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HRV as sole means of controlling interior humidity – fail!

Andy CD Zone 5 - NW Ohio | Posted in Energy Efficiency and Durability on

I’ve got one year of data in my new house, and I’m not pleased with the HRV’s ability to lower interior relative humidity. I’m hoping folks with more experience can clarify which is more flawed: my mechanicals or my expectations.

Here is the context. CZ 5, northern Ohio. Brand new SIP construction, blower door tests coming in well under 1.0 ACH50. Volume of house approximately 10,500 cubic feet on three levels, including full poured concrete basement. Heating/cooling are two 9kBtu/hr minisplit heat pumps. One sump well in conditioned space, sealed. No combustion appliances. No dehumidifier. Full-time residents are 2 middle-aged humans, a medium dog, and a cat. Here is the ventilation setup: Venmar Kubix HRV Plus Model 44102. Exhausts two small bathrooms, supplies fresh air to two upstairs bedrooms. Unit claims 40-80 cfm depending on speed and static pressures. Trying to get someone with the right equipment to confirm airflow, but it seems strong at all ports.

So far this winter, I have NEVER been able to get any of my seven hygrometers to drop below 47%. They typically hover around 60%. All my plumbing traps are good. We hardly take any showers. We cook some, not a lot. No dishwasher. No aquarium. No dryer–we dry clothes offsite. I figure all the entrained construction moisture is long gone.

Even in a worst case scenario of compromised or short-circuiting airflow, say 25 cfm, shouldn’t this unit be capable of doing several total air changes per day? It was a damp December, but we had some cold, dry days that didn’t move the needles much.

In such a tiny tight house, does the humidity produced by just the two of us require a dehumidifier IN ADDITION to the HRV? Is there some moisture source we’re overlooking? Or is there typically a long lag in getting the house’s contents–tons of drywall and flooring and interior framing and everything–to dry out so it’s no longer the source?

Edit – hygrometers show RH varies little from upstairs ceiling to basement floor, so I haven’t been able to identify any wet zones.

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  1. David Meiland | | #1

    Certainly time for some investigate work, as you allude to. What IS the airflow moved by the unit? What is the incoming air moisture content? One year seems a little short in terms of all construction moisture being gone, especially with a full basement.

    An interesting experiment might be to run a portable dehumidifier for a short period and see if you can knock the RH way down. I'd put it in the basement and see what happens upstairs. Then I'd move it upstairs and see what happens.

    If you have separate bath fans, you could try some long run-times on those and see what happens. Seems like winter air in OH should be fairly dry, so it should make a difference.

  2. Jin Kazama | | #2

    Using a dehumidifier during heating season is not a loss ( but rather abnormal? ) as you will end up using the produced heat neway, you will only loose the difference of COP from the minis.

    That said, i do not know the conditions at your palce, but as soon as we move in heating territory in here ( mid Quebec ) , it's hard to get anything above ~40%RH . As David pointed out, this calls for serious investigation.

    ( get a dehumidifier, you don't want mold or wood mites to grow too much, and 60%+ year long is perfect for both )

  3. GBA Editor
    Martin Holladay | | #3

    I think you should wait a little longer before you panic. Summertime ventilation doesn't lower the humidity much, and in many U.S. climates actually raises the indoor humidity. So you need to have a few months of winter ventilation before you can be sure what is going on.

    One possible source of indoor moisture is your basement. Of course, there is construction moisture in your curing concrete that has to evaporate. There is also the possibility that your contractor forgot to install 6 mil polyethylene (or horizontal rigid foam) under your basement slab. Is that possible?

  4. Flitch Plate | | #4

    Andy ... I am with Martin. My first thoughts were:

    - is there a watertight barrier in the basement slab? is it bulletproof?
    - releasing of concrete and wood frame construction moisture may not yet be finished

    If your house is one year old, I would not necessarily agree with this point: " I figure all the entrained construction moisture is long gone."

    "Entrained" means "made airborne". I would not count on all the construction moisture to be fully evaporated. I would monitor but not make any decisions until you have another year under your belt.

  5. Andy CD Zone 5 - NW Ohio | | #5

    Thanks for suggestions. There is 6-mil poly and 2" XPS below the slab--I put it there. It's not bulletproof, though, but based on previous posts by Martin I am under the impression that overlapping ploy joints was good enough. It is lapped up the walls and sealed with mastic, though. Basement walls have interior 3.5" XPS, 2" of which is continuous.

    My basement was actually poured in May 2013, so it's nearing 2 years old.

    Air outside today is 10 degrees and 64% RH. With my HRV running constantly on low speed (30 cfm) a few of my hygrometers are creeping closer to 40%.

    Today a friend was complaining about the dry air in his 100-year-old house, around 25% RH. It's a serious comfort issue for him, and he usually deals with it by unhooking his dryer vent so the heat and humidity stays in the house. All his windows fog up for hours, but at least his lips aren't chapped. If I tried that in my SIP house.....

  6. Expert Member
    Kohta Ueno | | #6

    Trying to get someone with the right equipment to confirm airflow, but it seems strong at all ports.

