Feedback on HRV (or ERV) retrofit plan for my home
It’s a lot of questions below– feel free to comment on any portion. I hope the answers help others as well! Thanks much for any advice– the content on this site is great.
My Goal:
My primary goal is to decrease CO2 levels in closed-door bedrooms in the California SF Bay Area. Running the HVAC fan extra isn’t cutting it, per CO2 meter readings (1000-1400 ppm). (Energy savings is just a nice to have.)
My Draft Plan:
The bedrooms are all on the second floor and have an unconditioned attic above them. Given that the air is fine on the open-plan first floor, I think the retrofit would much easier if I just added a fully ducted system drawing exhausting from bathrooms (and maybe an office) and ducted fresh air into the unconditioned space.
Q1: Tips on an HRV in a (sometimes) hot attic?
I read that an HRV in unconditioned space is inefficient, but if my primary goal is ventilating the upstairs bedrooms, it seems like this is a lot easier than putting it in the basement and running ducts through the first floor to get to the bedrooms. I figure if I just use insulated ducts it’ll be good enough to be worth the tradeoff. (Maybe some units are insulted or I should wrap the unit in insulation except where there’s filter access?)
Q2: Thinking HRV over ERV, right?
We live in the Bay Area which some maps say is “marine”– outside RH around 75 most of the year, with temps 35-100, and we run our A/C at least 6 months a year. Articles I read seems inconclusive, but seems like an HRV might be better?
Q3: Sizing – I use the ASHRAE calculation as the “medium” speed, right?
The ASHRAE calculation says 90 cfm for the house, which would perhaps be 25cfm into the 2 small bedrooms and 40cfm into the larger one or so.
Some HRVs give a range of speeds, like “The Zehnder CA200 exchanges 29 – 118 cfm of air at 0.80” wc. A ComfoAir 200 system is typically designed to exchange about 72 cfm in medium speed.”
So, if I were doing Zehnder ComfoAir, I should probably go up to the CA350, which says 29-218, and 125 cfm at “medium speed,” I think?
Q4: Which models should I look at?
I’ve only looked at Zehnder so far. They have a ComfoAir 350 and a Novus 300 which seem to do the same thing. I’ve heard of the UltimateAir RecoupAerator 200DX (an ERV), the Build Equinox CERV, and various Broan Models.
Q5: Tips on finding an experienced installer in the Bay Area?
If you have tips on finding an experienced installer in the Bay Area, I’m interested. Most HVAC people I spoke with seems to not have much experience with HRV/ERVs. One I talked most to suggested retrofitting it into our existing HVAC air-handler, which seems to be complicated to do correctly, and I don’t see much benefit from in my scenario. My current plan is to try to get a recommendation of the distributor of the system I choose with input from this site.
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Replies
I'll give your post a bump and suggest reading this article: https://www.greenbuildingadvisor.com/article/hrv-or-erv.
Okay, so I spoke with an Zehnder rep who answered these as follows:
Q1: The Zehnder Q units will shut off above 104 degrees F, so really are better in the basement / conditioned space, but it's not crazy to build a well insulated box around an ERV in the attic, and then run well insulated ducting out of that for a fully ducted system. (In this case I'd buy the ERV from Zehnder and the ducting and registers from a supply house.)
Q2: Zehnder rep pushed me pretty hard towards an ERV, when my reading of the article linked above (thanks Steve) would be that I'd be fine with either. The logic I understood was that ERVs don't change the humidity as much, so I'd be at risk of drying out the place with an HRV during drier times of the year?
Q3: Zehnder rep pushed me much higher, saying that the Q 450 ERV really should be designed to just up 130cfm (even though it can boost much higher, it's louder and less efficient there) and then the Q 600 ERV is the same size and goes up to 200 cfm. He recommended I go with the Q600 and design it for ~180 cfm for the best Co2 and VOC ventilation, even though the ASHRAE calculation says more like 90 cfm. Does that seem a little overkill?
Q4: He said that the Novus 300 is no longer sold, and that the Zehnder ComfoAir is obviously the best. :) Still open to other opinions on an alternative to look at.
Q5: I got a list of HVAC installers in the Bay Area that have at least indicated an interest in Zehnder equipment... so that's a start!
Would definitely appreciate additional thoughts on any of this. Thanks again!
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Did you end up finding installers in the Bay Area?
Any luck finding an installer?
It is my understanding that in your climate, an ERV is appropriate.
I think you are overthinking this. I would look into the Panasonic FV-04 ERV. Duct the supply and exhaust through the master closet. Install the unit sideways on the wall between the master closet and the hall, with the grill facing the hall. The FV-04 unit grill has a supply and return so you don't need to duct that side of the ERV. The fact that you have a return in that same hallway will move the fresh air around enough.
If one unit turns out to be insufficient you can install a 2nd one in the outside wall across from the entrance to Bedroom 3.
This discussion about the HRV (Heat Recovery Ventilation) or ERV (Energy Recovery Ventilation) retrofit plan is insightful, especially for those looking to improve indoor air quality and energy efficiency in their homes. Retrofits like these can significantly enhance comfort and reduce energy costs, making them a smart investment. It’s important to assess your home’s specific needs, climate, and existing HVAC system to determine whether an HRV or ERV is the best fit for optimal performance.