment, although not much of what you said surprises me. Ever wonder why manufacturer specs (and HVI ratings) don’t include latent capacity?
I’m curious about a couple of things… What’s your ventilation flow rate and conditioned floor area? Also, 60’s-era houses usually (always?) are way too leaky to require ventilation, so I’m guessing you’ve done some air sealing. If so, did you get a blower door test done to verify the work?
Terry,
As someone who has spent the better part of 5 years researching dehumidification and ventilation, I’m not an expert by any means, but I do agree with you that I now know more than 95% of the HVAC contractors out there. There is something fundamentally wrong with a trade that still works on 1980s assumptions — heat pumps make you freeze, won’t work below 45 degrees, “what’s an ERV?” …
I’m of the opinion that south of the Mason Dixon line, east of the Rockies, that the only thing that makes sense is a ventilating dehumidifier — skip the ERV/HRV. Unless of course, you have a modern, tightly constructed house. My reasoning is that anything pre 2010 is so loosely constructed that exfiltration through construction gaps will eliminate the need for mechanical balance and that the energy penalty for cooling some freshly dehumidified air is pretty minimal. If I were in your situation, I’d just pull out the HRV/ERV and eliminate the systems fighting against one another.
I’m sure someone more knowledgeable will come along and tell me why I’m wrong, but I can tell you that I’m fabulously happy with my whole house ventilating dehu and wish I had installed one years ago.
Matthew K, thanks for the feedback! I am intrigued with the ventilating dehumidifiers, and just starting to research them to answer the many Qs I have given my experience with ‘balanced’ ventilation ERV/HRVs. I do understand the risks of UNbalanced ventilation in our humid climate, so I am especially interested in understanding how one regulates and monitors ventilation rates and whether a ventilating dehumidifier will be better at avoiding negative air pressure than my current HRV system, enable a slight positive pressure, be as flexible as my HRV system re setting air flow rates, on/off, etc. Thanks again.
@Terry, ducting an HRV or ERV in series with a dehumidifier does present a challenge, especially considering that the dehumidifier will be cycling off when outside air is not too humid. I recommend the ventilation system be balanced with dehumidifier OFF. That way you’ll end up with positive pressure when it cycles on. But note that recovery efficiency will drop when the flows are out of balance.
OTOH, you can increase your dehumidifier’s capacity AND efficiency by treating hot/humid outside air before routing it to the ventilation system. That’s essentially what a ventilating dehumidifer is. I disagree with Matthew re: pulling out the ERV/HRV. You’ll still get good recovery efficiency when the DH is off, especially in cold weather.
In any case, forget about trying to maintain a balanced or positive pressure when the clothes drier or spot exhaust fans are running. Ain’t gonna happen.
BTW, not sure what is meant by “they can’t be synched with… air handler”…. The air handler shouldn’t impact ERV/HRV balance as long as ducts are reasonably tight, or fully inside conditioned space, therefore no reason to try to synchronize ERV/HRV with air handler. That is, unless ERV/HRV is connected to the main duct system. That is indeed a bad idea!
David B, thanks very much for your feedback. I apologize, but I’m not sure what you meant by several comments [maybe cuz i’m NOT a trained, credentialed pro]. So I’ll provide some more clarity re my setup and then impose on you by asking more questions about what you meant. (1) My system: HRV>dehumidifier>house return air plenum>all return air direct into full-airstream chemical & HEPA filtration chamber>heat pump air handler>supply duct system. (2) Are you suggesting installing a ventilating dehumidifier BEFORE the HRV so that all the incoming outdoor air is dehumidified BEFORE it goes into HRV and the HRV incoming air is put through a 120 pint/day dehumidifier? The idea being ‘double’ dehumidification of the incoming ventilation air, which then would presumably allow more ventilation on really humid days without pushing indoor humidity above 60%? If so, that’s an intriguing idea. Not sure I have the space in utility room to install another large piece of equipment & associated increase in ducts plus another condensate pump–but i could certainly analyze that. SO LET ME KNOW IF THAT’S WHAT YOU MEANT please. For me, energy inefficiency is less important than high quality indoor air with indoor humidity preferably under 50%. (3) FYI: for maintenance of high quality indoor air, I run my HVAC air handler fan 24/7 so the indoor air is always being filtered & I had the dehumidifier wired so that whenever the HVAC fan is operating, the dehumidifier is ‘on’ keeping the indoor air <50% humidity.
Terry wrote: “Are you suggesting installing a ventilating dehumidifier BEFORE the HRV so that all the incoming outdoor air is dehumidified BEFORE it goes into HRV and the HRV incoming air is put through a 120 pint/day dehumidifier?”
Not at all. You already created a “dehumidifying ventilator,” which is essentially just a dehumidifier that intakes outside air. What I’m suggesting is that you move your dehumidifier (DH) to the upstream side of the HRV. In that case, you’d need to move the HRV inlet filter to the inlet side of the DH in order to protect its coil.
This arrangement yields three benefits: (1) increases DH capacity, (2) increases DH efficiency (both capacity and efficiency are directly proportional to inlet air temperature & humidity), and (3) increases HRV efficiency, since the DH adds some heat to the air.
David, thanks for taking the time to clarify. Very interesting idea & it makes sense. I’ll need to figure out whether it’s feasible in my utility/laundry room given it’s size & configuration constraints. I suspect doing that also would require me to keep a DH for the house return air to control kitchen & bathroom-generated humidity from main living area. THANK YOU–I’ll def think it through.
