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Common Problems With Cold Weather Ventilation

Ventilation Energy Enthalpy Recovery Ventilator Erv Humidity

When I woke up Saturday morning, the temperature outdoors was -40 degrees. The wind chill was -100 degrees! It was just unbelievably, impossibly, inhumanly cold outside. Fortunately, that was on a mountaintop in New Hampshire and not where I was. I happened to have woken up on a mountaintop in North Carolina, where the temperature was a much warmer -3° F.

And when it’s cold outside, most people prefer to be in warm, cozy home with no drafts. Of course, we all know that no drafts means we have good airtightness, and good airtightness means we need mechanical ventilation. But how do you ventilate a home when it’s really, really cold outdoors without causing problems? And what are those problems with cold weather ventilation?

The 3 whole-house ventilation strategies

Exhaust-only ventilation. This one’s the cheapest to install and the most common. It relies on fans that are already in the home: bathroom exhaust fans and kitchen range hood. These fans are set to run continuously or intermittently with a controller. As they exhaust air from the home, the resulting negative pressure inside causes air from outside to come in through leakage sites in the building enclosure.

Supply-only ventilation. Like exhaust-only but the fans blow outdoor air into the house, creating a positive pressure. It can be done with regular bath fans or specially designed fans or by hooking up a controller to the central air handler.

Balanced ventilation. Most people automatically think of the energy recovery ventilator (ERV) and heat recovery ventilator (HRV) here, but balanced ventilation simply means supplying and exhausting equal quantities of air. I’ve written previously about different ways to do balanced ventilation.

When it’s really cold, bringing in outdoor air needs to be done with some forethought. There’s a reason we don’t just open the windows year-round. So let’s take a look at what can go wrong and what we can do to make things better.

Cold drafts

The problem. With any of the three strategies, you could end up with comfort problems if the occupants feel cold drafts. This can happen in a variety of ways. With exhaust-only, you could cause a draft through a leaky window or door near an occupied zone. With supply-only, you could be blowing untempered cold air right on occupants with a bad design. You could also use the central heating system to blow tempered air through all the vents in the home, but that air will feel cold to occupants, even if it’s close to room temperature. Same goes for ERVs and HRVs.

The solutions. This is mostly a design issue. Good HVAC contractors will tell you: Never blow air directly at the occupants. When ventilating with cold outdoor air, this is even more true. You want that air to be well-mixed by the time it reaches them. With exhaust-only ventilation, this is hard to do…and yet another reason to avoid that strategy. Another aspect of this is the ventilation rate. If you’re ventilating more than necessary, this problem is more likely.

Dry air

The problem. Cold air is dry air. 100% relative humidity sounds like really humid air, but when it’s at a temperature of 32° F, it’s actually pretty dry in terms of actual moisture concentration. If you warm up that same air to 70° F, the relative humidity drops to 20%. The colder it is outdoors, the lower the moisture in the air and the dryer your home becomes when that air comes inside, either through intentional whole-house ventilation or through infiltration. When you reach for that lotion, remember that the source of the problem is cold, dry outdoor air.

The solutions. Exhaust-only and supply-only ventilation with cold air will dry out the indoor air. That’s why indoor humidity runs lower in winter. In homes with a lot of air leakage, the indoor air can be bone dry. The same is true in homes with too much ventilation. Make sure you’re not ventilating more than you need to. 

The question of how much ventilation you need is a big one, and one that I’ve addressed here previously. The standard answer is to ventilate at a rate specified by the ASHRAE residential ventilation standard, 62.2. The building code is diverging a bit from 62.2, and the rate in the 2018 International Residential Code (IRC) is:

(0.01 cfm/square foot of conditioned floor area) + (7.5 cfm/person)

For code purposes, the number of people in a home is defined as the number of bedrooms plus one.

But of course, what rate you choose depends on where you are in the process. If you’re building a new house, you have to go by your local code. If you’re living in a house with mechanical ventilation system, you can run it how you wish. You can use the formula above to get an idea of  whether or not you might be overventilating. If the result of your calculation is that you need 50 cubic feet per minute (cfm) of ventilation air and you’re running your 200 cfm HRV continuously, I think it’s fair to say you’re ventilating more than you need to.

You also may be able to reduce the amount of ventilation air further when it’s really cold. Why? Because the stack effect increases with temperature difference. The colder it gets outdoors, the more infiltration you’ll have because of warm air rising inside the home. Some ventilation devices have controls that automatically cut down the amount of ventilation air when it’s cold.

Another way to reduce the drying effect of ventilating in cold weather is to use an energy recovery ventilator. They exchange heat and moisture, thereby allowing you to keep the humidity levels higher. Instead of exhausting the humidity you already have and replacing it with dry air, some of the water vapor transfers across the membrane and comes back into the house.

