An Energy Recovery Ventilator Is NOT a Dehumidifier

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ventilation energy enthalpy recovery ventilator erv humidity

A common misconception about the energy recovery ventilator (ERV) is that it's good for humid climates because it helps to dehumidify a home. It's usually the better choice for a humid climate when you're trying to decide between an ERV or an HRV (heat recovery ventilator) but not because it's a dehumidifier. It is not a dehumidifier. Here's why.

How ERVs and HRVs work

The photo above shows the inside of what could be either an ERV or an HRV. They both work on the same principle. The difference is what happens in the recovery core.

Basically, this mechanical ventilation device is just an insulated box with two fans, four ducts, and a recovery core. It pulls in outdoor air (red arrows) and sends it into the house. It also exhausts indoor air to the outdoors (blue arrows).

ventilation energy enthalpy recovery ventilator erv core 1

The two air streams both pass through the recovery core, shown above and below. Ideally, the two air streams do not mix at all but do exchange heat (both ERV and HRV) and moisture (ERV only).

ventilation energy enthalpy recovery ventilator erv core 2

Exchanging heat is definitely a good thing. It means that when you need to bring in outdoor air to ventilate a home in the winter, you can both preheat the incoming air and rescue some of the heat in the outgoing air. In the summer, you precool the incoming air.

The humidity confusion

It's easy to get confused over humidity, of course, because the mixture of dry air and water vapor exhibits such interesting behavior. (See two of my favorite books, Understanding Psychrometrics by Don Gatley and Water in Buildings by Bill Rose, if you'd like to crack that nut yourself.)

One point of confusion is the difference between relative humidity (RH) and absolute humidity. You might think that 100% RH is humid, but if the air is cold, there's really not much water vapor in it because cold air is dry air.

There are different ways to talk about absolute humidity, but I like dew point. You can't tell which is the more humid air by their relative humidities without also knowing the temperatures. Dew point, however, is a single number describing a volume of air that does tell you what might happen when you exchange indoor air and outdoor air.

An ERV doesn't dehumidify

The confusion over ERVs and dehumidification stems from their ability to reduce the amount of water vapor introduced into the house when the outdoor air has more moisture than the indoor air. But reducing the amount of moisture coming in is not the same at all as dehumidifying the air in the home.

In fact, the ERV will usually raise the humidity in the home when the outdoor air is more humid than indoor air because it doesn't exchange water vapor between the two air streams perfectly. Just as only some of the heat is transferred between air streams, only some of the moisture is  exchanged. So the incoming air will be more humid than the outgoing air on a humid day.

That's not a bad thing because the ERV is still reducing the amount of humidity that would have been brought into the home. If you ventilate with an HRV, supply-only, or exhaust-only ventilation sytem, you're bringing all the humidity in.

The ERV brings in less but doesn't dehumidify. If you want to ventilate and dehumidify at the same time, you need some type of supplemental dehumidification, such as a ventilating dehumidifier. That's one way to deal with the higher ventilation rates of ASHRAE 62.2-2013 in airtight, low-load homes in humid climates.

 

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Dew Point — A More Meaningful Measure of Humidity?

 

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Comments

Chris Dorsi

Thanks yet again, Allison, for clarifying the role of basic physics in the design and maintenance of our buildings.  
 
In this case, it may be tempting to look at that drain line attached to the ERV and think that it's a dehumidifier. But it ain't so!

Roy

I agree that an ERV is not a dehumidifier in humid climates during cooling conditions, but it is during heating, i.e., when the outdoor temperature and humidity is lower than the indoor temperature and humidity.

Curt Kinder

Nice article. Keep banging the dewpoint drum, although there is an argument to be made for wet bulb temperature, especially since HVAC systems are rated at conditions that incorporate wet bulb figures. 
 
At some point I'd like to see an article addressing the fact that separate dehumidifiers (the real ones) add quite a lot of sensible heat in exchange for moisture removal, and are therefore somewhat of a bummer in hot humid climates. 
 
In a related vein, I'm in the preliminary phases of specifying an Ultra-Aire (Therma-Stor brand) SD12 split dehumidifier, which is rated to essentially turn the typical sensible heat ratio of 75 / 25 on its head (~25/75). 
 