@Terry, in my earlier comment, I questioned what you wrote about syncing with air handler. Allow me to elaborate…
In general, I recommend against connecting a balanced recovery ventilation system (I’ll use ‘BRV’ to refer to either ERV or HRV) to the main duct system, since an air handler blower can easily pull BRV fans out of balance, which in turn kills efficiency. Some mfrs address this concern by specifying a minimum separation between blower and BRV connections. But this doesn’t necessarily eliminate the problem, especially when combining a large air handler with a small BRV. These little ventilation fans are relatively weak and if they’re pulled substantially out of balance, you might as well not have that expensive heat recovery core (and can lead to comfort issues in cold weather). So I typically recommend separate duct systems.
Of course, that’s not always possible in retrofit situations. In your case, the rather large 0.5 static drop across your filtration media should act to isolate the HRV supply fan from the main blower, but the dehumidifer blower can also push (or pull) your HRV out of balance. That’s why I suggested that the HRV be balanced with the DH turned off. That way it will have maximum recovery efficiency in cold weather when heat recovery matters most. During the summer, the DH will create a positive pressure imbalance, which is obviously better than a negative pressure imbalance, especially in your climate.
As for running the air handler 24/7, that could actually exacerbate your humidity issue. A lot has been written about that. You might start with Allison’s 2015 article. (I duly warn that the extensive and overly techy comments following this article could make your head explode!)
David, THANK YOU again, for this followup suggestion. (1) my HRV was balanced with the DH OFF; (2) yes, 24/7 op of HVAC fan would increase humidity. It was supposed to be ‘wired’ into the HVAC system so that the DH was operating whenever the HVAC air handler fan was ‘on’. So in effect, the DH is running 24/7 whenever the HVAC air handler is running. This seems to work well in controlling indoor humidity [IH]. (3) yes, you’re correct again: in retrofit, there is no way to install separate BRV duct system –that would require major reno & $$$$$$$. (4) thanks for ref to Allison’s 2015 & the warning re tech brain fog [been there often, & M.S. degree doesn’t help much!]. I’ll give it a try. (5) All these challenges of retrofit ventilation is why I am very concerned that many of those who RXing high rates of residential ventilating [for existing residences] don’t really understand the real world challenges & potential adverse IAQ & health results when things don’t work well. And there are far too few pros like you who understand them. If you were in DC metro region, I would have already found you years ago to design & install my system from the git-go, my friend. Gratitude to you. Stay SAFE.
I know I am going against the grain here. But, Im not sure continuous fan increase indoor RH. That assumes the fan speed for continuous drops the face velocity across the evap coil to 250 FPM or less. 550 FPM is dangerously close to water blow off. Lots of ECM motors are programmed to run at 1/2 speed of less with fan only call.
David & Mathew, thanks for the comments and feedback. Couple of things in response. (1) ERV/HRV retrofitted into 1960s ‘leaky’ house [1800sqft] with typical very leaky metal duct system. (2) leaky ducts can’t be solved affordably in most of house due to having to rip out walls to do so & I won’t use any liquid/spray duct sealants bcuz they are toxic health hazards & often don’t seal well anyway according to studies I read. (3) as typical with older houses, limited space in utility room & limited access to return duct systems, so have done max possible redesign/modification of accessible duct syste. (3) ‘tightening’ house: done some. Sorry can’t find my blower door test results due to loss w/ computer drive crash. (4) More info on my system. Heat pump. HRV hard ducted direct to 120 pint dehumidifier that’s direct ducted to whole-house return plenum & all return air & dehumidified HRV air directed thru whole-airstream chemical & HEPA air filtration chamber BEFORE air flows into HVAC air handler and distributed thru supply ducts. BTW: my whole-airstream filtration unit only creates 0.5 static pressure due to new tech PTFE HEPA membrane [awesome!]. (5) I refused numerous HVAC advisors who RX’d dumping ventilation air direct into SUPPLY side of HVAC air handler–absurd idea re health bcuz obviously that air is then not filtered to any high standard for either particulates or chemicals. (6) Put in ventilation & heavy duty air filtration due to medical condition requiring healthiest air feasible [tho house does not have mold/damp foundation or major chemical emissions from building or furnishings]. I share my experience with challenges of retrofitting established house to stimulate discussion because public health officials, ASHRAE, some IAQ experts, etc are advocating high humidity & ventilation rates for Covid & other pandemics likely to come due to climate change, & bcuz of climate change many sources are emphasizing necessity of ‘tightening’ houses for energy efficiency . That’s fine, but I think the practical problems/challenges of doing so with existing building stocks are NOT being identified and recognized; & IMO, that has the potential to create a lot of UNHEALTHY houses & indoor air illnesses unless those practical problems are addressed and solved. Allison–deep gratitude for offering this forum & I hope the questions I raise are constructive.
Wow Terry, You say you’re not an expert. But you’re experiential knowledge easily pushes you into that category. David B brought this discussion to my attention because we had talked about ERV’s and mold previously. One of my good friends is a top notch mold hound in Madison WI. She told me that she has NEVER tested an ERV core where she didn’t find mold. (Also said the same for flex duct) Of course those findings is a bit tainted since she wouldn’t be testing unless there was a suspicion of an issue.
