Remember
when your dad told you to “Close that door! We’re
not cooling the great outdoors!” How we’ve grown.
We wouldn’t think twice about wasting energy like that
anymore, would we? I mean it’s simply not done. Who would
be so wasteful?
Would it break your heart to find out that we probably are?
Commercial heating\cooling ventilating systems have the ability
to introduce fresh outdoor air into the served spaces. The quantity
of this outdoor air is based in the maximum expected or design
occupancy. Dampers are set to a prescribed and measured minimum
position to allow for this.
The minimum outdoor air intake rate is 15 to 20 cubic feet per
minute (cfm), per person. So if the design occupancy is 100
people, you would set the system to deliver at least 1,500 cfm
or 90,000 cubic feet of outdoor air per hour. That is the “Solution
to Pollution is Dilution” approach. That’s enough
to completely replace all the air in a 1,125 square foot home
10 times per hour…dad would have blown a gasket!
Providing all 100 of those people fresh air is a good thing.
Problem is, 100 people are there at maximum occupancy. The remaining
time there is less, maybe a lot less. Still those dampers are
set to provide the same amount of outdoor air.
You see where I’m going with this thing? Yeah, that’s
right, let’s close that damper. But how? And how much?
If I close it too much it would not provide adequate indoor
air quality and if I leave it open I’m conditioning the
“Great Outdoors”.
It surely wouldn’t make sense to count heads in and out
and make constant manual adjustments. That would be too labor
intensive. So does that mean you’re stuck with the maximum minimum
all the time? No. As a matter of fact there is a good way to
test for proper ventilation.
A newer control strategy called Demand Control Ventilation (DCV)
uses a space CO2 (Carbon Dioxide) level monitor to adjust the
outdoor air intake. CO2 is a very good indicator of occupancy
level. Technology has made CO2 sensors accurate, stable and
affordable. Now outdoor air intake can be automatically readjusted
based on the level of CO2 in the space. This will insure that
we use the minimum outdoor air intake at minimum occupancy while
providing for maximum intake at maximum occupancy.
Now that would have made dad happy.
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