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.