And that brings up the next problem of cold weather ventilation…

Frozen ERV cores

The problem. I don’t live in a cold climate and don’t have direct experience with this, but if you talk to people in the really cold places, like IECC climate zones 6 and higher, they’ll say you have to use a heat recovery ventilator rather than an energy recovery ventilator. The reason behind their claim is that when that water vapor transfers through the energy recovery ventilator membrane, it encounters that incoming cold air and freezes. As the mass of ice in the ventilator grows, you lose not only the moisture recovery and heat recovery but also the ventilation air. Ice, as it turns out, is a pretty good air barrier.

The solution. I’m sure some people will tell me in the comments to this article about actual freeze-up cases. What I know is that frozen ERV cores don’t have to happen in cold climates. How do I know? Because the Cold Climate Housing Research Center (CCHRC) has done some research on it. They studied 14 houses with 8 models of energy recovery ventilators in Fairbanks, Alaska in the winter of 2013-14. None froze, even though temperatures got as low as -40. (I love that temperature! Or at least the interesting factoid about it.)

The solution is simple: a defrost cycle. Different models have different methods for preventing ice, so there’s not just one to do it. Controls could close and shut off the intake side every once in a while, thus turning your ERV into an exhaust-only system temporarily. Similarly, the intake side could have a damper that switches from outdoor air to room air periodically. Another option is that the incoming air can be tempered with room air to keep the core above the freezing point. Frozen ERV cores aren’t a given in cold climates.

Frozen HRV condensate lines

The problem. An energy recovery ventilator can get freezing at the core, where the interior air’s water vapor permeates through the capillary core membrane to the cold side. A heat recovery ventilator can freeze up, too. In an HRV core, the moisture can condense on the cold, impermeable membrane. That’s why HRVs have a drain in the bottom and a condensate line to carry away the liquid water. And that’s where the trouble happens. If the end of that condensate line is outdoors, you can get a chunk of ice in it.

The solutions. You can fix this in a few ways. You could wrap the condensate line with heat tape, which uses electric resistance heat to prevent freezing at the end when the temperatures are low enough. That’s an inelegant solution that uses more energy, though. Better would be to run the HRV condensate line into a sewer drain line somewhere inside the house. That way the condensate should never encounter below-freezing temperatures. (But make sure you get a qualified plumber for this as there are right ways and wrong ways to do it.) If the condensate line must run to the outdoors, design it so all condensate runs out quickly.

Condensation on air intake ducts

The problem. This was a problem I ran into on one of my very first jobs, way back in 2004. My client was building an ENERGY STAR home so I worked with him to make sure the HVAC contractor installed mechanical ventilation. The intake air duct ran from a wall cap on the outside wall through the basement ceiling to the air handler. In their first winter, they discovered a problem: a line of water spots on the basement ceiling, right below where that intake air duct ran. Turns out, they put no insulation on it.

When cold surfaces run through humid air —  and in winter, the indoor air is where the humidity is — condensation is possible. That’s exactly what happened here. That cold ventilation duct turned into a condenser, dropping its condensate on the basement ceiling. This was a case of accidental dehumidification.

The solutions. Condensation on the ventilation intake duct, or even frost if it’s cold enough, from having uninsulated intake duct. It could be totally uninsulated, as was the case with my client, or just poorly insulated. Make sure the duct is fully insulated and, as with ducts in crawl spaces, the outer insulation jacket is sealed up.

Condensation on the intake duct also could happen because of an intake duct that isn’t properly sealed. That duct is under negative pressure. There’s a fan somewhere pulling air in from outdoors. If the duct isn’t sealed, it can pull humid interior air in, resulting in condensation. So when you install ventilation intake ducts, you’ve got to seal them completely, insulate them completely, and seal the insulation jacket completely.

Condensation in walls

The problem. Water vapor likes cold surfaces. It will condense or adsorb on them with glee. (Or is it with gusto? I can never remember.) A supply-only ventilation system in a cold climate can result in positive pressures indoors, which can push humid, indoor air into wall cavities. When that humid air finds cold sheathing on the other side of the insulation, it starts accumulating. If enough accumulates over a long enough period of time, you can start a biology experiment in the wall. That’s bad for durability and for indoor air quality. I don’t recommend it.

The solutions. In warmer climates (generally zones 4 and lower), cold weather rarely lasts long enough for this to be a problem. In cold climates, this issue has garnered a lot attention because it’s real. It’s one reason some people recommend using exhaust-only ventilation rather than supply-only. That’s far from a guarantee against moisture accumulating in your sheathing, though. Those exhaust fans could be pulling air in down low while the stack effect is pushing it out up high. The photo below (by Joe Lstiburek) shows moisture damage from positive pressure on the second floor of the building. The first floor is fine. (This is from his article, How Do Buildings Stack Up?)

The real solution here is fix the building enclosure. You can do that by installing exterior insulation that keeps the sheathing warmer or by keeping humid indoor air from leaking into the wall cavity.

