Sharing ERV and HVAC supply ducts
I’ve read the https://www.greenbuildingadvisor.com/article/ducting-hrvs-and-ervs which describes integrated (erv return and supply goes to hvac return) and exhaust-ducted hybrid (erv supply goes to hvac return). Both of these seem to require the hvac air handler to be on when the erv is on, and that seems less desirable.
Assuming I have a dedicated erv return ducts, I was wondering why the erv and hvac cannot share supply ducts? Would the air pressure from the hvac overwhelm that of the erv? My desire is to ensure we’re getting fresh air to the bedrooms.
We’re renovating 2 story high ranch. It currently has in-wall ac units and baseboard radiators. We’re planning on having an air handler (and ducting) in the attic (after insulating at the rafters) connecting to a heat pump and hydronic heating coils connected to the existing boiler. For downstairs, we’re going to keep the baseboard radiators, add wall-mounted mini-splits, and maybe add a WhisperComfort. The upstairs has 3 beds, 2 baths. Downstairs has 1 bath, 1 study, den, and 2 car garage (under the upstairs bed/bathrooms).
Any suggestions on how to most cost effectively do this, considering initial and long term costs?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
wag2692, I am not an HVAC tech but lots of this comes down to some basic physics. You can double check these responses with an HVAC company when you have them come over to give you a quote for install. They could also tell you how much more it costs for dedicated ducting :).
I am responding because I did a ton of research when i was trying to better understand the options/solutions for these same issues for a recent build. The ducting for my own ERV unit (explained below) was about $1000.
You asked: Assuming I have a dedicated erv return ducts, I was wondering why the erv and hvac cannot share supply ducts? Would the air pressure from the hvac overwhelm that of the erv?
Response: The air handler blower fan - normally - has a much larger capacity and is pushing a much largervolume of air (CFM) with a higher amount of pressure (water column).
You mention: My desire is to ensure we’re getting fresh air to the bedrooms.
My response: If feel that this most recent build of mine is less than optimal because we used:
> Dedicated return air ducts for the ERV;
> Dumping the fresh air from the ERV into the return air duct of the air handler.
I also wanted to make sure that my bedrooms received sufficient fresh air. I am not sure I am satisfied with the reassurances provided by the HVAC engineer. The HVAC engineer strongly suggested/insisted to avoid the ERV fresh air being ducted directly into the bedrooms. He said that the fresh air in the winter season is usually too cold to be ducted directly into bedrooms. There are people who dump the fresh "cold" air into the closets so it is not directly washing over the occupants of the room. Not sure I like this solution because the closets would be the coldest spaces in the house.
If a person has enough physical space, I would suggest oversizing the ERV to run the unit at the lower CFM setting. By running the CFM at the lower end, the "heat/cold" exchange in the winter would be much more "effective". (This also helps improve filtering, reduce noise, improves water vapour exchange in the ERV.)
Why I think this matters to you. If you have direct dedicated fresh air ducts from the ERV to the bedrooms, a larger unit running at lower CFM would mean that fresh air going the bedrooms would be warmer than a small unit running at the high end of the CFM.
You asked: Any suggestions on how to most cost effectively do this, considering initial and long term costs?
Comment: I believe that the best alternative for myself would be dedicated ducting for ERV
> Return air
> Fresh air distribution.
I would also have a large ERV that would run at the lowest fan speed (i.e. CFM).
This option costs more because it means more ducting and because I would have a larger ERV unit. In the longer term, I would have better air flow to the bedrooms.
Using the air handler system ducting is obviously an accepted solution but the ERVs are often only running when the air handler is running. In my case, I run the ERV 50 minutes per hour and it is not wired to the furnace. I am not sure if this will create condensation in the furnace when the furnace is off because I want that fresh air coming in and I can't change the ducts now. This is my decision and I will learn from my stubbornness if the furnace dies out.