Home Ventilation Advice from HVAC Researcher Jeffrey Siegel: How Many CFM is Ideal?
What this covers
Spoiler alert: Corbett's wearing a retainer. Yes. You don't have to comment about it. Video forthcoming, as a matter of fact. Jeff Siegel is a superstar of HVAC research, and you can learn more from him in his original interview with me (https://www.youtube.com/watch?v=hSFQQpgvgeo) and at his research site (https://civmin.utoronto.ca/home/about-us/directory/professors/jeffrey-siegel/). We're in the University of Toronto's rooftop climate chamber, shooting the bull about how many air cleanings per hour (aka air changes per hour, which is confusing to blower door folk) we should aim for in our living spaces.
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Home ventilation requires balancing outdoor air dilution and indoor air filtration through responsive systems that residents can control; the standard ASHRAE 62.2 provides a starting point but practical implementation depends on specific risk factors, equipment efficiency, and user behavior.
- Homes are generally poorly ventilated until retrofitted; effective systems combine dilution, filtration, and portability rather than relying on a single approach
- Air cleaning targets range from 0.5 to 5-6 air changes per hour depending on occupancy, pollution sources, and circumstances; this should be tunable rather than fixed
- System automation is technically feasible but barriers are user interface design and behavioral psychology, not technology
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Fully automated responsive ventilation control is technically feasible but faces barriers in user interface design and behavioral implementation, not in technology itself.
“yes the technology is not the issue the issue is the same thing that we always face with complicated systems is the implementation and so we have a lot of work to do on user interfaces”
Responsive ventilation systems that can be controlled up or down are more important than hitting a single fixed target number, and users need clear guidance on what factors should trigger adjustments.
“we want to be able to have a system that's responsive which is where you started this but we also have to have really good guidance for people about how to control it and you know what are the factors that should lead them to go up or down”
Air changes per hour measured via blower door test (envelope leakage rate) is distinct from air cleaning rate (filtration-based air change); these are commonly confused.
“there's two there's air changes per hour 50 which is blower door metrics and then there's air x what do you call it I call it air cleanings yeah that's I I call it air changes per hour and of course that's confused”
Not all HVAC fans are equally efficient; Permanent Split Capacitor (PSC) fans and Electronically Commutated Motor (ECM) fans have substantially different power consumption, so choosing an efficient fan reduces operating cost and energy waste.
“not all fans are the same right so there are some permanent split capacitor fans some electrically commutated motor fans they have very different efficiency”
Most homes are not set up to be well ventilated with outdoor air until professional ventilation specification is applied.
“we have homes are not set up to be very well ventilated with outdoor air until I come along”
When outdoor air pollution is present (e.g., near a highway) or indoor sources are active, air cleaning targets should be increased above baseline; outdoor air intake fraction can be reduced to lower total incoming pollutant load.
“if you have some incident going on in outdoor or maybe you live near a highway and there's outdoor air pollution then you know it's appropriate to go to a lower number on the outdoor air”
An economizer control strategy automatically increases outdoor air intake on favorable weather days to provide free cooling and reduce air conditioning load; this is standard in commercial buildings but rarely implemented in residential.
“on a nice day you know there is no you get free cooling under some circumstances by bringing in more outdoor air so that's called an economizer it's done in many commercial buildings even most commercial buildings and so you know that hasn't really been explored that much in residential buildings but makes a ton of sense”
In newly constructed airtight homes, outdoor air cannot simply be introduced without mechanical balance (ERV/HRV); passive infiltration cannot meet ventilation needs in tight construction.
“in homes that are new and the kind of people that I'm talking to generally are going to build very airtight you can't just bring in a bunch of fresh air you need to have a balance system”
Historical ventilation controller systems from the 1980s-90s had the capability for user adjustment, but users were confused by the interface design rather than the concept of adjustable fresh air intake.
“there were ventilation controllers that people could turn up or down depending on how much fresh air they wanted and the issue wasn't people knowing whether they wanted fresh air or not the issue was people being confused by the controller”
Effective home ventilation requires two complementary strategies: dilution of indoor air with outdoor air, and filtration of indoor air; neither alone is sufficient.
“we've got two things we've got dilution of outdoor air and we've got filtration of indoor air”
Outdoor air dilution and indoor air filtration are additive strategies: you can achieve a target air cleaning rate through a combination of lower outdoor air intake and higher filtration, rather than maximizing one or the other alone.
“we can add the ventilation the outdoor dilution air and the filtration like we can there we can separate them and then add them together to kind of get what that is right”
Social psychology, user behavior, and interface design issues are as important as technical pollutant concentration analysis in determining ventilation system success.