Don’t forget the V in HVAC

Homes are getting a lot more airtight these days. New homes have to get blower door tests showing they hit certain thresholds. Older homes are getting air-sealed by home performance companies. That means we have a lot more whole-house ventilation systems in use. And no matter how it’s done, there’s technology involved. There’s design. There’s variation in conditions, both interior and exterior.

Buildings are put to the test when it’s really cold outdoors, as it has been recently with the ridiculously named Bomb Cyclone. (It sounds like a dessert at a chain restaurant.) Building enclosures fail. Heating systems fail. And ventilation systems fail. Things happen at really low temperatures that don’t happen when it’s a more normal cold.

If you’re experiencing any of the cold-weather ventilation problems I described above, the first step is figuring out where things went wrong. Was it the technology? The design? The installation? Knowing some building science can help a lot as you figure it out.

My hope is that eventually whole-house ventilation will be accorded the status it deserves. The initials HVAC seem to accord it equal weight to heating and air conditioning, yet that’s not currently the reality. But for now, I’m anxious to hear your cold weather ventilation stories and problems. What problems did I miss? What solutions have you found?


Related Articles

Why Do Airtight Homes Need Mechanical Ventilation?

4 Ways to Do Balanced Ventilation

Cold Air Is Dry Air

Accidental Dehumidification – A Preventable Mess



Now really, I don’t have to give units for that temperature. Not because you already know it but because in the two temperature scales in common use, -40° F is equal to -40° C. Even the science nuts who insist on Kelvin know it’s impossible ever to reach -40 K because of the third law of thermodynamics. And the same is true of the Rankine scale, which is to Fahrenheit what Kelvin is to Celsius and for which you should not bother creating any neural connections. Therefore, it is never necessary to give units for the temperature -40.


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This Post Has 37 Comments

  1. I don’t agree with your
    I don’t agree with your comments about ERV’s and HRV’s relative to frosting. It is my understanding that HRV’s also frost and do require a defrost cycle in cold climates, in fact, HRV’s will frost sooner than ERV’s since ERV’s will allow water vapor to pass through at lower temperatures before it freezes. I know of at least one manufacturer of ERV’s that claims that they do not need a defrost cycle.

    1. I’m glad you mentioned that,

      I’m glad you mentioned that, Roy. Here in the Southeast, freezing ERV or HRV cores is rarely an issue, although it certainly could have been our recent cold snap. But I’ve folks up in CZ 6 telling me no way, never ever put an ERV in their area because it’ll freeze whereas an HRV won’t. And as I was writing this up, I started thinking about that. You’ve got the same warm humid air going through the exhaust side and the same cold air on the intake side. The only difference is that in the ERV, as you pointed out, some of the moisture switches sides. So I agree with you in your disagreement with me. If that water will freeze going through the membrane, it should also freeze going near the membrane.

      So where does this cold climate confusion about ERVs and HRVs come from?

      1. I think that the confusion
        I think that the confusion comes from the HRV/ERV manufacturers. Some prefer to sell one over the other for various reasons. Adding a defrost cycle to either does add first cost, results in some energy penalty, and makes the equipment less reliable. Thus, some researchers are looking at the possibility of turning off the ventilation during potential frosting conditions if it can be justified by higher infiltration due to stack effect as you mentioned.

        Another thing to think about is that all of these moisture issues will be even worse in northern climates when humidifiers are used. I spent my first 45 years living in Illinois, Iowa, upstate New York, and Minneapolis, and have never had a humidifier and never felt the need for one. If you really think that your humidity level is too low in the winter, I suggest tightening your house first before installing a humidifier, and even then, I would think strongly about whether you really need higher humidity.

        1. Still other manufacturers
          Still other manufacturers seem hesitant to give solid recommendations for HRV vs ERV in a particular climate, relying on the local HVAC techs to make that call. Venmar used to have a map, but they got rid of it and Broan has been somewhat cagey about giving specifics. That has resulted in me getting into a least one “ERV vs HRV” disagreement with a local HVAC tech. I don’t want to argue too much with the installer because they’re the one who has to feel comfortable installing and servicing for the homeowner, but, they aren’t always up on the latest, either, especially if they are in a CZ either one could work and it can be a case of weighing cooling season benefits and risks vs heating season benefits and risks. That is a really great point about winter humidification from RoyC, too, yet another consideration.