I haven't yet figured out how it does that without supercooling the air passing through it - I'm told it incorporates recovery media like an ERV.

Allison Bailes

Chris D.: Yep. That condensate drain is mainly for the HRV version, not the ERV. 
 
Roy: Yes, an ERV can dehumidify in winter when the air is dry, but an HRV does a much better job of that. In fact, some friends of mine in North Carolina who installed an ERV decided they needed to switch out to an HRV core in the winter for better dehumidification.

Roy

Curt, 
I am not familiar with the split Ultra-Aire, but I have looked at their other products. They use the air leaving the evaporator to pre-cool the air entering the evaporator with an HRV. This helps energy efficiency a lot compared to conventional vapor-compression dehumidifiers. Looking at their website, the SD12 appears to be a split-system air conditioner with this same HRV on the evaporator which results in a one-ton AC with 2/3 latent and 1/3 sensible capacity (SHR=0.33), thus it is still providing some sensible cooling. It looks like it is equivalent to about 10 SEER in terms of total cooling. I wonder if one wouldn't be better off using one of their standalone dehumidifiers, especially if the house already has a higher SEER AC that would remove any unwanted sensible heat? 
 
Maybe one of the "buiding science" guys know the answer to this?

Debbie

hot humid comment here... 
I have an Ultra Air 70 pint whole house dehumidifier, on my 1000 sq ft home 15 SEER VS AHU heat pump, in Louisiana. the vs ahu handles the humidity in 52% range. when it is raining outside, and not much call for a/c (shoulder seasons) the dehumidifier maintains 50%...or lower, depending on setting. good stuff, in terms of comfort. but it costs me $30 a month to run dehumidifier, and..this is the biggie...the heat it rejects while dehumidifying is noticeable. 
for this reason the Ultra-Aire SD12 split dehumidifier would make better sense, as heat is rejected to ambient. of course price is more, but it makes sense..efficiency/comfort wise. 
if I had it to do over, I'd have waited & gotten the split dehumidifier. 
just my opinion based on my own personal house.

ted

Nice post Allison. 
 
I don't recall seeing anything from Curt I didn't think was brilliant.  
 
Shall we call bath and kitchen fans dehumidifiers? How about windows? How confused do we want to get the consumer? 
 
Erv's and hrvs are NOT dehumidifiers. 
 
They are latent and/or sensible heat recovery devices. Dehumidifiers have compressors. 
 
 

Roy

Debbie, 
 
When you say the rejected heat is "noticeable", what do you mean? Is the house heating up? If so, is it because the AC has been turned off?

Roy

ted, 
 
I respectfully disagree with you. ERV's and especially HRV's are "dehumidifiers" when used during cold outdoor air conditions. It is my understanding that many builders in norhtern climates install them for this specific reason in order to prevent humidity issues in the winter in tight houses.

Tim O'Brien

In reply to Curt and Roy regarding whole house dehumidifiers: 
 
Curt is correct that typical refrigerant dehumidifiers can add significant heat into a space. A dehumidifier removes energy from the air causing water vapor (in the air) to change state into liquid water. The change of state from vapor to liquid is energy intensive (about 1,000Btu per pound of water). This energy (along with the electrical energy consumed by the dehumidifier)is reintroduced into the air as sensible heat (temperature rise) before the air leaves the dehumidifier. 
 
In some applications this additional heat is a plus (basements and crawl spaces, shoulder season when the conditioned space needs a little heat), but it can be a negative if your AC system must operate to remove the dehumidifier heat. 
 
The Ultra-Aire SD-12 is a split refrigeration system similar to an air conditioner and transfers the (dehumidifier) heat outside the conditioned space. Roy is correct that a recuperative heat exchanger is used to enhance the dehumidification. The heat exchanger is not 100% effective so there is some sensible cooling from the SD-12 in addition to the dehumidification. The supply air temperature from the SD-12 is significantly closer to the return air temperature than a typical AC system, but it is still a lower temperature than the return air temperature. 
 
Roy - if your AC must run to remove all of the sensible heat created by a dehumidifier in your conditioned space, then you are paying to operate the dehumidifier (once) and to remove the dehumidifier heat from your space with the AC (twice). The SD-12 rejecting the heat outside your conditioned space will be more efficient in that case since your AC will not need to operate to remove the dehumidifier heat. If you live in an area or have an application where the sensible heat (temperature rise) from a dehumidifier is useful much of the time, you will be better off with a "traditional" dehumidifer which rejects its heat into the conditioned space. 
 