“there's a whole lot of like social psychology and user behavior issues that are just as important as the pollutant concentration issues that I think about”
Running a thermostat fan continuously in 'fan on' mode, combined with a good well-maintained filter, provides measurable air cleaning benefit.
“running the fan continuously if you have a good filter that's well installed and you're on top of changing it is offering you benefit”
Portable HEPA filtration units can be added to a home ventilation strategy to provide localized high air cleaning rates in occupied rooms without requiring expensive central system upgrades.
“we can add in portable filtration too so if you have everyone's in the living room then you can have a lower number with your central system and have a portable or a room-based system in the space where people are”
To calculate air cleaning rate in CFM for a given space, multiply square footage by ceiling height and divide by 60 to find the CFM needed for one complete air change per hour.
“so 30 000 cubic feet and what you do is divide that number by 60 to find out how many CFM we'd have to run in order to clean the air one time per hour”
In a 3,000 square foot home with 10-foot ceilings, achieving 5 air cleanings per hour requires running 2,500 CFM continuously—equivalent to a 5-ton air conditioner at full speed.
“five airplanes per hour would be that 500 CFM times five which is 2500 CFM that's like a five ton air conditioner running on its highest speed non-stop”
MERV-13 and MERV-16 filters are appropriate for residential ventilation systems; MERV-15 filters are too large and create excessive pressure drop for standard residential equipment.
“something like that Murph 13 or Murph 16 filter or if you really insist a half a filter which has to be gigantic to to make up for the pressure drop”
Current residential ERV/HRV equipment typically maxes out at 2,500 CFM, which limits the outdoor air component of total air cleaning to that capacity; 5-6 ACH targets cannot be met through outdoor air alone.
“ervs unfortunately don't run you know you can't get any RV in residential it's going to run up to 2500 CFM so you can't necessarily get that Max air cleaning out of just your outdoor”
In COVID-19 classroom settings designed for pathogen control, air cleaning targets of 5-6 air cleanings per hour are recommended; the difference between 5 and 6 is not material.
“best case scenario I had thought was five air cleanings per hour in like a covid classroom setting but you're you said six just a minute ago yeah but there's no great science so functionally the difference between five and six I'm not going to split hair”
Under normal (non-pandemic) residential conditions without active pollution sources, 0.5 air cleanings per hour via filtration is a comfortable minimum target.
“the short answer is I'm comfortable going down to a number like a half air change per hour uh if the risk is such but um you know if you have a source going on in the space cooking something else going on in the space then you want a bigger number”
Automatic or user-controllable fraction of outdoor air intake is an important feature; ideally homes can modulate between minimal outdoor air (relying on filtration) and maximum outdoor air (on favorable weather days for economizer benefit).
“the other piece on this is what is the fraction of outdoor air that's coming into the space and so I think ideally there would be either automatic control of that or a user control of that so that on a nice day you know there is no you get free cooling under some circumstances by bringing in more outdoor air”
The practical formula for baseline home ventilation is: (square footage × ceiling height ÷ 60) × [0.5 to 5-6], where the multiplier is adjusted based on risk factors like occupancy, pollution sources, and outdoor air quality.
“make sure that you are keeping your head on straight using math very simple math take your your square footage multiply by your ceiling height divide that by 60 that's your air cleaning one time per hour and then use what Jeff is telling us which is half to five or six be able to turn it up and down based on what you know and monitor”
Optimal home ventilation strategy for families spending time in a single room is one central system at low filtration (0.5 ACH) plus one portable HEPA unit in the occupied room, rather than high central system capacity.
“probably just one HEPA portable unit that's less a good filtration system for your house to say Merc 13 or something like that with the ability to run between half an air cleaning per hour which in that analysis is 250 CFM of just filtration that's not a lot and then up to five or six but that's depending and and I think probably localized is the right way to do it”
ASHRAE 62.2 is a ventilation standard that many people cite, but there is general uncertainty about what it actually specifies and whether it is well understood even by those who wrote it.
“I'm specifying like ashrae 62.2 if you don't know what that is I'm linking on screen another video but that's like a number that maybe means something and maybe doesn't everybody kind of agrees even people who write it”
Some commercial engineers are recently specifying 1,800 CFM ERVs for residential applications, suggesting a shift toward higher ventilation rates in new home design.
“I have seen commercial Engineers specking 1800 CFM ervs on homes uh recently so they're like it seems like people are getting really excited about it so we'll see how that works”
Pressure drop across filters is a critical constraint in HVAC system design; excessive pressure drop reduces airflow and increases fan energy consumption.
“if you don't know where the pressure drop is I'm linking on screen now but this is for the super nurse”
Distribution of air throughout a home is an important engineering consideration; uneven distribution can create 'dead zones' with poor air quality.
“distribution is important”