      2. I find that most of the
        I find that most of the confusion comes from people who regurgitate something they heard somewhere and have no actual knowledge of what goes on. I’ve been working with these units in a cold climate for over 11 years and can tell you that both ERVs and HRVS with a good defrost strategy don’t run into problems with low temps. Poor installation, maintenance and lack of understanding can always lead to problems with any mechanical system.
        What irks me the most is when “experts” allude to “you don’t want to over ventilate” and never define that. Why we think that a MINIMUM ventilation standard is the goal and that anything over that is “TOO MUCH” is just short sighted and is causing so much confusion.
        Go ahead, call me a radical. I think that the health of the people in the building is the most important issue. Energy loss, comfort, humidity (too high or low) are all LESS IMPORTANT issues than health. You want evidence that minimum ventilation rates are way too low, read Jan Sundel’s paper from 2011. 27 different studies with correlation to ventilation rates and health and performance impacts. End result. Healthy ventilation rate 53 cfm. And we talk about 7.5 to 15 cfm. Stop creating confusion by only telling part of the story. Use ERV’s in most cases in cold climate. Use a brand that has demonstrated they can deal with extremes like we had last week. Hire trained professional to install who know what they are doing. AND educate the consumer what is in their best interest. From all the research I’ve seen, healthy air is most likely attained somewhere between an air change every one to 3 hours. Shooting at anything lower than this puts the occupants at risk.

    2. Does anybody know how long
      Does anybody know how long defrost cycle of hrv continues when once it started ?

  2. 1st, Great Conversation with
    1st, Great Conversation with RoyC. I fully agree with the tight house issue. When we designed this house, the great experiment was if being tight (0.9ACH50) would yield comfortable winter humidity with no humidifier… Yes, RoyC is exactly right! Without a humidifier, only an Ultra-Aire XT-105, my humidity stays in the 30-40% during winter.

    1. ejkessler: You mentioned
      ejkessler: You mentioned that you have a dehumidifier in your house, but you don’t have to run it in the winter, do you? I was just curious, because a lot of people do not realize that if you do need dehumidification and heating at the same time, standalone dehumidifiers work pretty good since the latent heat of the water removed from the air is returned as sensible heat. Thus, you get a sensible heating COP that is greater than 1. I am not saying that this COP is better than a conventional heat pump, but it is better than electric strip heat.

      1. Good question RoyC,
        Good question RoyC,
        I don’t use Spot De-Humidifiers but yes that synergy would be good for those that do use them. I built the house 2yrs ago and I designed it with input from Andy Bell at Energy Vanguard but also by studying under Dr. Bailes in his 1st offering of his Mastering Building Science course. I actually wish I took course a year earlier, I would have done a few things different.

        To answer your question, I do run the dehumidifier year round, though in the winter, even here in the mixed climate of Virginia, I don’t expect it gets much use; I just always leave it on.
        My home HAC+V+Deh uses the Mitsubishi Mini Split system for the HAC; the Mitsubishi Lossney ERV for the V; and the Ultra-Aire XT-105 for De-Humification. The weather has been regionally record cold for the past couple of weeks and I did see a 30 & 31% humidity; good days to take a long shower and make soft boiled eggs for breakfast. haha
        In the winter I have the De-Humidifier set for 45% in hopes to let it get as high as it wants and it typically stays in the mid 30’s%, but in the summer I keep it at 35% just because I can and it allow me to set my thermostat at 76-78 degrees and it feels extremely comfortable & refreshing.

        1. ejkessler: That is very
          ejkessler: That is very interesting. Your house must be quite tight if that dehumidifier can maintain 35% RH during the summer in Virginia. You say that your cooling setpoint is 76-78 F during those conditions. What do you think it would be if you were only able to maintain 50% RH indoors in the summer?

          1. Another Good Question RoyC.
            Another Good Question RoyC.
            Yes it’s fairly tight but did not meet my design goal; I achieved 0.9ACH50 when they did the HERS blower door test, my goal was to get in the 0.6 range. Perhaps I actually did better the the 0.9 because I could not fully seal off:
            – the 2nd story 6″ intakes for the ERV and Dehumidifier,
            – I also only closed the Dryer Door which lead to a 4″ hole and
            – the Oven/Range Hood & Whole house Vacuum system may not have been sealed fully
            – I had one Spot ERV which I completely forgot to seal as well…

            Anyway, I keep it at 35% because the Ultra-Aire XT105 only draws 4.9 amps and it allows me to set my A/C at a higher temp than the 70-72 degrees I normally would like. Also I keep it well below 50% so I don’t grow dust mites; I have an allergy and lung issue with them. But your question is a good one, I had to go to the Psychrometric Chart to see an equivalent, haha… seems like a homework question from Prof. Bailes during my building Science Class. Now if I dusted the cobwebs from my brain and if my crude table and eyes read right…
            … my 78 degrees at 35% RH would equal about 63 degrees at 50% RH. Gee maybe I am not reading or doing it correctly because that 63 degrees from the Psychrometric chart kind of surprised me, non the less it feels great in the summer and it is amazing when I walk in from lawn work.

