Ventilation and Oxygen/CO2 levels in multifamily apartments

Hi, Sara. Yikes!?!  There is probably plenty of useful info at PHI and PHIUS for explaining the fundamental importance of a balanced ventilation system in occupied rooms (vs exhaust-only strategies commonly found in bathrooms and kitchens). This might be one useful doc: https://www.phius.org/phius-mechanical-summit-2021-balanced-ventilation-design-principles

Dan / Sara: Maybe we should separate out the need to bring in outdoor air vs the desire to use Energy Recovery to soften the energy cost of bringing in that outdoor air.    I don't think you need an expert to run calculations on whether enough fresh air can leak through the envelope.  Almost any housing built in the last 20 years will likely be suffciently tight that either mechanical ventilation or perfect occupants (opening & closing windows at exactly the right time) would be required to maintain air quality.  Nice overview article at this link and Joe Lstiburek at Building Science Corporation has written a lot.  Relying on a leaky envelope to provide fresh air means that occupants get a lot more fresh air when it's really cold, really hot, or really windy outside (since those conditions will increase leakage) and a lot less otherwise. Doesn't seem right. NYC's codes requires either natural ventilation (e.g. window openings > 4 - 5% of floor area being served) or mechanical ventilation.   If mechanical ventilation, Section 403 requires that it be balanced--that is, you can't use exhaust-only approaches that negatively pressurize the spaces.  And while it's great to give people the option of opening windows, we shouldn't assume that everyone operates them perfectly all the time.   Not everyone is physically able to open the windows, and other conditions (noise, smoke...) may preclude it.   So we should provide balanced mechanical ventilation as a baseline and allow occupants the joy of opening the windows if they like. For dwellings, NYC Mechanical Code Table 403.3 is calling for fresh air at 0.35 ACH or a mininum of 15 cfm/person whichever is greater (first bedroom assumes 2 people, other bedrooms at 1 per.  So unimaginative.)  So an 800sf 2BR apartment with 9' ceilings would require 800 x 9 x 0.35 / 60 = 42 cfm of fresh air from the ACH requirement, or 3 x 15 = 45 cfm from the per-peson requirement.     Now, a normal adult exhales CO2 when sitting at a rate of about G=0.010 cfm.  The indoor CO2 levels saturate at a level above outdoor levels that is just the ratio of the rate at which the occupant is adding CO2 vs the rate at which outdoor air is being brought in and stale air is being exhausted.  So if the air change rate F=15 cfm, the CO2 levels will saturate at G/F = 0.010 / 15 =0.000667 or 667 ppm above the outdoor level.  So if the the outdoor CO2 level is 420 ppm, then the indoor level will saturate at 420 + 667 = 1087 ppm.  These levels of ~1000pm are typical of what one observes when applying the ASHRAE 62 ventilation standard. Now these ventilation & exhaust rates were set not by what we need for human health or brain function, but just by the level of ventilation below which there are complaints from odors ("bio-effluents").  Research published by Harvard (the "CogFX" study) has found that brain function gets impacted at CO2 levels above 650 ppm.   They recommend providing fresh air at a rate of 40 cfm/person -- so the CO2 level is 0.010/40 x 1,000,000 = 250 ppm above outdoor levels, or 670 ppm. They studied the cost of providing this 40 cfm instead of 15 - 20 cfm of outdoor air, looking at climates and utility rates across the country, and found that *without* energy recovery, this enhanced ventilation cost $14 - $40 per person per year in added utility costs, and that this cost dropped to between $1 - $18 per person per year with energy recovery.  Roughly speaking, they find that adding energy recovery cuts the energy cost impact by at least half, and sometimes as much as 90%. Whether you're going to provide the ASHRAE mininimum or the CogFX-recommended level of ventilation, the up-front cost of adding ERV's is non-trivial.  Suppose we were only trying to provide 15-20 cfm per occupant, but (based on the above) an ERV saves $20 per person per year.  Then that 2-bedroom, 3-person apartment saves $60/yr with an ERV, or $1200 over 20 years.  The savings would be doubled if we were doubling the ventilation rate above these code minimums.  So the economics are of the right order of magnitude, and it's probably why IECC 2021 C403.7.4.1 requires Energy Recovery for dwelling units.  [Fun fact:  All the old science fiction shows taught us to think of a sealed space "running of of oxygen", but it's the raised CO2 levels that impact (and eventually will kill) us, long before oxygen levels drop measurably.   Each molecule of CO2 we exhale was traded with one molecule of oxygen (O2) we inhaled.  So a room that has CO2 levels that are 600ppm (0.06%) higher than outdoors will have O2 levels 0.06% lower than outdoors.  So if normal outdoor air is 21.00% oxygen, then the indoor air would be at 21.00 - 0.06 = 20.94% oxygen.   It's sort of a metaphor for our age that we tend to think the problem is not enough of something we consume (in this case, oxygen), when it's the buildup of our own waste that will kill us.] Hope this helps, Z

Really enjoyed Z's input, as usual. Standard infiltration of a curtain wall is 0.06 cfm/sf (.3 In H20 in differential pressure, which is about 25 mph wind.  Chicago average wind speed is 10 mph).   Even with 25 mph wind, which is much stronger than average Chicago wind speed and may come and go, it is 7.2 cfm infiltration only for a 15'x8' curtain wall on the exterior of a bedroom.  So, agreed with Z, infiltration to get enough ventilation is not easy to depend on. While there are multiple papers discussed regarding high CO2 level during sleeping and the performance the day after, there are discrepancies among the limits, see below two paper, one suggested below 1500 ppm is acceptable health impacts, the other suggested even below 1000 ppm could have different impacts. https://pubmed.ncbi.nlm.nih.gov/26452168 https://www.iqair.com/us/blog/resources/the-co2-levels-are-high-in-my-bedroom-during-the-night-what-should-i-do Similar for daytime performances, check out the ASHRAE Positon Document on Carbon Dioxide, see below.  The Harvard study provided one perspective, several other performance studies suggested otherwise.  More research will be required. https://www.ashrae.org/file%20library/about/position%20documents/pd_indoorcarbondioxide_2022.pdf