Nonetheless, the conditions in the core are completely compatible with microbial growth. I wonder what we would find if we started looking. I am afraid the industry is content with don’t ask, don’t tell.
Thanks for that info, Danny G, re mold on ERV cores. I am not surprised given my own experience. In past I had several HVAC & ERV pros tell me it was impossible for mold to develop on ERV synthetic cores bcuz “the core membranes are treated with antimicrobials”. Yeah, mine was supposed to be also, but that didn’t prevent lots of mold. Perhaps the antimicrobial ‘wore off’; the mold didn’t ‘grow’ on the membrane but on the macro debris caught by the ERV core? Some one recently mentioned that ERVs now come with aluminum cores–I’ll have to investigate that.
Terry, Perhaps that is the issue – Mold food. With the right water activity, fungi will grow on glass or steel if there is enough food on the surface.
MERV 8 filters seem to be the standard for many ERV’s. And they are not very air tight around the edges, allowing air bypass. That could deposit a lot of particulates, i.e. mold food on the core.
I am no longer of fan of better living through chemistry. So antimicrobial usually generates a sinking feeling. in my gut. Most are short lived. I’m especially unnerved if the container has skull and crossbones on the label.
I agree Danny. One thing we all need to advocate for is getting ventilator manufacturers to provide hi level particulate & chemical filtration on ventilation air. As you know, some tech challenges & cost factors, but they’re solvable. Drives me bonkers when IAQ, IH, ASHRAE, HVAC , etc pros & gov officials call for hi levels of ventilation without linking it to hi levels of FILTRATION [chems & particles]. They apparently ignore the fact that in the ‘real world’ in most locations, the OUTDOOR AIR is heavily contaminated with both particulates & chemicals! It’s no longer valid to generally assume that outdoor air is better quality than indoor air. Rural outdoor air: ag chemicals, dust, climate wildfire smoke/toxic chems, etc. Urban outdoor air: chemicals, particles, climate wildfire smoke/toxic chems, TOXIC ‘SCENTED’ LAUNDRY fumes from many houses’ dryer vents [see research by Dr Anne Steinemann https://www.drsteinemann.com/ ], etc etc.
@Terry wrote:
> Rural outdoor air: ag chemicals, dust, climate wildfire smoke/toxic chems, etc.
…and pollen! Pollen is a BIG problem for anyone with allergies, but easy to catch with filtration. I found that placing a basic media filter behind my window fan makes a huge difference.
Terry, would you mind providing product information about your “whole-airstream filtration unit only creates 0.5 static pressure due to new tech PTFE HEPA membrane.” One way to reduce the static pressure is to make the filter larger (area), but of course you must already know that. I’m interested in the health effects you have attempted to achieve.
Bill, yes, I am happy to share that info. Keep in mind, I have a custom-designed filtration cabinet that I had installed between the house return air plenum & the heat pump air handler cabinet. That air filtration cabinet is approx. 24+” x 24+” x 7ft. tall. It consists of: (1) particulate prefilter section in V-formation holding MERV 11 prefilters; followed by (2) chemical filtration section in V-formation holding 4-1″ thick x 23+ x 23+” refillable steel trays that are filled with combo of granular coconut activated carbon + KMnO4; followed by final particulate filtration ‘box’ chamber 11.5″x 23 & 3/8″ x 23 & 3/8″. So the house entire return air plenum is directed into the filtration chamber and then flows into the heat pump air handler. The 99.99% HEPA ‘box’ filter I purchased was custom manufactured by AFF Flanders & is their model “MEGAcel 1” consisting of ePTFE membrane. ePTFE membrane is known as being capable of providing submicron filtration with VERY LOW AIR FLOW RESISTANCE. My unit was tested at 1900 cfm to provide resistance of only 0.8 at the factory. But of course with a residential heat pump max cfm of @ 1200-1300, the air flow resistance per my magnehelic gauge is only 0.5 even with the entire return air flow going thru prefilters & AC/KMnO4 chemical filtration trays. Pretty damn amazingly good. Previous trials of various manufacturers HEPA ‘box’ pleated filters always drove the air resistance to 0.9>1.1 which of course put very heavy demands on the heat pump system rated for a max air resistance of 0.7. Before deciding to lay out the big $$ for the MEGAcel ePTFE HEPA, I spent a lot of time searching science & med sites re potential adverse health effects of ePTFE, given that it’s in the TEFLON family of ‘forever’ chemicals with extensive documented adverse health effects. Current research indicates it is inert when used as filter media, though there was no research yet on whether its use as air filtration media could result in nano-abrasion that would ultimately release ultra fine particles. It seems unlikely. But there was research indicating serious adverse health effects from PTFE fumes released during the production process; the fumes containing ultra-fine particles that are toxic if inhaled. But that’s unlikely to happen during normal air temperatures & filtration usage in a residence. Also lots of research on adverse health effects of ePTFE when used in surgical sutures or implants. So at least for time being, I concluded that it was an excellent HEPA filter for use in my residence. Hope all this info is helpful.
Bill, I needed best possible chemical & particulate filtration system because all my body systems were impaired [immune, respiratory, CNS, ANS, mitochondrial function, etc] by occupational toxic building whose indoor air was heavily contaminated with chemicals, mycotoxins & endotoxins. [EPA’s previous HQ bldg which due to cover-up by administrators made over 1000 employees ill including many with disabling complex diseases]. The high level air filtration in my healthy home, helped stabilize my condition and slowly improve.
