ERV vs. HRV
jw54
| Posted in GBA Pro Help on
I have to choose which ventilation system to install in climate zone 5. After reviewing and monitoring humidity data, it would appear that outside year round moisture is too high, 65% for indoor comfort of 45 %. This makes the hrv with some dehumidification the choice unless I am missing something. The house is new construction design just above net zero. What is the best, most economical way to dehumidify the incoming feed air ?
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Hi Joseph,
In climate zone 5, both an HRV and an ERV should work fine. I would actually lean towards an ERV, as cold winter air is inherently dry (even when the RH is high), and heating days outnumber cooling days (however, the size of the house makes a big difference here). ERVs also are a better choice if you do plan to use active dehumidification. These are minor quibbles though, like I said both will work. Some relevant articles:
https://www.greenbuildingadvisor.com/article/hrv-or-erv
https://www.greenbuildingadvisor.com/article/choosing-equipment-for-a-whole-house-ventilation-system
If you plan on cooling your home, a properly-sized air conditioner is the best way to dehumidify. If not, there is little benefit to constructing a dehumidifier into the house, simply buy a standalone model or some window AC units if humidity does end up being a problem.
Hi Aedi, Thank you for the reply. I am familiar with the referenced articles and had chosen to go with the ERV. Then after monitoring and researching I came across the RH charts for Mass. which show 63% average daily RH in my area during the winter months. My existing house shows 30% RH in the winter. I figure if 63% average is being introduced why do I need to transfer any of the outflowing moisture. I also remember a chart showing HRV's are more efficient than ERV's. There seems to be an either or works consensus.
Take a look at this case:
Outdoor: 32F, 65%RH --> dew point in these conditions is 22F (dew point a common way of indicating absolute moisture content)
Let's see how much moisture that really is. If you raise the temperature to 68F, a dew point of 22F gives you a relative humidity of 17%. So even if your house is at 25%RH, bringing in 32F, 65%RH air is actually going to lower your indoor humidity.
Remember that the figures you are referencing are Relative Humidity - that is the amount of moisture the air is carrying relative to it's temperature. The cold outside air can't carry as much moisture as the warm air inside, so as Trevor said, That 63% outside air is quite dry. Brought inside and warmed it would be in the low teens.
Exactly. Check out this link:
http://www.dpcalc.org/
Solving for Dew Point, set your outdoor air temperature and humidity condition. Then switch to solve for % RH and watch the Relative Humidity of that air drop as the temperature increases.
An ERV makes a lot of sense in a heating dominated climate.
Conventional wisdom is that indoor humidity should be kept lower in the winter, for protection of the house rather than human comfort. That said, the more air tight the house, the less of a concern this becomes. 45%, if that is a minimum, is possibly a little too high to shoot for in the winter. As already pointed out, due to the air's lessened ability to hold moisture at lower temperatures, there's really no such thing as moist and cold air. 65%, or even 90% near or below freezing temps is so low you might as well consider it zero. It's unlikely you'll want to actively dehumidify in the winter.
In the summer, an ERV will do a bit better at keeping the humidity down (being careful not to say reducing humidity, because they don't do that, they just introduce less). As long as there is more moisture in the outside air than in, some of the moisture on the incoming stream will be transferred to the outgoing stream. If you reach a point of equilibrium (based on absolute moisture content, not relative humidity), then no moisture transfer will take place.
A dedicated dehumidifier in the summer is somewhat counterproductive. You're removing moisture, but you're adding heat, and you're using energy to make that trade. If humidity is a problem, it makes sense to also cool the air while getting rid of it (i.e. AC).
Thank you for the education. Is there an inexpensive, efficient appliance which would monitor and have the dual purpose of humidifying in winter and dehumidifying in summer the incoming fresh air from the ERV. I plan on using mini splits for heat and ac.
I've never heard of a device as you describe. The mini splits will likely address the dehumidifying in the summer, and it's unlikely you will actually need to humidify in the winter. Selecting an ERV would help retain some humidity in the winter. My house runs between 35-45% with an HRV, and we don't even have a shower hooked up yet.
I agree that you are unlikely to need such a unit. With a tight house and erv, winter humidity will stay high enough. In the summer, the cooling will dehumidify. If you want excellent humidity control you might want a dehumidifier for shoulder seasons, but that's unlikely to be needed.
You can also consider a unit with swappable erv vs hrv cores, and use the hrv core in moderately cool shoulder seasons if the erv isn't removing enough humidity.
Charlie,
That's a great idea. Do many of the units have that ability to switch cores?
Zehnder in the only one I'm aware of. Interestingly, the sales guy told me he swaps the cores seasonally, but it's the HRV he puts in during the shoulder seasons, ERV on the winter and summer.
I, too, was considering an ERV (in the Pacific Northwest climate zone 6) but then read this article on this site:
https://www.greenbuildingadvisor.com/article/ventilation-small-home
The article states:
"An ERV is probably not what you want."
Even in the Pacific Northwest, outdoor air may seem cold and damp but it’s usually much drier than indoor air in absolute terms, Dana Dorsett responds. So pulling in outdoor air doesn’t create an indoor humidity problem. It will, however, create an energy and comfort problem.
Further, Dorsett says, LH would be better off with a heat-recovery ventilator rather than an ERV.
“An ERV usually only makes sense in locations that have a fairly substantial latent cooling (outdoor humidity) problem in summer,” Dorsett says. “In the PNW the latent loads are almost always negative even when it’s hot out. An HRV (sensible heat only) makes more sense in your climate.”