    Just to eliminate a variable, if you're tired of waiting for somebody with a flow hood--you can try the LBNL "garbage bag" method:

  7. Flitch Plate | | #7

    Actually you do not seem so far off the 60% summer and 45% winter targets. Maybe due to lake effect weather you're facing exterior challenges rather than interior?

    Look at Cleveland's extreme humidity spikes; even extreme for Ohio's average. Scroll down till you find it:

    Did you see this current thread on mini split performance? Perhaps you need to look at your heating system?


    A wood stove dries things out nicely; always good to have one if you live near a large body of water.

  8. Aj Builder, Upstate NY Zone 6a | | #8

    I too agree with Martin. Check back as to all in a month. My interior humidity runs high and my home is a leaky 1980s build. It will finally get much lower with the onset of real winter cold as we have now. My summer humidity runs high as I leave all windows open for six months a year and I have no mold that I am aware of other than the usual places... shower tile not set on proper substrate... not my doing but will be my redo.

  9. David Meiland | | #9

    Flitch, a woodstove in his tight house would have to have an outside air connection, and would do very little to lower humidity. I know mine doesn't.

  10. Nick Welch | | #10

    Are there exhaust fans in the bathrooms and over the range?

  11. Flitch Plate | | #11

    David ... you I guess are also assuming the issue might be infiltration of outdoor humidity?

  12. Aaron Becker | | #12

    Andy never mentioned whether the kitchen exhaust fan is vented to outside or is just a recirculating setup. Also, it wasn't clear whether the bathrooms had individual seperate exhaust vents or if the bathrm humidity venting is supposed to be completely handled by the HRV (I would assume the latter, given the cost and theoretical purpose of the HRV). How about some simple tests - run the showers (one at a time) and place a humidity gauge in the outdoor exhaust stream vent to see if there is a difference before and after. Do the same thing with the kitchen exhaust venting. Aside from the bathrooms and kitchen, the next obvious source is the basement. Take a humidty reading near the sealed sump pump to confirm it isn't the prime source. I, too would be concerned about the humidity numbers, I would expect lower numbers than stated for winter time. One more thing to check: the condensate lines for the split minis - are they in place and functioning? Could they be leaking moisture back into the building? (Or is that only when the AC part is functioning, not the heater?)

  13. Charlie Sullivan | | #13

    Aaron, you are right--the condensation line on a minisplit indoor unit is in fact only needed for A/C mode.

  14. Jill Fussell | | #14

    I moved into a tight "green-built" raised-slab single story 1860 sq ft house with spray foam insulation during the wet winter of 2009-2010 in the Austin, TX area and had a terrible time with high RH during the first year. The slab had been poured and only allowed to dry a day or so before framing started, which I attribute to the high relative humidity (60% or more). Luckily I had a portable dehumidifier that I ran the first year, and finally the RH dropped into the 40-45% range. As a side note, I consulted an architect friend who had completed a separate residential design for me of a 1920 sq ft house with a full 1920 sq ft basement, and asked him to compute the amount of moisture that would be given off from the basement in the first year - his answer was about 600 gallons of water. So, based on the size of your house and probably the size of the basement associated with it, I would venture a guess that the curing of the basement cement is the reason you have high RH. I would suggest running a portable dehumidifier in the basement for at least a full year as the cement cures. You might also want to check the frequency that your HRV is running and for how long. My was set at the factory setting and was coming on too frequently, bringing in too much relatively moist air for the rather efficient HVAC to condense and dispose of (I don't have a dehumidifier on the HVAC). Good luck!

  15. Andy CD Zone 5 - NW Ohio | | #15

    Posting an update with more data. January has been relatively cold and dry compared to December. I borrowed a dehumidifier. I got better airflow data. I am now keeping the inside RH in the mid-40s, which is still too high. Some have stated 45% is a reasonable winter level, others say 35% is max. I lean toward the latter. (Another current thread is dealing with a tight house that can't get ABOVE 20%!)

    The basic question remains: why can't I get the RH down? [I described the house in my initial post. Again, my HRV draws from the two bathrooms and exhausts to the upstairs bedrooms. My house is 1,400 sq.ft. on three levels, counting the conditioned basement. There is no other exhaust fan, no kitchen exhaust. We don't cook that much.]

    The dehumidifier running in the basement can get the room down into the high 30s after a day or so, but shut it off and RH springs back toward 50 within a half day. The rest of the house seems unaffected, tracking 45-50% with part-time HRV. In fact, my five hygrometers show that RH varies little from upstairs bedroom to basement. There are no "wet spots."

    NEW DATA: HRV has a measured airflow at house exhaust of 81 cfm on high speed and 40 cfm on low speed. Another more accurate blower door test confirmed my house is tight - close to 0.5 ACH50. Guy did IR study, found no evidence of any places with excessive air intrusion that would be sucking in moisture.

    It comes down to an energy thing. I never suspected I'd have to fight to get my RH down in the winter. Never thought I'd have to run high-speed continuous HRV, or operate a dehumidifier, with the energy penalties those things carry.

    The ONLY explanation that works is the uncured concrete theory, but that seems so dubious!

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