Terry,
Have you tried any chemical detox treatments? Very few allopathic practitioners know much about environmental illness.
I just talked to a client yesterday about a visit to her doc over a lesion which appeared when she was exposed to some unknown chemical toxin. He wrote “self inflected” on her chart. They just don’t know anymore than what they learned in pharmaceutical industry funded med school.
My experience has shown that you will need to seek non consensus clinicians to get the most effective treatment for your “terrain” (Credit to Antione Bechamp). There are lots of therapies available.
@Terri Sopher Sr Thank you for answering with such specificity. I can totally empathize with your situation, as I once met a similar fate that forced me to discover what I now consider the only “real doctor” I know, an open-minded and thinking MD who took me seriously, and diligently worked (tested and tested) to discover a multitude of factors that were contributing to my (then) disabling condition, eventually diagnosed as CFS (but included Candida, EBV, mitochondrial dysfunction, blood clotting disorders, et al), when “traditional” MDs would not even attempt to diagnose CFS, because they say there is no “indication” to prescribe to treat that condition. So, I do understand where you are coming from, and know it is terribly disheartening.
I am designing a new HVAC duct system for our 1997 house, wanting to upgrade to very high efficiency equipment for the HVAC system, which requires me to bring my house up to the latest code for energy conservation. That’s hard!
I took all the HVAC courses they had at the local Community College, then began to teach myself the “air side,” because they didn’t teach it. I’ve been in this mode for a year already, hacking at it from my desk every day, starting when I discovered a hole in our heat exchanger in the 24 yo Bryant gas pack. On the positive side, I have made it through the Manual J process (though my city — Durham, NC — threw away the building permit documents) and the Manual D process, up to the point where I decided to use Writesoft’s commercial software because it gives me more flexibility when retrofitting. And now I am learning the ASHRAE duct fittings after learning the ACCA ones.
Anyway, all of that is to say that it’s the filter that has been giving me the greatest headache, the high static pressure drop as well as the conflict between a desire to get the kind of filter you felt you needed (for our health) and what would be realistic for me to afford.
As soon as you mentioned the “forever chemical,” images of that FRONT LINE episode about (was it Dupont?) and the leaking of their Teflon enabling indestructible chemical that poisoned many people in the region around their plant (was it West Virginia?) immediately popped into my head. They say that chemical is virtually everywhere today, and you cannot exist without getting a few molecules of it inside your body. But, then to use it in your filter medium ? I really don’t know, Terri. I read a little about that material that you mentioned initially and how they construct it to get more surface area to use collecting the sub-micron particles and all, but I just don’t know if I could sleep knowing that chemical was in my super-high-filtered airpath. I cannot forget the black teeth of the cows that died from exposure on that man’s farm adjacent to the Dupont facility.
When I commented to your post, I wanted to know how you had gotten the 0.5 ESP when using such a demanding filtration set-up, because I too want well-filtered air. Now, I am going to read and re-read all you explained (thank you so much) and try to see if it might apply to myself. You no doubt understand how easy it is to become “gun shy”?
FYI: Allison Bailes has some excellent information posted on how to get the best filtration without having to break the static pressure bank.
@Terry Sopher Sr One other thought, Terri. I was brought back from being nearly dead, just like you, and I am wondering, because you have been so thoughtful and diligent in creating a protective IAQ, and state that, because you have, your condition has been improving, as did mine, to a degree, maybe you don’t need to keep your filtration standards set quite so high going forward. Surely it has contributed to your better health. I am thinking about maybe removing that [awesome] toxic thing hanging in your filter closet. I am thinking MERV-13, or maybe a step above, so you can stop worrying about anything getting through your defenses; Allison has great ideas about how to deal with them. With a less than HEPA quality air filter in your HVAC air path, you can reduce your static pressure enough to use a more efficient system for your HVAC. And, if you did that, and if you placed a couple of HEPA quality floor filters in key places around your home, I am thinking that approach might be better than using the material you are currently using. It would certainly be better on your HVAC. Time progresses and things change. Sometimes it takes an “awakening” to tell you that you can “go back out and play” again.
Please do not think I am trying to push you into something you don’t want to do. I don’t know you well enough to even be talking about your situation, so could well be far off the track in what I have said. If so, please forgive me. I only want to be helpful. I know you are a highly intelligent person.
Terry and Bill, thank you for your well considered observations. This discussion is really intriguing. I work on many sides of the equation, the building environment, HVAC and support to the practitioners diagnosing these illnesses.
I have to believe we were not created to live in computer clean room-like homes. Although, I do understand their value when trying to recover from toxic exposure. Dr Bill Rea, now deceased, understood this as well and tried to provide on campus dorms for patients during treatment. But at some point there has to be a root cause diagnosis and appropriate treatment. I’ve been taught that the body will heal itself if you give it the right stuff.
As I’ve gotten older I’m convinced we really don’t know much about human physiology. (And even less about psychology, especially the female species) There is just so much we don’t know.
Richie Shoemaker theorized that the HLA 24 gene is key to mold sensitivity. But I’m not convinced that the three Gs, Germs, Genes or God, are the cause for all illness and disease.
Have either of you looked into medical papers on the causes MCS? Some suggest it’s associated with non native EMF exposure, which changes the structure of water in the body. There are studies which do show it depresses immunity.
