36 | July 2020
How Often Should You Change
the HVAC Filter?
One of the topics I've hammered on for years is the in-
adequacy of using rules of thumb for sizing air conditioners.
The standard one ton of air conditioning capacity for each
400 to 600 square feet of conditioned floor area just doesn't
work. We know that a load calculation based on how a par-
ticular house is likely to perform is much better than a rule
of thumb. Another area where a rule of thumb dominates is
in replacing the filter in your air conditioner (or heat pump or
furnace). Change it every thirty days, right?
Is 30 days the answer?
When you buy the standard one-inch fiberglass filters for
your HVAC system, the instructions on the package usually
say to change it every 30 days. This advice gets repeated
dutifully by a lot of people and organizations trying to help.
And the truth is, you won't cause any problems by changing
your filter every 30 days and you might even prevent some.
A few years ago, I spent a day going on service calls with
Phil Mutz of Moncrief Heating & Air Conditioning here in At-
lanta. As you might expect, some of the problems we found
were low air flow due to dirty filters.
Changing the filter every 30 days is a hassle, though.
In spring and fall, the filter generally won't get as dirty be-
cause there's not as much need for heating and cooling,
so you can get by with not changing it as frequently. And in
a house that doesn't have much infiltration or duct leakage
and a low level of dust overall, the filter can stay clean lon-
ger even when being used a lot (unless your return vents
are in the floor).
And then there's the issue of the type of filter. If you've
been following this blog over the past year, you know I'm a
fan of high-MERV filters that are thicker (at least two inch-
es) because they do a much better job of cleaning the air.
If you want good indoor air quality (IAQ), you need better
filtration and that means a MERV-13 filter. But high-MERV
filters also happen to be more expensive so changing them
every 30 days is not the so good for your pocketbook.
The good news with high-MERV filters, though, is that
they usually hold more dirt. There's more surface area
with thicker, pleated filters so when they capture the same
amount of dirt as a standard one-inch filter, the change in
pressure drop isn't as much. So how do you know when to
change them? Well, if your filter looks like the one in the
photo at the top of this article, you've waited too long. But
there's a better metric than visual inspection.
How about using the pressure drop
across the filter?
I recently wrote about how to get a low pressure drop across
a high-MERV filter, and the issue of when to change the filter
came up in the comments. Here's what John Semmelhack
wrote there:
Our general rule is that when the pressure drop across
the filter doubles from the initial, clean pressure drop,
it's probably time for a change. With big, deep filters
this can take a loooooooong time, and the filters will
look absolutely disgusting well before the pressure drop
would indicate that it's time for a change.
The photo above shows how to make the measurement
of pressure drop. You need a hole in the return ductwork on
each side of the filter. In this case I used two static pressure
probes, one inserted in each hole. Each probe has a piece
of hose connecting the probes to the input and reference
pressure taps on the digital manometer. In the photo above,
the pressure drop across this MERV-13 filter is a low 0.06
inches of water column (i.w.c.).
If you're not an HVAC or home performance profession-
al and don't want to spring for a digital manometer, you
can set up your system with a magnehelic gauge, shown in
the photo above. You can get a decent magnehelic gauge
from Dwyer for less than $50. Make sure to get the lowest
range they sell: 0 to 0.5 i.w.c. If you ever have a filter with