          2. ejkessler: Your claim that
            ejkessler: Your claim that raising the RH from 35% to 50% results in about a 6 F required reduction in dry-bulb temperature for the same comfort level is interesting to me. According to the comfort envelope in ASHRAE Std. 55, this change in RH only results in about 1 F change in dry-bulb for equivalent comfort. Can you give me a reference to Prof. Bailes source for this information?

          3. Oops, I was rushing or my
            Oops, I was rushing or my glasses were not on well… not 63degrees but 67 degrees.
            Perhaps Dr. Bailes will weigh in and re-teach me how to use the table.
            I used the chart:
            Starting at the bottom at 78 degrees, I went up to the 35% RH curve and then across to the 50% curve and straight down to see what temp it yields… my eyes see 67 degrees.
            So the big IF … is IF I am understanding proper use of the Psychrometric table from 2+ yrs ago and also am I using it correctly.
            If I did it correct then it says that 78Degrees at 35%RH “Feels” like 67degrees at 50%RH. I do know intuitively that less humidity relates to a much more comfort at a given temperature.

          4. The method you used does give
            The method you used does give a valuable answer, but not the answer you are looking for. Moving horizontally on the psychrometric chart is holding constant the specific humidity, the absolute quantity of moisture in the air. It tells you if you could take a snapshot of the air in your house and drain all the water out of it, how many ounces of water there would be, regardless of temperature. It doesn’t reflect comfort, or “feels like” temperature. A somewhat more accurate (but not perfect) method would be to follow the northwest-pointing lines, on the Enthalpy scale, to compare temperature and humidity changes at similar comfort levels. The ASHRAE 55 calculator is the most precise method. From 17 years of doing IAQ surveys, the perceived answer is probably somewhere between 2 and 4 degrees F, assuming no change in air movement.

          5. Ah ha Bobby,
            Ah ha Bobby,
            Thanks so much for helping me! You and RoyC are an excellent nudge to re-leaning this. Dusting my cobwebs and looking at the Chart did seem a bit easier than I recalled. By the way, 2-4 degrees delta does seem more in line with what I would expect. Now on to locating a good free access link to ASHRAE 55 calculator… would you direct me if one exists?

            Also, Perhaps just a bit more help if you don’t mind on the Psychometric Chart if you would. I would like to better understand what you were teaching me!

            So as a starting point in RoyC’s excellent question to me… if I select 78degrees on the chart, , and follow parallel to the nearest northwest-pointing diagonal line…
            – What do I do next?
            – I do note that this NW line intersects both the 35%RH curve and the 50%RH curve and finally intersects the 100%RH curve about where it reads 67.5 degrees.
            I am lost at this point on how to conclude this question of using the Chart to determine what 78degrees at 35%RH compares in comfort if I allowed the Humidity to raise to at 50%RH set point?

          6. Ok Bobby,
            Ok Bobby,
            Let me try that again… Let me guess since you already told me the answer was a delta of 2-4 degrees.
            My NW line starting at 78degrees actually intersects the 100%RH line 47.5degrees. Now, if when I intersect both the 35% & 50% curves I drop back straight down I land on 60degrees and 56degrees respectively; a 4 degree delta. This is likely not the correct method but … 🙂

          7. Bobby, I am not aware of any
            Bobby, I am not aware of any comfort studies that show that constant air enthalpy corresponds to constant “comfort”. There have been comfort studies by Fanger and others that show that constant comfort lines do move in the “northwest” direction on the psysch chart, but with much less slope. You can see the slope of the constant comfort lines on the ASHRAE Comfort Envelopes in ASHRAE Standard 55 or in the Fundamentals Handbook. If you pick some points off of these constant comfort lines on the sides of the envelopes, you will see that it takes about a change of 15 to 20 percentage points of RH to result in a change of dry-bulb or operative temperature of 1 F in the opposite direction. The ASHRAE comfort calculator should show about the same result, since it was used to generate these lines.

          8. Just to followup and be sure
            Just to followup and be sure that I wasn’t reading the comfort envelope wrong, I went to the Thermal Comfort Tool at If you enter 50% RH and 25.4 C operative temperature, 0.1 m/s, 1.1 met, and 0.5 clo, you should see a calculated PMV of 0.0, corresponding to “Neutral”. Then if you change RH to 35%, it will change PMV to -0.12 which is slightly cooler. Now raise the operative temperature to 25.8 and the PMV will go back to almost zero, actually, 0.01, but I couldn’t get any closer. So that is just a 0.4 C increase in operative temperature (0.7 F) for a 15% decrease in RH for equivalent comfort. Most people don’t believe that the effect is this small, but that’s what the research showed.

          9. Thank you RoyC!
            Thank you RoyC!
            I cannot wait to study that site somewhat. From what you say, it does seem to yield a value rather insignificant compared to how it feels.
            None the less I love the Ultra-Aire XT-105 and the control over humidity it provides me and the relief it provide the A/C. That said, if I were not using a Mini-Spit but instead using an ole type A/C unit which turns on/off rather than the VRF cycling slower or fastest helping to more effectively pull humidity out, I imagine the XT-105 would have a better payback.