You might find this helpful if you aren’t familiar with it. This organization has done a lot of work on the topic. http://www.chemicalsensitivityfoundation.org/index.html
I know both Claudia Miller and Jeff May. They are extremely knowledgeable.
As we continue to clean the indoor environment, I hope we can stay vigilant to seek causes and cures.
Allison, Terry, David & Mathew,
Thanks to all of you for your contributions to my better understanding the issues to address in my “forever home”. It is why I came here
“Allison–deep gratitude for offering this forum & I hope the questions I raise are constructive.”
We install Panasonic Intelli-Balance 100s in all of our homes (WhisperComforts in another home, but that was a one off).
CZ 4c, new construction, typically 2 plus or minus ach50. Kitsap County, WA state (I think we need to spend more time on site-specific factors, CZs are too broad for good design). When our homes were leakier w/o balanced ventilation, the hardwood floors would cup in summer, gap in winter.
So we’ve gone with ERVs to reduce the cupping/shrinkage. Relatively low latent heat for most of the year, but with climate change . . . We’ll see. I didn’t want to go with HRVs in the PNW because we’ll more humidity indoors in summer, drier in winter, which I figured would exacerbate the finishes issue.
VRFs for heating/cooling, so there is some dehumidifying when latent/sensible heat gets high enough. We typically have one zone of the home that is heated/cooled via a duct system (also vrf).
So far so good, but I can’t say that the buyers end up running their systems properly. In a perfect world, we’d have maintenance contracts with our buyer to help keep things tuned up and filters changed. Alas, just not enough time in the day.
I’m serious, aren’t HRVs and ERVs already obsolete?
The energy they recoup can be provided less expensively by a minisplit heat pump powered by solar PV, and the ventilation they provide is accomplished much easier with smart bath fans.
Add the fact that the future owners of a house with an HRV/ERV will have no idea how it works, and their HVAC contractor will probably have no idea how it works and what you have is worthless and will almost certainly be removed.
Here’s hoping that they are never required by code!
From a Martin Holladay article: “if you want to lower your energy bills, install a photovoltaic array, not an HRV.”
https://www.greenbuildingadvisor.com/article/misconceptions-about-hrvs-and-ervs
https://www.greenbuildingadvisor.com/article/are-hrvs-cost-effective
I guess Martin Holladay was assuming the only goal was to save energy. He made a lot of good points in his article, but FAILED to correctly state the main role of ventilation: to dilute indoor air chemical & particulate contaminants! Martin stated ERV/HRV purpose is “…to exhaust stale air from stinky, humid rooms.” That appears to be the old, discredited ASHRAE concept of indoor air ‘comfort’. Many ppl are now advocating that we need to “increase ventilation of buildings & ventilation rates” to DILUTE the concentration of pandemic infectious viruses, &/or dilute indoor air contaminants that occur with tighter, energy-efficient buildings. IMO, in too many cases, they are failing to recognize that simplistically ventilating buildings &/or increasing ventilation rates can easily create WORSE indoor air & health problems. IMO: to avoid these problems, ventilation must be only a part of a broader healthy indoor air strategy; it must NOT be the primary or only strategy. PROBLEM #1: outdoor air is no longer necessarily ‘fresh’/ unpolluted, & to enable healthy indoor air, it must be effectively filtered for both particulates & chemicals to avoid creating worse indoor air health issues. Why would we want to create new or worse indoor air quality issues by ventilating with unhealthy outdoor air? When we ventilate, shouldn’t our goal be to ELIMINATE potential health hazards by high level filtration of ventilation air BEFORE it is distributed inside a building. That’s seldom done now for residences, & too often not done in commercial buildings. Vast majority of current ventilation equipment has inadequate or no filtration for chemicals and particulates. PROBLEM #2 stated well by Building Science expert, Joe Lstiburek: “The solution to pollution is NOT dilution!” Elimination of unhealthy building materials, furnishings, & activities is the most effective & efficient means of creating healthy indoor air for people [i.e., ‘source control’]. We should use ‘dilution’ only when elimination & filtration haven’t been able to provide healthy indoor air. PROBLEM #3: in many locales [& increasingly with climate change], outdoor air may have high levels of humidity that, when brought inside buildings in ventilation air, will create conditions for mold & associated health effects. Many if not most buildings are NOT designed currently to dehumidify high rates of ventilation. And many existing residences may not be able to be retrofitted correctly for problem-free ventilation. IMO, ventilation can be an important, & even essential, component of achieving indoor air quality & healthy buildings…but only when part of a carefully developed overall IAQ strategy that first focuses on (1) elimination of unhealthy building materials, furnishings, & activities, and (2) high level filtration of indoor air and ventilation air.
Terry, you continue to discredit your claim of “not being an expert”. Your observations are spot on. Joe also says house don’t need hyperventilation systems. Best practice is 4 to 5 ACH, but not with outside air. 4-5 ACH in the conditioned space with effective filtration to remove the stinky bad stuff. This is a challenge with low load homes at some 1500 to 2000 sq ft per ton. The concept is discussed in section 15 of ACCA Manual LLH, entitled Engineered Bypass Air.
