ASHRAE Journal - November 2019 - 75

COLUMN BUILDING SCIENCES

FIGURE 1 HVAC System Pressures. Typical pressure difference between bedrooms
and common areas in houses with forced air distribution systems.

Supply

Return

Supply

Room

Common Area

Room

go under a door, it moves downward. The particulates
in the moving air have their own momentum (as do
the air molecules). The air makes the curve and bends
under the door, but the particulates don't. They travel in
more or less a straight line and end up hitting the carpet
directly under the door, leaving a dark line. Go back and
look at Photo 1 again. So why does the air want to go down
under the door? Ah, you all know why-the pressure difference between bedrooms and common areas (Figure 1).

PHOTO 6 Electric Resistance Baseboard Heaters. What a mess-an energy-efficient
respirable particle generator.

PHOTO 7 Smiling Guy. Chuk Bowles cleaning the key surfaces in an air-handling
unit 30 years ago. Good to be correct, eh?

Filtration
The last deposition mechanism, and the most common, is the ﬁltering of particulates by carpeting.
When air ﬂows through a carpet the carpet acts like a
ﬁlter and removes particles from the air that passes
through it.
Air moves under baseboards in exterior and interior
walls due to air pressure differences caused by wind,
temperature differences and the operation of heating
and air conditioning systems. As the air moves under
baseboards, the carpet at these air pathway locations
becomes discolored if the air contains particulates.

The Big Picture
Most of the deposition typically observed is carpet
ﬁltration. The carpet ﬁltration issue is one of the most
common complaints from homeowners. Why is this
now such a big issue? Why didn't we have the problem
before? We've had carpets for a long time and the air
pressure differences are also not new. So what is different? Simple:
* We have more of the right-sized particulates;
* We have lighter colored carpets; and
* We have consumers with higher expectations.

We are not about to argue against light-colored carpets
or argue against consumers with higher expectations.
However, let's talk about the particulates. I estimate (a
Joe WAG [wild a-- guess]) that in the typical home we
experience a ten-fold increase in the quantity of indoor
microscopic particulates in the less than 10 micron
range principally due to cooking, candle smoke and
electric resistance heating elements.
Electric resistance baseboard heaters are often respirable particle generators (Photo 6). Guess what? We are
seeing more and more heat pumps. What is in every heat
pump? Ah, electric resistance heating elements that kick
in when the heat pump no longer can do its thing. Guess
what comes off those heating elements-because ﬁlters
are so good. Have a good look at Photo 7. That smiling guy
is Chuk Bowles who predicted all of this 30 years ago. It
is good to have legends as friends. Thank you, Ron Bailey
and Chuk Bowles.
N O V E M B E R 2 0 19

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ASHRAE Journal - November 2019

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