          10. +1 to what Roy & Bobby said.
            +1 to what Roy & Bobby said.

            ASHRAE defines the RH comfort range between 40% & 60%. Within that range, there’s a small comfort trade-off between RH and dry bulb temperature. But in my experience, that only applies at the high end of the range.

            When I moved from Charlotte to SE Arizona back in ’06, I thought I could increase my cooling set-points a couple of degrees. RH in my Charlotte home ranged from the low-to-mid 50’s in summer. For years, I was perfectly comfortable with the stat set at 77F during waking hours. I turn it down a few degrees while sleeping. Here in AZ, the RH in my home ranges from 30% in May/June to the high 40’s during the July-Sept monsoon season. Guess what. I haven’t been able to raise the stat by even one degree without experiencing some discomfort. It might have been different had it been the other way around, moving from dry to humid.

            In general, as indoor RH gets close to 60, especially above 60, most people experience discomfort and will compensate by lowering the stat. I don’t put much credence in the notion that this effect extends much if at all below the low 50’s.

            I recommend setting your dehumidifier at 50% this summer and see if you’re not still comfortable at 76F+. Setting a DH at 35% guarantees it’s going to run pretty much 24/7 throughout the summer. That works out to roughly 400 kWh/mo. That’s more than my central A/C consumes!

  3. Professor Bailes,
    Professor Bailes,
    Thanks for this timely article for me here in the uncommonly frozen King George, VA. While I’m in Zone 4, it has been so cold here that our local section of the Potomac River froze across. I’ve been nervous about my Mitsubishi Lossnay ERV core freezing. After reading your article I am somewhat relieved! Thanks for providing the thought provoking article and especially for your “Frozen ERV Core” section and “Solutions” where you reviewed helpful defrosting methods.
    I am comforted because it seems I stumbled into a defrosting activity. Since my balanced & distributed Lossnay ERV unfortunately over ventilates (100 cfm on lowest setting), I am in the habit of turning it off typically on cold evening and even off/on during coldest days. I wish I could say that I thoughtfully came up with this by intellect and clear thinking, lol.

    I need advice, perhaps you would be willing to help me further with my ERV ventilation problem. During your 1st Mastering Building Science Class a couple year ago, while I was building my homes HAC+V+Dehumid, I learned enough to know that with a balanced and distributed ERV, the ASHRAE 62.2 would over ventilate my home. Using BSC-1, I only need 49 cfm however Mitsubishi said they had NO circuit mods or Control mods to offer me, what a letdown because other new ERV systems seem to offer a type of Duty-Cycle program.
    Do you have any ERV control Ideas to suggest? Perhaps someone makes an ERV control with a programmable duty-cycle?
    Thank you.

  4. This article is well timed
    This article is well timed for me having just gone through this cold spell in northern Wisconsin. And one of the problems I felt I was having is I felt I couldn’t my humidity low enough. Your company designed our HAVC system and I am very satisfied with the system, Alexander Bell did the design work. No real complaints I believe part of my problem is learning how to use the EVR. I wish the controls were more automatic in controlling the inside humidity with the outside temperature. My window were icing up every night. We were getting lows below -20f. I wanted to get the humidity down to 30%. I finally turn range hood on at night. Our house is very tight I believe it is less than 1 ACH. I was thinking about changing the EVR core for an HVR core. I did have some icing and condensation on my air intake is about 20 feet long and some kinks to get around some of the plumbing, I have a rerouting plan after the heating season I never did check to see if my core was icing up.

    1. @Walter, even on low speed,
      @Walter, even on low speed, running a range hood all night will significantly increase your heating costs, especially if you have a heat pump with supplemental heat strips. You’d be better off increasing the ventilation rate through the ERV (assuming it has reserve capacity). However, after spending good money getting your enclosure below 1 ACH50, solving a moisture problem with sub-zero air is adding insult to injury. Instead, you should identify and the moisture source and work from there.

      Showers, cooking and clothes drying are likely the largest moisture generators. Make sure you have adequate spot exhaust for shower moisture, and that family members operate those fans without fail. Ditto for cooking. Also, it’s best to keep the bathroom door shut during and after a shower so the exhaust fan can do its job.

      BTW, if your ERV has a ‘stale air exhaust’ inlet in a high-use bathroom, make sure you don’t rely on that when taking showers as the ERV will recycle much of that moisture back into the house. Always use the dedicated exhaust fan. In fact, I advise clients with high performance homes not to install ERV exhaust inlets in bathrooms. That’s just asking for trouble.

      Lastly, you mentioned condensation and ice on the outside air duct. Any portion that passes through condtioned space must be insulated with a continuous vapor barrier.