I appreciate your kind words, Danny, but my areas of ignorance are large, my friend. I have sooo much yet to learn, especially HVAC tech [plan to dig into IAQA’s HVAC course series this fall]. Thank u for the additional feedback re ACCA LLH–I see it’s $135 so I’ll chew on that. Just finishing Joe Allen’s “Healthy Buildings” book & starting Joe Lstiburek’s “Moisture Control in Residential Buildings”, & just finishing Joe’s great Zoom course on bldg science. I just messaged u on LinkedIn–i can learn a lot from you. NCarolinians are fortunate to have your expertise.
Kevin, I don’t think see it solely about energy savings. In my climate (CZ4), humidity is always an issue. My experience has shown that non ERV systems will drop indoor humidity below 30% in the winter. The complaints include, nose bleeds, dry eyes, sinuses, overall respiratory complications, and static electricity. Then the building shrinks. Hardwood floors, moldings, raised panel doors and furniture shows substantial evidence of shrinkage. This demands a humidifier and steam is the only one I have seen that is effective. Those cost as much or more than the ERV. There are some cases where we have huge houses with 2 people that require humidifiers even with an ERV.
In the summer, we see high indoor RH. Of course that can be addressed with a dehumidifier. But wintertime operation doesn’t improve.
Not really. There’s a point where the air tightness of a home requires mechanical ventilation. For example imagine if your home was a plastic bubble. You’d still suffocate with just a heat pump.
I’ve been surprised at the number of people who think that their heat pump provides ventilation.
Where do you get plastic duct work? When did they approve that?
Replying to Danny Gough’s uncertainty about continuous fan operation of a central AC air handler…in all but the driest climates that practice will increase indoor humidity substantially during cooling season. It isn’t about the droplets being blown off, but rather condensed water reevaporating whenever compressor cycles off and coil warms to return air temperature.
In my own experience the hit (increase in %RH) is about 10 points…a penalty I’m not willing to pay. Of course having a separate ducted dehu overcomes that hit, but at considerable energy cost.
Awhile back I did a back-of-envelope calculation of the total area of exposed metal in a modern higher SEER air handler coil and it worked out to somewhere around 100 square feet per ton – that’s a lot of wet metal dumping its moisture back into ducted air stream during every compressor off cycle – easily dozens of times per day at industry-standard 3-cycles-per-hour thermostat programming.
Curt, I’m not at all disputing your explanation at all. But I just can’t seem to get my head around the physics. The compressor shuts off. But there is still water on the coil. In order for that liquid to change state, there must be an energy exchange. It seems to me that exchange is inevitable with or without the blower running. Although, the blower would accelerate drying, the end result is the same. The resident water that does drain off will end up in the discharge air as vapor. Please teach me how the actual physics support the increased RH..
Your understanding is correct – in fact running the blower greatly accelerates the drying. With the blower off, very little drying takes place since the air immediately adjacent to the coil saturates and no further evaporation takes place – the coil simply stays wet until the next compressor cycle. In addition, some continued dripping and runoff into the drain pan and condensate drain line continues during the off cycle…that’s where we want the water to go in humid climates.
If the blower stays on, room temperature air of moderate humidity (50 – 60% RH) passes over the fins and coils at high velocity, promoting rapid evaporation and increasing indoor humidity.
To experience this personally, set up a window or pedestal fan in a room adjacent to where you typically take a shower…take a shower and then stand in front of the running fan while you are still dripping wet – you’ll experience the high cooling effect of rapid evaporation!
In semi-arid areas where indoor humidity is not a concern (or, if anything, too low) , it is actually advisable to keep the blower on, if not continuously, at least for a few minutes after compressor cycles off. The re-evaporation of condensate (if any was produced during the on-cycle) provides extra free cooling via the energy transfer…about 1000 Btus per pound of re-evaporated water.
Taking that effect yet another step brings us to the “swamp cooler” These produce cool air solely via blowing hot dry air across a coil kept wet be being supplied with tap water…actually it more resembles a sponge.
About 25 years ago on a summer road trip through central Nebraska I stopped for lunch at a small town diner. I noticed what appeared to be a great big window AC unit producing lots of somewhat cool air – it kept me comfortable while I ate, but something was “off”…Its cabinet was the size of a very big window AC unit, on the order of 1.5 – 2 tons, and the airflow was consistent with that big a unit. What caught my attention was the tiny power cord – it looked like what might be used for a floor lamp – 16 or maybe even 18 gauge 2 conductor with a light duty plug.
I wasn’t buying that, so I went outside in search of a much beefier electrical connection consistent with a big window unit – 15-20 Amp 230 volt circuit – none found! I finally realized it was a swamp cooler – all the cooling derived from evaporating water, the only electrical load being the blower..perhaps 3-5 amps at 120V.
David wrote: “…and pollen! Pollen is a BIG problem for anyone with allergies, but easy to catch with filtration. I found that placing a basic media filter behind my window fan makes a huge difference.”
I bought a custom sized 2″ MERV-13 filter that fits a convenient downstairs window. I slide open the windows, place the filter in, and close the window frame “snugly” on the filter frame. Airtight? Absolutely not! But the gap between the filter and windowframe is surprisingly small (the paper filter frame gives a little, so it does fit kinda snugly). Then I turn on the exhaust fan of my choice, many times kitchen range hood – finally got a use for that 600 CFM fan. I also try to pop open said window when my ACs are running, balances out negative pressure in my house (I hope). Of course this only works when the outside temperature and humidity are somewhat ok. So in a way it’s a partial (as opposed to whole) house mini fan with filter.
