62 | March 2020
This is a good example because
it's such an extreme event. We hear
people size HVAC systems for par-
ties like this, which is frankly silly be-
cause it's a couple of hours per year,
and as you can see if the HVAC sys-
tem is done well, the spikes don't
last that long.
We've seen similar results in the
other client homes we monitor. They
do need some fresh air as we get
them tighter, but not an incredible
amount. Don't take my word for it
though, go measure yourself!
Our Conclusion
In our projects and monitoring, we've
found that 10-30 cfm of "fresh air" aka
mechanical ventilation has been ad-
equate to keep carbon dioxide levels
moderate and air quality good, at least
through the proxy measurements we're
using. This supports what Joe Lstibu-
rek of Building Science Corporation
found during the Canadian R2000 pro-
gram which built tight homes and used
a similar fresh air strategy to BAD ASS
HVAC.
Two caveats going back to how
people and combustion applianc-
es need to breathe. First if there
are more than 3-4 people living in a
home, more fresh air will be needed.
Second if you have a "natural draft"
water heater or furnace that pulls
air to burn from inside your home, it
will need additional air too. Since we
only install heat pumps or furnaces
that draw combustion air from out-
doors, this is not a large factor in our
projects, and is also an argument to
get rid of natural draft appliances.
Fresh Air Ain't Fresh
One last thing, fresh air often ain't fresh.
Check out the dark corner on this
filter of a client HVAC system. This
is where the outdoor air stream hits
this filter (it's from a ventilating de-
humidifier, which is how we keep
Again, these are tVOC levels, so I'm not 100% sure what the precise CO2 levels
are but suffice it to say they got way above the 600-800 ppm target. This next chart
shows the peak at 6:30 PM as we broke up to go look at the HVAC in the basement
and moved away from the Foobot monitor. Shortly thereafter the event ended, and
people began going home.
https://photos.app.goo.gl/vSyET8daEwpJ4ZpRA
This next chart shows that by 2:15 AM, almost 8 hours after the peak, 30 cfm of
fresh air (less than half the 62.2 recommended level for this home), tVOC levels were
back to normal, which indicates CO2 levels were back to normal as well.
https://photos.app.goo.gl/RkjJbpeCcQJVqcc96
