HRV size
This may be an overly simple question but it’s one I’m having a hard time finding the answer to so I thought I would ask it here. If my house requires 100 cfm of ventilation, let’s say according to Ashrae, what size HRV do I need to buy? The Zehnder 350 says “Comfort ventilation up to 218 cfm”. I always assumed that meant 218 cfm of supply and 218 cfm of exhaust – it will provide my house with 218 cfm of ventilation. I just talked to the Zehnder rep in my area and he told me that the 350 supplies 218 cfm combined, half exhaust and half supply. So that unit would supply 109 cfm of supply and 109 cfm of exhaust if balanced properly. Is that correct? Is that the same for all HRV and ERV suppliers?
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It's not correct. The rating is not split in half, it's 218cfm supply and 218cfm exhaust. That's a good size for your needs. You want at least 50% over capacity, and double is better, for boost capability.
Trevor, Thanks. It didn't seem right to me. And you also answered by unasked question about how much over capacity I should have.
> my house requires 100 cfm of ventilation
Also pay close attention to distribution. Say 15+ CFM/person per breathing zone. A closed door TV room where 3 people sit for hours - that's 45+ CFM.
ASHRAE 62.2 usually results in "achieving minimally acceptable indoor air quality". I'd want better than that.
I think ASHRAE actually requires over ventilation - see BSC-01 discussion from the building science group. My house will need 90 cfm according to ASHRAE and only 45 cfm according to BSC-01. But I'm not going to have separate exhaust fans in the bathrooms so I'm probably somewhere in the middle. I'm shooting for 70 cfm.
Revisiting this question, I am trying to spec the HRV for my Zone 3 northern Calif build. 2500sf, 3 bedrooms. Using 6" hard pipe duct because I have it on hand and I'm comfortable working with 26 gauge duct... and i have 3 elbows in 1 of the 3 bedroom legs.
According to ASHRAE I need either 55 or 105 cfm depending on which multiplier gets used (.01 or .03); the BSC proposed calculation comes in at ~50cfm. In either case, these seem like pretty low numbers to me - especially given that there are very few HRV's on the market that move less than 70cfm.
I was going to buy a Fantech or Broan that moved 105 cfm until I read a few more articles and, realized I might be oversizing . Thoughts? Would I really be moving enough air with a 55-70 cfm system? TIA for your thoughts and recommendations.
Recent studies have been exploring quite measurable human cognitive impairment at remarkably low CO2 levels.
https://ehp.niehs.nih.gov/doi/10.1289/ehp.1510037
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3548274/
https://orbit.dtu.dk/en/publications/human-responses-to-carbon-dioxide-a-follow-up-study-at-recommende
https://www.yaleclimateconnections.org/2016/07/indoor-co2-dumb-and-dumber/
This has led some people recently to conclude that ASHRAE recommendations are if anything substantially below ideal.
Having continually monitored the C02 in my house for the last year, I've come to three conclusions. 1. BSC and the 2013 Ashrae vastly underestimate the ideal ventilation rate. 2. The ventilation rate should be determined mostly by number of occupants, rather than square footage. 3. Only use an HRV/ERV with a boost function, and you can't have too much boost capacity. My 2400 square foot house gets about 60cfm on low setting, 100cfm on medium setting. Low setting is enough for one adult and up to one small child and a small dog. For two adults, it needs the medium setting or C02 starts to run away. On a couple of occasions we've had 5 or 6 adults over for a get together, and the HRV cannot keep up even on high setting, which is somewhere around 200cfm.
I have to concur. An HRV/ERV should be matched to the number of people and pets and not to the size of the home. I recently had to move some heavy furniture into a bedroom and was the only person at home at the time. My CO2 monitor started beeping as the CO2 level went over 1000 PPM. It had been around 600 before engaging in that activity. I was really surprised. CO2 levels are amazingly affected by number breathing animals and level of activity, so plan the size of your HRV/ERV accordingly.
Trevor, Eric;
This is really helpful. Just curious, what CO2 monitor are you guys using? Do you keep it in a bedroom? I'd like to test our home as well.
Here's mine on Amazon:
https://www.amazon.com/Temperature-Humidity-Mountable-Detector-0-2000ppm/dp/B00MB93CK8/ref=pd_sbs_469_t_0/134-6096334-7718040?_encoding=UTF8&pd_rd_i=B00MB93CK8&pd_rd_r=75648477-4d57-4702-812e-67c43b5f0a1c&pd_rd_w=un2Qw&pd_rd_wg=aGAn8&pf_rd_p=5cfcfe89-300f-47d2-b1ad-a4e27203a02a&pf_rd_r=149RDWHCKC5J2RWTR853&psc=1&refRID=149RDWHCKC5J2RWTR853.
I keep it in the master bedroom.
Thanks Eric!
Looks a solid little machine. I just ordered one.
Thanks again-
Apropos nothing in particular, we are "windows open" people, so most of the year in our climate the hrv and HVAC won't even be turned on... But when they are, co2 mitigation is the only real reason to make this investment. How much fresh air does a 15*15*8.5 foot bedroom need to keep two adults reasonably healthy?
Trevor,
That's very interesting data. I haven't seen much on the correlation between the ventilation rates and the CO2 levels. What level of CO2 do you consider to be too high?
Kevin
I go by the current consensus standard, 1000ppm. I start to get concerned when I see it creeping up over 800ppm, because once it's climbing it will tend to continue climbing unless something is changed (reduced CO2 production or increase ventilation).