This is my (very) poor man’s version of an ERV, so my real question is: Can an ERV be meaningfully installed in a 2013 California house as retrofit? 3.5 ACH50 envelope leakage, 5% duct leakage.
@Deniz wrote:
> I also try to pop open said window when my ACs are running, balances out negative pressure in my house…
Why would your AC blower cause house to go negative?
> Can an ERV be meaningfully installed in a 2013 California house as retrofit?
No way anyone can answer that without reviewing the particulars of your home, but if it has an unfinished attic, crawl space and/or unfinished basement (I realize the latter two are unusual in Calif), then a reasonable retrofit ERV project is a lot more likely.
BTW, if you live in a warm, dry part of the state, you might consider installing a balanced ventilation system without the recovery core (e.g., Fantech AEV80, which sells for $299 at supplyhouse.com). It’s hard to justify the additional cost of an ERV without high latent and/or heating loads.
@David I had not thought of just a balanced ventilation system. My climate is dry, yes, but warm has turned into hot the last few years. Many days above 90F, which used to be a rarity.
> Why would your AC blower cause house to go negative?
https://www.energyvanguard.com/blog/The-Sucking-and-the-Blowing-A-Lesson-in-Duct-Leakage
After reading this excellent article, I did the test and some math. Duct leakage on the supply side octopi on two 3-ton units. Assume the fans suck the return air in at 1200 CFM each. 137 CFM is lost if the supplies are added up (actual measurement). Duct leakage to outside is at least 66 CFM (measured).
@Deniz, being deeply familiar with zonal pressure impacts of duct leakage (but unfamiliar with the specifics of your test), allow me to offer the following thoughts for anyone who might want to try to quantify a duct leakage imbalance:
As I’m sure you know, the typical duct pressurization test doesn’t determine supply vs. return leakage. As Allison noted in the article you linked, a manometer can show if duct leakage is net positive or negative, but the only way to determine the magnitude (cfm) of the imbalance with a duct tester is to isolate the supply and return sides and run separate tests on each side (requires fabricating a temporary dam at the AHU).
More importantly, the standard 25-Pascal test pressure (25 Pa = 0.1 inch water column) is intended to represent the AVERAGE pressure inside a duct system. In reality, the pressure on each side always ranges from 0 Pa at the grilles to whatever pressure the blower develops at the air handler. That means actual leakage during operation is likely to be substantially different. If leakage paths are mostly near the air handler where pressures are highest, actual leakage will be higher than the test indicates. Likewise, if leakage paths are mostly out in the branch lines and boots where pressures are lowest, actual leakage will be lower than indicated. Moreover, dominant leakage paths can be biased toward the same or opposite ends of their respective sides. Bottom line: a duct pressurization test provides an objective assessment of duct sealing but the results shouldn’t be assumed to represent actual leakage.
OTOH, it sounds like you may have used a flow hood to measure supply airflow at the diffusers and compare to nominal airflow…? I see a couple of problems with that. First, unless you measure airflow at the return grille(s), you can’t really know if either system is truly removing 1,200 cfm from inside the pressure boundary. Second, flow hoods and most other field-suitable airflow measurement methods are roughly +/- 10% at best, which is larger than the imbalance you’ve imputed.
Of course, there’s no harm in opening a filtered window to take advantage of natural ventilation when conditions are moderate. But keep in mind that unless you operate a mechanical exhaust fan (or verify duct leakage is seriously supply-side dominant), wind and/or stack effect can swing your house positive by inducing infiltration through the envelope leakage pathways you’re trying to avoid. In fact, unless it’s cold outside, an open downstairs window in a multi-level home is more likely to be an exhaust port than a supply port.
Regarding recovery vs non-recovery ventilation equipment… Keep in mind that HRV/ERV recovery efficiency is significantly lower in warm weather, as recovery efficiency is proportional to delta-T. The additional cost of a recovery type ventilation system clearly makes sense when there’s a 70 degree delta-T, or persistently high latent loads. Not so much in a climate with low latent loads and maybe a couple of hundred hours a year with a delta-T exceeding 20F.
@David thank you for the detailed explanation, as always very informative.
> the only way to determine the magnitude (cfm) of the imbalance with a duct tester is to isolate the supply and return sides and run separate tests on each side (requires fabricating a temporary dam at the AHU).
Indeed, we were unable to isolate the supply and return sides. I observed, however, that when my system runs, air rushes into the house when I crack a window. I should be able to verify with an anemometer.
> If leakage paths are mostly near the air handler where pressures are highest, actual leakage will be higher than the test indicates.
The main returns are very close to the air handlers/plenums. The supply lines are (obviously) many and lengthy. There is some leakage after the air handlers, that can be felt by hand and seen via IR cam. You might not believe this, but the ducts were attached to the plenum, insulation around them, and then mastic used to “seal” the insulation sleeves (not the duct) to the plenum. Every time I look at it I want to redo it myself but not sure how to get rid of mastic.
> OTOH, it sounds like you may have used a flow hood to measure supply airflow at the diffusers and compare to nominal airflow…? I see a couple of problems with that. First, unless you measure airflow at the return grille(s), you can’t really know if either system is truly removing 1,200 cfm from inside the pressure boundary. Second, flow hoods and most other field-suitable airflow measurement methods are roughly +/- 10% at best, which is larger than the imbalance you’ve imputed.