Please be aware that efficient dehumidifiers reject less heat into your space than inefficient dehumidifiers. An inefficient dehumidifier consumes significantly more electrical energy which is converted into heat when the dehumidifier operates. Thus an efficient dehumidifier will reject less heat into your conditioned space for a given amount of water vapor removal.

Debbie

Roy,  
I realized I could give up part of a closet located centrally in my house to put ahu, return & supply plenum inside conditioned space. ducts in furdowns also in conditioned space, to serve the house. 
 
with a lot going on in this little closet, the dehumidifier is on the top shelf of the closet with duct into supply plenum. 
the duct for the kitchen is directly opposite the dehum duct. 
 
dehumidifier only comes on when system is off,and when it is on, the air leaving the kitchen supply (opposite on plenum from dehum duct) is 92 degrees. 
not blowing, like when a/c system is operating...but you can definately feel the hot air exiting the supply grill. 
 
I didn't believe it was as hot as temp gun reading, so I put thermometer in the supply grill for kitchen. it says 92 whereas temp gun is 90...so just average it out to 91 degrees. 
 
if my house were bigger...I probably wouldn't notice it as much, but when in the kitchen with dehumid on...it is warmer. if you are standing at the stove cooking..it is hot. (with or without stove vent on) 
 
it isn't because a/c is off, 
but that the rejected air temp is so high. house is tight & well insulated. 
 
for this reason, I'd opt for the other dehumidifier were I do do it over.

Roy

Debbie, 
 
I feel your pain. Space constraints are always a problem. However, there are other additional issues with going to a split system dehumidifier. I am sure that the product cost is higher, installation costs are higher, and then you have to worry about proper refrigerant charging after field installation. Also, for your particular application, do you think that having cool air blowing on you in the kitchen with a split dehumidifier would be less uncomfortable than warm air?

David Butler

+1 to what Tim said. 
 
@Ted, I get your comment about homeowner confusion, but we need to cast aside the notion that dehumidifiers must have compressors. For example, we can dehumidify with chilled water coil. And exhaust fans are most definitely dehumidifiers! Commercial systems are available that use a desiccant media. And yes, windows are great dehumidifiers. The problem is what happens to all that water! 
 
@Debbie, there are a couple of ways you can reduce your DH costs. (a) Raise set-point to the high 50's, still within comfort range. No point in paying extra to get to "50% or lower", when you can be very comfortable at a higher setting. 
(b) Move the unit to a location where you won't notice the extra heat, or a cooler part of the house. As long as can communicate with the rest of the house (e.g., no closed doors), moisture levels will equalize fairly quickly wherever you put it.

Ryan Moore

Does anyone have advice on how to model the SD-12 in REM/Rate?

Debbie

David, 
moving the dehumidifier isn't an option. with limited space in closet & on the supply plenum...this is the only location that fits. 
as I will be adding fresh air intake...again part of the whole plan...redesign isn't going to happen. 
 
what saves me, unlike most homeowners, is that the vs ahu handles most of the humidity. as it is, I'm only using dehumidifier in the shoulder seasons. where as normally unit wouldn't run & RH inside would rise, the dehumdifier takes care of the humidity.  
not ideal...but workable. 
its ok with me, I figured all of this out before the install, just didn't expect to feel the dehumdifier heat. 
 
Roy, cooler air while cooking isn't a bad deal for me.  
 
I'm not really looking for a 'fix', just sharing what I've learned. 
 
I've been in the efficiency business for 15+ years & each step of making my house more efficient was well planned. the afterthought was the dehumdifier, too good of a deal to pass up.  
 
in a different configuration the heat from dehumidifier wouldn't be as noticeable. but putting equipment & ducts in conditioned space is the big savings for me.

Tim O'Brien

Reply to Ryan Moore - 
 
I am not familiar with and have not used REM/Rate, but I would describe the SD-12 as Curt above - a one ton air conditioner with an unusually low sensible heat ratio (SHR). I'm not sure if the unusually low SHR value will cause problems with the REM/Rate software calculations. 
 
Feel free to contact me if you wish to discuss in more detail. 
 

