at all uncommon, you can't let that filter load to the point
where the pressure drop is 0.5 i.w.c. That would mean the
pressure drop across the dirty filter by itself is equal to
what's allowed for the total external static pressure. That
is, the filter has used up all the pressure that's supposed
to be used up in the drops through the whole duct system
(and the air conditioner coil in the case of a furnace).
If you want to go to a performance-based method for
deciding when to change the filter, you've got to make
sure your duct system and filter design are appropriate.
If they're not, then you should fix those problems so you
can get it to the point where a doubling of the filter pres-
sure drop is your signal to change the filter.
Allison Bailes of Atlanta, Georgia, is a speaker, writer,
building science consultant, and founder of Energy Van-
guard. He is also the author of the Energy Vanguard Blog.
You can follow him on Twitter at @EnergyVanguard.
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a pressure drop even approaching the upper limit of that
gauge, you have a problem.
What about the total system pressure?
One thing needs to be pointed out here: Waiting for your
pressure drop across the filter to double will work only for a
filter that's designed properly and has a low pressure drop
with a clean filter. And you also need a duct system that's
designed properly, without excessive resistance to air flow.
If you put in a MERV-13 filter and the pressure drop
across a brand new, clean filter is 0.25 i.w.c., which is not
i
