HRV versus ERV
user-4401404
| Posted in General Questions on
I’m a retired builder and have a HRV in my 10 yr old home. I’ve heard a lot about ERV’s. My question is what does an ERV do and how does it differ from an HRV? I live in Golden B.C. Canada and heat my home for 8 mths. of the year.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
The short answer is that, in addition to transferring heat from the warmer airstream to the colder airstream, an ERV also exchanges humidity from the more humid airstream to the less humid airstream. For a more thorough answer, see: https://www.greenbuildingadvisor.com/blogs/dept/musings/hrv-or-erv
In any B.C. climate the average latent-cooling loads are negative, an ERV buys you nothing in cooling energy use (which is already pretty low), and nothing in comfort. ERVs makes some sense in places like Florida or the gulf coast of Texas where outdoor dew points average 22C or more, and moisture is a large fraction of the cooling loads, but even there it's only "worth it" if you need/want to support high ventilation rates without adding a lot to the latent cooling energy use.
Dana, Would Jerry be able to maintain comfortable indoor humidity in winter easier with an ERV?
Jerry,
Dana gave you good advice. In general, stick with the HRV unless your house is unusual and you are finding that the indoor air in winter is very dry. Even if your indoor air is very dry during the winter, there are usually better solutions than an ERV -- in most cases, the first step for a house with that problem would be air-sealing measures in the basement and attic.
For more information, see these articles:
HRV or ERV?
Designing a Good Ventilation System
Misconceptions About HRVs and ERVs
Reid,
Q. "Would Jerry be able to maintain comfortable indoor humidity in winter easier with an ERV?"
A. Some homes in winter have dry indoor air; others have humid indoor air. So the answer to your question about "maintaining comfortable humidity in winter" depends on the circumstances.
In general, homes with many occupants (crowded homes) tend to be humid; so do very tight homes. In general, homes with few occupants tend to be dry; so do leaky homes.
But there are exceptions to these generalizations.
The usual solution to high indoor humidity during the winter is to operate the ventilation system for more hours per day.
The usual solution to low indoor humidity during the winter is to perform air sealing work in the basement and attic.
The amount of moisture returned by the ERV in winter isn't going to make much of a difference in a place as cool & dry outdoors as Golden B.C. in winter. An ERV makes a bigger difference when there is a consequential amount of moisture in either the incoming or outgoing air streams. At a comfortable & healthy 30% RH @ 20C the amount of moisture in the outgoing air stream isn't much about 4 grams of water per kilogram of air. At the mid-winter average dew point of ~-9C in Golden the outdoor air runs about 1.7 grams/kg. The ERV will be returning only about half of it, splitting the paltry 2.3g/kg difference.
This is very different situation from the Gulf Coast, where they see days of 35C 50%RH outdoor air which has a moisture content ratio of about 18g/kg, which almost twice as much as comfortable 25C/50% RH conditioned indoor air at about 10g/kg. The difference in indoor & outdoor air's moisture content there is 8g/kg. Splitting the difference there with an ERV makes a measurable difference in the (latent) cooling load.
In a tight house the usual way to raise indoor humidity is to limit the ventilation rate on the HRV.