Tim O'Brien

Reply to Debbie -  
 
The Ultra-Aire dehumidifier creates a warm and dry supply air stream when dehumidifying. It sounds like most of the dehumidifier supply air is exiting thru your kitchen supply grille. I will assume the following constraints exist: 
 
Dehumidifier will not be relocated 
Supply ducting will not be modified 
 
The dehumidifier processes less air than your air handler so the supply ducting is oversized for the dehumidifier air flow (alone). This allows the dehumidifier process air to be "lazy" and find the shortest path from the supply to the return. This is typically not a problem unless you occupy a spot near the supply in the short path (which sounds like the case in your house). You can check this theory by reviewing the air flow from other supply grilles (while the dehumidifier is operating)in your house and comparing to your kitchen supply. If your kitchen supply seems to be getting all/most of the warm dry air, than it is the short path. 
 
There are some options to address this comfort issue. These options may increase the time of operation and cost to operate your HVAC equipment, but should improve your comfort in your kitchen. 
 
If the Ultra-Aire supply duct directly opposes your kitchen supply drop, the dehumidifier supply air velocity may be carrying most of the dehumidifier supply air to that drop. Modifying the dehumidifier supply duct connection to direct the dehumidifier supply away from the kitchen supply drop is a passive solution which will reduce the amount of warm dry air emanating from your kitchen supply. 
 
Have you tried interlocking your air handler fan with the Ultra-Aire dehumidifier when it runs? The Ultra-Aire DEH3000 controller can be configured to do this. This will help in two ways: 
 
1 - by adding more air to the supply and pressurizing the supply plenum air will better distribute to all of the supply grilles in your house 
 
2 - mixing the warm dry dehumidifier supply air with cooler return air from the air handler will reduce the temperature of the mixed supply air making the warming effect of the dehumidifier less noticeable when it reaches you 
 
Since you have a variable speed AC you could also operate the AC on low speed when the dehumidifier runs if you want cooling in addition to dehumidification, but I suggest you try running just the air handler fan first. 
 
Feel free to contact me if you wish to discuss these options. 
 

David Butler

In REM, you can set up a custom AC and set the total (sensible + latent) capacity and the SHR according to the mfr specs. Depending on how the specifications are given, you may have to back into total capacity.

Ryan Moore

Tim & David, 
 
The dehumidification side makes sense to set that up as a custom A/C in REM/Rate, but the ventilation is more difficult. It's obviously supply only, but what CFM? What wattage? What is the duty cycle? In short, how am I meeting ASHRAE 62.2 with this unit? All the literature I can find online is mum on the subject. I'm going to call the manufacturer tomorrow and see if I get anywhere.

David Butler

@Ryan, sorry I missed your intent to use SD12 for ventilation. According to the specs, the ventilation feature is just an option, not a primary function. The ventilation rate would be determined by the static of the outside air duct, fine tuned with a balancing damper. In REM, it would be modeled like any other supply-only ventilation system (in addition to the AC function). 
 
In order to satisfy 62.2 with a supply-only system, the blower must be set to operate off-cycle to ensure a consistent ventilation rate. In this case, you're operating what's essentially a one ton blower between cooling and DH calls to draw in a much smaller volume of OA necessary to meet your design ventilation CFM. The penalty can be substantial, depending on blower watts and typical operating cycle, which for this product, may be 0 for much of the year. 
 
According to the specs, the SD12 blower consumes 160 watts (348 to 406 CFM depending on static). That's a LOT if it has to operate 8760 hrs/yr! 
 
The blower penalty can be reduced somewhat by using a cycle timer (as opposed to continuous fan) and increasing the OA volume accordingly to achieve the desired effective rate (see cycle-timer ventilation option in REM, and refer to the 62.2 table on duty cycle adjustments). Note that running the SD12 blower off-cycle with a high OA ratio can cause comfort issues in cold weather. You might be better off bringing in your ventilation air separately with a 30 or 40 watt ERV, allowing it to mix with house air, then let the SD12 handle the diversified DH and first stage cooling load.

George Reynolds

The CDC has a lengthy discussion about ebola and AGP (aerosol generating procedures) for patients. I am well aware that many pathogens cannot exist in air. However, an aerosol is not air, it is an airborne solution.