HRV/ERV – lots of plants
Anders_Bostrom
| Posted in Mechanicals on
The HRV or ERV selection question again but with a twist.
We are finishing a 1600 SF PGH far north in zone 6B for two occupants. We heat the building with a ductless mini split and a couple supplemental resistance heaters in the bathrooms.
Most recommendations for unit selection points to an ERV, the entire Canada included. My issue is that we will end up with plenty of houseplants. Not a jungle like setting but still, lots of plants.
I’m concerned to the humidity level will be too high since an ERV will add to the humidity level.
Does anyone have experience to this scenario?
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I'm in the ERV research stage myself, but my understanding is that an ERV doesn't exhaust as much humidity as an HRV, but also doesn't bring as much in, as it keeps it closer to what it already is inside, making the air less dry in the winter than it would be with an HRV and less humid in the summer than it would be with an HRV. The HRV brings humidity levels closer to whatever they are outside while the ERV helps keep it closer to whatever it is inside. We often have months of 80-100% humidity outside. If outdoor humidity is usually closer to what you want then maybe an HRV makes sense.
My wife has maybe 20 plants in our house right now and we have two kids. Interior humidity levels didn't get below 30% all winter last year despite it getting down to 15F outside at times, and even last month when it was in the 40s our dehumidifier occasionally still came on. I think it's good to plan for having a dehumidifier in addition to whatever ERV/HRV you get, as neither the ERV or HRV is designed to consistently reduce humidity.
In cold weather, an ERV will retain more moisture inside the house than an HRV.
In the summer, if you run air conditioning or a dehumidifier, an ERV will retain more dry air inside the house than an HRV. If you use fans and open windows instead of air conditioning, you can turn the ventilation system off.
Maintaining houseplants will tend to raise the indoor humidity in all weather, so an HRV might be preferable in cold weather and an ERV preferable in warm weather. If you install an ERV, you can increase the ventilation rate in winter until the humidity levels are where you want them. Maybe consider using an ERV with higher CFMs than you think you need so you can fine-tune humidity levels depending on the season.
Or, just capture the condensate water from your HRV in winter and feed it back to the plants :-) In that case, best of both worlds.
ERVs are not anywhere close to 100% efficient with regard to moisture exchange so as Michael points out, increasing the ventilation rate will also reduce exchange efficiency. If you plan on having a lot of plants though I'd be looking more to C02 level monitoring, particularly at night when your respiration + the plant's collective respiration (although small) may spike co2 levels :-) I'd go with the ERV in any case.
Are these plants going everywhere, or they being concentrated on a south exposure? You may want to think adding supply/return options to help manage moisture in that area.
Good points, Michael and Dennis.
Looks like I will upsize an ERV unit to have the ability to increase the flow of needed depending on CO2/humidity.
As far as the plant location I have the feeling that the gardener in the house would like them to take over the house. I see a greenhouse project in the future.
ERV's aren't moisture adders like you think.
I have a couple small ERV's, the plants pretty much all die of slow deaths from low moisture.
The t-stat generally states ranges around 40-45% RH. I run the ERV on low continuously.