  5. A talk was being given on The
    A talk was being given on The Fundamentals of Flow: Ducts Done Right in Saratoga yesterday. I was a bum and skipped out. Instead doing an energy audit on the outskirts of Saratoga. I found the 14 year old house to be quite air tight, 890cfm and about 2500sqft if counting the basement. The owner has a concern about condensation on the windows. At first I was thinking an HRV but as I’m researching tonight I see ERV’s are starting to be recommended in our cold climate. I think I’ll offer the Panasonic FV-10VEC1 ERV unless anyone suggests otherwise..?

    1. @James, unlike an HRV, an ERV
      @James, unlike an HRV, an ERV acts to keep moisture inside during cold weather, although it will still eject some portion of the the moisture (half, more or less, depending on outside-inside dew point differential). Whether or not an ERV would remove enough moisture to keep the windows dry is difficult to predict. A lot depends on how the type of windows (frame material & u-value) and the extent of the problem. For example, if condensation only occurs on the coldest nights, then an ERV may be sufficient. More diagnostics, especially site monitoring, would be wise.

      Also be aware that if the windows in question are isolated from room air circulation by drapes or shades, it may be possible to resolve the problem by leaving the window coverings partially or fully open on the coldest nights.

      Another possibility is that the bath exhaust fans are under-performing (undersized or duct restriction) or not being allowed to run long enough, etc. This would be obvious in a multi-day graph from an RH logger.

    2. James,
      I believe the 100cfm max is adequate, I wish my Mitz Lossney ERV would go to 50cfm, mine wastes too much at the minimum.
      check out the ASHRAE 62.2 for the formula for your house size and number of bedrooms. I suspect the 50-100 range on your home will be good cuz you’re limited to how distributed you can make it. I prefer Building Science Corp. BSC-1

  6. I am new to the HVAC unit. I
    I am new to the HVAC unit. I have been in one room of my house and constantly have a draft (temps outside in the 30’s). Otherwise I wake up like the life has been sucked out of me. Stuffy, cant breathe, skin like an alligator. Had service to my unit and they flipped a switch on the unit. Now it doesnt blow on me anymore. Woke up able to breathe, skin less dry.
    Is that safe to keep that way? What about in the summer?

  7. I got an HVR for a relatively
    I got an HVR for a relatively air tight room that is about 130 sq feet. I do have some passive vent plates, but no circulation other than the HVR. I live in Colorado and when it is between 20 and 30F, everything is fine, but last night it got down to 11F and the HVR at that temperature only goes on for 10 minutes of intake and then 1 hour of exhaust only. I don’t have a meter, but is seemed that either there was a high buildup of CO2 or there was a low level of O2 due to all the exhaust.

    Has there ever been an issue with oxygen levels in cold weather use such as 10F or even below zero into -10F. The whole thing of it going into defrost mode at that temperature and only bringing in fresh air for 10 minutes and then exhaust only for one hour seems not good. Do you need a special passive vent to the outside that has to be a specific size?

    1. Are you sure it’s operating
      Are you sure it’s operating in exhaust only? By definition, for every CFM of house air exhausted, a CFM of outside air is brought in through induced infiltration. If an exhaust fan pulls the house negative, any difference between exhaust and induced infiltration is transient. Otherwise, the negative would continue to grow over time.

      1. Yes, according to the manual,
        Yes, according to the manual, that is how it works below 30F and even more so below 20F. I am using a Panasonic FV-04VE1 WhisperComfort. I agree with your logic, it should be passively pulling through the vent plates. When I opened the door this morning, there was a woosh of air into the room. I know there can be slight pressure differences between rooms, but this was different. It was almost as if there was less air in the room and fresh air rushed in. I definitely feel like a train hit me and my lungs are sore today. When I immediately went outside breathing fresh air was a boon.

        I am sure the HVR is working as it had been fine until last night, and last night was the first night it had to work below 20F which is when it goes into its defrost mode. Regardless, I am going to order a CO2 meter/alarm and an O2 levels meter/alarm to be sure what actually happens.

      2. Ok, I’m familiar with that
        Ok, I’m familiar with that ERV and how it operates. I’d be surprised if it caused the pressure differential you describe. The fan is only rated at 40 to 50 CFM, which is less than the average bath fan.

        Nevertheless, regardless of what’s causing the differential, your home is not being robbed of O2. For every cubic foot of air being sucked out of the house (by fans, winds, stack effect, whatever), there’s a cubic foot of outside air entering, somewhere. Otherwise the differential would continue to grow indefinitely!

        When something causes the house to go negative (or positive), the imbalance between infiltration and exfiltration is only transient until the two reach equilibrium. If the shell tight, a fan will activate leakage paths that otherwise may have been static. If the shell is tight enough, the fan’s flow volume might decline. But one way or another, the “in” and “out” will be always be equal over time. The transient imbalance is probably on the order of seconds, although admittedly that’s not something I’ve tested.

