28 | BUILDING SCIENCE Rx — December 2018
Here's a chart I saw in a presentation at the North Amer-
ican Passive House Conference in Boston a few months
back. It was put together by my friend John Semmelhack of
Think Little in Virginia from data he found in various places.
As you can see, you've got to step up your filter game to at
least MERV-10 to get even half of the PM2.5. But you really
need to be at MERV-13 because you want to remove more
than 90% of those particular little bitty invisible pieces of stuff.
But there's a caveat here: Many (most?) systems with
high-MERV filters decrease the air flow because of poor
design. My next three articles will explore this issue in depth
and show you how to do it right.
6. Not filtering outdoor air for
mechanical ventilation
Some ventilation systems are designed to use the heating
and cooling ducts to distribute outdoor air, too. With the
central-fan-integrated-supply type of mechanical ventila-
tion, a duct from the outdoors is connected to the return
side of the duct system. Occasionally, a designer or installer
doesn›t pay attention and connects that outdoor air duct to
the return side downstream of the filter.
Oops! When that happens, you're putting unfiltered
outdoor air straight into your ducts, where it can make the
ducts, blower, and heating and AC components dirty as well
as sending more particulate matter into your indoor air.
By the way, I haven›t said this yet, but the biggest source
of PM2.5 in most homes is outdoor air. (If you smoke in-
doors or burn candles or incense regularly, those would be
bigger.) So make sure you filter that outdoor air before intro-
ducing into your home.
7. Filter in the wrong place
I have to think this one is anomalous but with the strange stuff
I›ve seen in the wild, who knows. The photo below was sent
to me by my friend Jamie Clark in Kentucky. The air moves
through that system vertically. There's nothing coming in from
The system in
the photo at left
had both prob-
lems. The pres-
sure drop across
the dirty filter and
coil in this sys-
tem was a super
high 0.9 inches
of water column
(i.w.c.). With a
clean filter, it was
still a too-high 0.6
i.w.c. (Read more
about this home.)
That pressure
drop is about ten
times too high.