It would seem that the issue between choosing an HRV or an ERV has more to do with the moisture content in the air and not with how cold the air is. With both the PNW and Mass being near the ocean there will always be lots of moisture in the air, summer and winter, so maybe an HRV is best for both climates despite the outdoor temperature?
>"with both the PNW and Mass being near the ocean there will always be lots of moisture in the air, summer and winter, so maybe an HRV is best for both climates despite the outdoor temperature?"
The outdoor dew points in MA average about 65F in summer, straying into the mid to high 70s from time to time. This is a significant latent cooling load. The climate is not at all comparable to the PNW, where the latent loads are usually negative.
This is largely due to the prevailing winds in the PNW coming off the Pacific, where the ocean currents are coming out of the Gulf of Alaska. In MA it's coming off the continent,, but often out of the warm Gulf of Mexico. Even when the wind is off the Atlantic it is crossing the warm Gulf stream. The water temperatures in MA are much higher in summer than in WA/OR due to the Gulf Stream, but can also be much lower in winter due to the effects of the arctic winds coming out of Canada half the time.
Scott,
Isn't your project in Nelson? That's almost 700kms from the sea in the mountains. It's closer to Calgary than Vancouver.
An ERV is less likely to get frost in winter than an HRV. Frost occurs when the outgoing air stream cools below its dewpoint and also below freezing. With an HRV, the dewpoint stays constant as the air travels through the core. With an ERV, the dewpoint decreases (in winter) as the outgoing air travels through the core due to moisture transfer to the incoming air stream. Renewaire is confident enough in this effect that they do not include defrost controls.
Malcolm, yes, it is but the southern part of the province is still much warmer than the north, and the moisture from the ocean usually stays on this side of the mountains and doesn't get into Alberta as much.
Of course, manufacturers of HRV's and ERV's provide broad zone maps showing large swaths of the continent as being suitable for their particular product (zone 6 covers a lot of ground) so I'm guessing the ERV would still be the better choice?
What they can't predict is how much moisture you are going to produce. If Winter interior humidity is too high, a HRV is better. If too low, then an ERV is better. Either is possible in a wide range of climates.
I am in Zone 3 Galt Ca which would work best EVR or HVR. Also is there any preferred manufacture that you could recommend?
We use the Panasonic intellibalnce ERV in climate zone 5 on a regular basis and have been very happy with its performance. It has become our go to model for some time. They have a cold climate version which is what we have always installed. In a low load house you will almost undoubtedly need a dehumidifier for the spring and fall and probably parts of the summer also. The cooling load is so small in some of these houses that there are a lot of situations where our customers want to reduce the latent load without reducing the sensible load. A humidifier will also probably be required in the winter no matter what option you choose. For example today at my house the outside RH is 73% and my indoor is 36% - that is with my intellibalance running at close to 100CFM all the time. I have found the air conditioners are not great dehumidifiers unless they are running under near full load conditons or if you are ok running them below set point in order to manage the latent load. In code min houses it becomes less of an issue but the better insulated and air sealed the house is the more important proper dehumidification and humidification becomes
Based on our experience here in climate zone 5, Chicago suburbs, in a Passive House with a Zehnder ERV and a 3 head Mitsubishi Hyper Heat HVAC set-up, we learned our first summer that a dehumidifier is essential in order to avoid jumps in humidity above 60%.
Luckily, this only seems to happen on the warmest and most humid of summer days/nights. We've been averaging about 30-40 days a summer where our 2 stand-alone dehumidifiers are necessary. They're both set at 50%, but seem to turn off around 55%, which works for us (in terms of comfort).
There's a random day, here or there, in spring or fall when they're needed, but it's uncommon. Winters we've been maintaining 35-45% indoor humidity without issues.
When I asked in a CPHC training session this past summer about this issue (worried that I had done something wrong with our HVAC set-up), the instructor explained that in their work as PH designers they do indeed specify ducted whole house dehumidifiers (as jrpritchard notes above) because the heat pumps don't run hard or long enough to manage the latent loads.
Another example proving this point, the heat pump compressor is virtually silent even on the hottest summer days, but chugs along on the coldest winter days (similar to the noises made by a refrigerator).
When clients ignore the advice (to avoid the added cost; they think it's overkill), it almost always results in a unit being retrofitted some time after construction is complete (not fun for anyone at that point).
I go into more detail about our system here: https://kimchiandkraut.net/2019/02/21/hvac-for-passive-house/
You could also consider one of the 'magic boxes' like CERV or Minotair, but these are newer companies, so even if effective I would worry about what happens if the company goes out of business after an installation. Also, if issues do arise after construction, how confident are you that a competent service professional shows up to successfully troubleshoot the problem? I would argue that it's not an insignificant amount of risk involved. They're worth considering, but with caution.
And even if you get lucky for some reason and never need a dehumidifier, an installed unit is cheap insurance during construction to avoid problems later (to me anyway).
I haven't responded before this because I don't know the ins and outs of the difference between ERV and HRVs. I guess I better pipe in here. The latest comments are going entirely in the wrong direction if they are directed to LodiDave. Summers in the central valley of California, where LodiDave lives, is dry and is never humid that time of year. So if the advice is being given to LodiDave then you are giving him, and any other person living in that area a bum steer. You would never need a dehumidifier in the central valley at any time of the year in a properly functioning home.
I was responding to OP.
Sorry if that wasn't clear.
I am sorry if my response was misleading in some way I was responding to the original question from Joseph in which he was asking for advice on ERVs vs HRVs and the need for supplemental dehum in CZ5