The modeling here is assuming a somewhat leaky residential house that gets 2 ACH of natural ventilation, and is modeling the effect of adding purifiers on top of that.
If you want to evaluate additional ventilation, either open windows or mechanical ventilation, you can use the model here with the HEPA line. Because HEPA is so close to 100% thorough in removing particles, it is essentially equivalent to outside air from a covid perspective.
Yes—it is quite leaky—the rule of thumb the American Society of Heating, Refrigerating and Air Conditioning Engineers for low rise residential is more like 0.3 ACH. This would make your filtration look a lot better.
The ASHRAE recommendation is from https://www.ashrae.org/File Library/Technical Resources/Standards and Guidelines/Standards Addenda/62-2001/62-2001_Addendum-n.pdf for “Bedroom/living room” areas in “Hotels, Motels, Resorts, Dormitories” it gives an Area Outdoor Air Rate of 0.06 CFM/ft2. If the room has an 8-ft ceiling, that is then 0.008 CFM per CF, or 0.45 ACH (0.06 ÷ 8 x 60) They then want you to combine that with a minimum amount of airflow per person, which is not an ACH convertible number. Still, if we assume that a bedroom is 1,000 CF and is designed for two people then their 5 CFM/person is another 0.6 ACH (10 ÷ 1000 × 60). Total is 1.05 ACH.
For residential they have:
0.35 air changes per hour but not less than 15
cfm (7.5 L/s) per person
For calculating the air changes per hour, the volume of the living spaces shall include all areas within the conditioned space. The ventilation is normally sat-isfied by infiltration and natural ventilation. … Occupant loading shall be based on the number of bedrooms as follows: first bedroom, two per-sons; each additional bedroom, one person. Where higher occupant loadings are known, they shall be used.
A 1,000 CF bedroom with two people would need 30 CFM, which is 1.8 ACH (30 ÷ 1000 × 60). I suspect the 0.35 comes from assuming much larger rooms?
Yes, 0.35 ACH is for the whole house. Most houses do not have active ventilation systems, so that’s all you would get for the bedroom. But that is true that if you are worried about CO2, you should have higher ACH in bedrooms. But this recommendation is not just about CO2, but also things like formaldehyde. Also it is roughly the amount that houses get on average. I have seen studies showing that the cost of sick building syndrome is well worth having higher ventilation rates. So probably more houses should have active ventilation. But if you don’t have active ventilation in a house, I think 0.35 ACH is a reasonable average. Apartment buildings will have active ventilation and higher occupant density, so the ACH will generally be higher, as you point out.
I think it probably depends when your house is built? I would expect older houses to generally be a lot leakier? When I get home and have access to my laptop again, I think I can use the air quality measurements I took, which include CO2, to determine how leaky mine is as an example. I think the way to do this is look at how quickly CO2 levels fall off in an empty room?
While this is an old house, this particular room is essentially new construction: it was gutted in 2017 and redone with foam insulation and new windows, so it’s not surprising that it isn’t very leaky.
The modeling here is assuming a somewhat leaky residential house that gets 2 ACH of natural ventilation, and is modeling the effect of adding purifiers on top of that.
If you want to evaluate additional ventilation, either open windows or mechanical ventilation, you can use the model here with the HEPA line. Because HEPA is so close to 100% thorough in removing particles, it is essentially equivalent to outside air from a covid perspective.
Yes—it is quite leaky—the rule of thumb the American Society of Heating, Refrigerating and Air Conditioning Engineers for low rise residential is more like 0.3 ACH. This would make your filtration look a lot better.
0.3 is very low! I wonder why so much lower than recommendations for commercial spaces?
I’ve also seen higher numbers: https://en.wikipedia.org/wiki/Air_changes_per_hour gives 2-4 ACH for bedrooms, the lowest on the list. They cite https://moaablogs.org/air-changes-per-hour-calculator-formula-recommendations/ which cites https://www.vent-axia.com/sites/default/files/Ventilation Design Guidelines 2.pdf which seems to be from a manufacturer who wants to sell ventilation equipment and is incentivized to give high numbers.
The ASHRAE recommendation is from https://www.ashrae.org/File Library/Technical Resources/Standards and Guidelines/Standards Addenda/62-2001/62-2001_Addendum-n.pdf for “Bedroom/living room” areas in “Hotels, Motels, Resorts, Dormitories” it gives an Area Outdoor Air Rate of 0.06 CFM/ft2. If the room has an 8-ft ceiling, that is then 0.008 CFM per CF, or 0.45 ACH (0.06 ÷ 8 x 60) They then want you to combine that with a minimum amount of airflow per person, which is not an ACH convertible number. Still, if we assume that a bedroom is 1,000 CF and is designed for two people then their 5 CFM/person is another 0.6 ACH (10 ÷ 1000 × 60). Total is 1.05 ACH.
For residential they have:
A 1,000 CF bedroom with two people would need 30 CFM, which is 1.8 ACH (30 ÷ 1000 × 60). I suspect the 0.35 comes from assuming much larger rooms?
Yes, 0.35 ACH is for the whole house. Most houses do not have active ventilation systems, so that’s all you would get for the bedroom. But that is true that if you are worried about CO2, you should have higher ACH in bedrooms. But this recommendation is not just about CO2, but also things like formaldehyde. Also it is roughly the amount that houses get on average. I have seen studies showing that the cost of sick building syndrome is well worth having higher ventilation rates. So probably more houses should have active ventilation. But if you don’t have active ventilation in a house, I think 0.35 ACH is a reasonable average. Apartment buildings will have active ventilation and higher occupant density, so the ACH will generally be higher, as you point out.
I think it probably depends when your house is built? I would expect older houses to generally be a lot leakier? When I get home and have access to my laptop again, I think I can use the air quality measurements I took, which include CO2, to determine how leaky mine is as an example. I think the way to do this is look at how quickly CO2 levels fall off in an empty room?
Looks like the room I’m testing in is only 0.45 ACH / 9 CFM: https://www.lesswrong.com/posts/eMDuTAA9uA7vFKeEA/ceiling-air-purifier?commentId=G6Addw3f7TpLwHCsD
While this is an old house, this particular room is essentially new construction: it was gutted in 2017 and redone with foam insulation and new windows, so it’s not surprising that it isn’t very leaky.