Yes the rater did use a flow hood and measured the return grilles as well. They weren’t exactly 1200 CFM (as expected). But if they’re +/- 10% then yeah, I’m not sure why he bothered (would have been nice if he gave me the individual measurements so I could get rough CFM supply by room). Looked expensive, though.
Is there a mechanical ventilation system that will introduce fresh outside air based on actual pressure differential instead of calculated needs? Or is that = open a window if you know the house is negative?
> But keep in mind that unless you operate a mechanical exhaust fan (or verify duct leakage is seriously supply-side dominant), wind and/or stack effect can swing your house positive by inducing infiltration through the envelope leakage pathways you’re trying to avoid.
If the house turns positive, don’t the dampers/flaps of the various mechanical exhaust systems account for that by letting the excess air out? Between 7 bathroom fans, a laundry room fan and the kitchen vent hood, should be plenty of 3+ inch holes..
> Is there a mechanical ventilation system that will introduce fresh outside air based on actual pressure differential instead of calculated needs? Or is that = open a window if you know the house is negative?
I can’t imagine how something like that would/could work.
The problem with opening a window as a pressure equalizer is that it will increase (dramatically) the air exchange with the outside. Wind and stack effect guarantee that. The impact of wind is typically lateral (positive on one side, negative on the opposing side) and the impact of stack is vertical (top to bottom or bottom to top, depending on the season). That means the house can be positive and negative at the same time (and usually is, when all fans are off).
Theoretically, the only time a window (or any other leakage path) is neutral, or acts to benignly neutralize internal imbalances such as duct leakage, is if outside air is perfectly still and precisely the same temperature as house and the house is uniformly the same temperature (i.e., no stack effect).
Extrapolating from this, even if I open all windows, the pressure will still not be uniform. So in essence it seems local and overall pressure differentials between various areas of the house and the corresponding outside areas are unavoidable. Back to: either provide forced make-up air if an exhaust fan is running, or open a (nearby) window and pray.
If your objective is to ventilate the house with filtered air when outside conditions are favorable or moderate (‘poor man’s ERV’) then your best bet is to use a filtered window fan as the supply. The filter doesn’t need to cover the entire window opening, just the fan itself, since any openings around the fan’s frame will be at negative pressure. Forget about trying to neutralize pressure differentials when your HVAC and/or other fans are operating.
I’m in a tiny house with a substantial negative air pressure problem when I use my portable AC, all the windows howl. Would a ERV help balance the air pressure?
The exhaust fan over the stove and in the bathroom don’t have too strong of an effect, but the portable AC unit causes the home to lose air, so I’m trying to figure out what kind of fresh air intake system would help. Currently I have to keep a window open but that creates an extremely strong air current from the window and messes with the temperature in my 300 sqft tiny house. Anyone have advice?
@Alice, ERV’s by design are intended to maintain pressure balance. Although it’s possible to adjust the intake and/or discharge fans to achieve an imbalance, doing so would significantly reduce the ERV’s recovery efficiency, thus defeating the purpose of having the ERV.
Not sure what you mean by ‘portable AC,’ but it definitely shouldn’t cause your house to *lose* air. You need to determine why that’s happening and fix that problem.
Portable AC’s plug into an outlet and sit on the floor with a small duct that blows the hot air from the condenser out a nearby window. The cheapest ones with one duct get the inlet air for the condenser from the room. There are other models with two ducts connected to the window with one being a supply to the condenser and the other an exhaust from the condenser. The one duct system will depressurize the room which will cause outdoor air to be sucked in elsewhere, thus adding to the cooling load. The two duct system will be less efficient due to the condenser rejecting heat to the hotter outdoor air. So the net effect is one increases your cooling load at higher efficiency and the other has a lower cooling load at lower efficiency. Since you are noticing the depressurization, you probably should use a two-duct portable unit.
David,
The portable AC to which Alice refers typically operate not by being installed in a window but by being ducted to a window. The unit itself sits on the floor, typically on wheels. Nearly all (~90%) are single hose variants, which are horrible and should be outlawed, IMO. Air needed to cool the condenser / hot side is drawn from within the house, resulting in both a substantial imbalance of air pressure within the home and of course a huge amount of outdoor air sucked in through every available leak…I can’t imagine what one of those would do in a small tight house.
The better portable units have two hoses that serve to bring in outdoor air to cool the condenser and then discharge it outdoors. That is the only design anyone should purchase / deploy. They are horribly inefficient relative to conventional window units, but they do work, sort of. I have one in the loft of a ~300 SF shed whose windows are way too small for a conventional window shaker – it works, but draws roughly double the power of a similarly effective window unit.
Thanks guys. I should have googled ‘portable ac’ before replying. I’ve seen two-hose units, especially those mobile outdoor units used for tents, construction, etc, but I wasn’t familiar with one-hose portables. Wow… every cubic foot of room air used by the condenser gets replaced by a cubic foot of outside air. That seems like remarkably like chasing one’s own tail 🙂
RoyC,
I’ve tested a two-duct portable unit and it was also terribly leaky and inefficient.
https://greenbuildingindenver.blogspot.com/2014/09/dual-hose-portable-heat-pumps-dont-work.html
Window air conditioners are highly efficient by comparison.
https://www.youtube.com/watch?v=_-mBeYC2KGc