        Bottom line, if you actually have sustained elevated CO2 levels, it’s likely because the fan CFM + natural leakage isn’t enough for the volume and occupancy of the home, not because the fan is operating in exhaust only mode.

      3. @NS, here’s another
        @NS, here’s another interesting factoid: The pressure differential you described is probably on the order of a few tens of Pascals at most. A blower door is typically operated at a 50 Pa differential, which is enough to slam a door shut or cause ears to pop. As a point of reference, an equivalent reduction in atmospheric pressure (and oxygen density) occurs when you walk up a 10 or 15 foot hill!

  8. I have a totally sealed two
    I have a totally sealed two zone home counting the basement and attic(which are all conditioned areas) about 6200 sf. We have a 200 CFM erv unit in the attic and live in Chicago suburbs. Have 1 aprilaire 700 series in basement on goodman, and one on 2nd floor. Heat doesn’t run a lot because the house is so tight. Have erv set between 10-20%. The problem we have is low humidity! After searching for hours I cannot find anyone with this problem!!! Lol. We have 4″ closed cell I’m all walls, and 14″ open cell in rafters. With both humidifiers can’t get over 25-27% humidity in home!! I just ordered an aprilaire 800 steam to install and would like to put on 2nd floor as that’s where all bedrooms are at. I’m just not sure if the steam unit would cause any problems with the erv air exchanger… Any info greatly appreciated!!! So dry in house we are experiencing cracking in walls. Trim corners opening, and waking with coughs because of dry mouth/throats!

  9. @Mark, given the size of
    @Mark, given the size of your home, what you describe is not at all surprising. It sounds like your family is simply not generating enough moisture to offset the volume of dry outside air introduced via infiltration and ventilation.

    Although it’s true that super tight homes can have the opposite problem (high wintertime RH), that’s generally only true for smaller homes in terms of the occupancy-volume ratio. And even those homes can have low RH during colder weather (the colder the air, the drier it will be). Keep in mind that even a tight home will exchange its entire volume with the outside at least once or twice a day, plus contributions from central ventilation, clothes dryer, range hood & bath fans. That’s a lot of dry air in cold weather!

    As for your question — a steam humidifier will not cause any issues with your ERV. The only air that passes through its core is outside air and exhaust air from the house.

    As an aside, in my considered opinion, an ERV operating at 20 to 40 CFM is largely irrelevant in a 6,200 ft2 house given the natural air changes through your building shell. I generally don’t specify or recommend ventilation systems on homes that size unless required by code.

  10. Hello,
    We have always had a moisture problem in our house during the winter months. When most would run humidifier we would run dehumidifiers. In the last 5 years we added on to our house and installed an air exchanger with our new furnace. This had helped but has not solved our problem. We still get moisture on our windows if it is zero or below. Our basement is terrible, a very dry basement in the summer has water dripping from the walls in the winter. We are at a loss of what to do, we have asked numerous professionals and no one has been able to give us any ideas in how to solve our problem. Any ideas?

    1. @Kristin, moisture on window
      @Kristin, moisture on window glass at sub-zero temperatures isn’t all that surprising (depends on window vintage). Water dripping from basement walls is something else altogether. If you can identify the source of the moisture, the solution will flow from that.

      In what part of the country are you located? How old is the house?

      If basement is dry in the summer, as you indicate, this suggests an internal moisture source since exterior moisture (from groundwater and/or improper site/roof drainage) would likely manifest throughout the year. Inadequate spot ventilation (from showers, laundry, and/or cooktop) is the most common interior moisture source, but this would also manifest year round. OR perhaps you have elevated indoor RH throughout the year but don’t realize it since there’s no condensation?

      Do you have any unusual moisture sources such as a steam shower? Is it possible a humidifier is installed on the furnace that you’re not aware of?

      Is the furnace (and presumably, a gas water heater) atmospheric or closed combustion? (The former will have a metal flue, while the later will have a PVC flue.) Gas combustion appliances generate a LOT of water vapor, but that moisture gets vented to the outside with other combustion gases. Your CO detector should alert you if there’s a problem… the fact that you’re alive suggests that’s not the case 🙂

      Do you happen to have an unvented gas fireplace insert?

      A humidity & temperature data logger (e.g., Onset Hobo) may reveal clues, especially if a pattern emerges.

  11. I don’t have enough humidity
    I don’t have enough humidity in my house. This began when my fresh air pipes were replaced. My humidistat on the furnace says there’s 41% humidity but the humidistat upstairs says 21%. I have had “experts” tell me nothing is wrong but my nose knows when it’s too dry. Even a portable cold air mister isn’t helping. I have closed the damper because we just hade extreme cold for a week (Alberta). I think it’s related to the cold air intake pipes. Nobody agrees.

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