ASHRAE Journal - August 2022 - 28

BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
(left) Cold Climates. Warm air would be blown up against the cold glass from floor registers located under windows. The " blown " air would rise to the ceiling and flow
across the ceiling, be cooled, fall downward and be pic across the ceiling, be cooled, fall downward and be picked up at the bottom of interior walls by a return register.
IGUR
ked up at the bottom of interior walls by a return register. FIGUR
against the warm glass from ceiling registers located above windows.
be pic
be picked up at the top of interior walls by a return register.ked up at the top of interior walls by a return register. FIGUR
climates and fall to floor, thereby requiring high returns. climates and fall to floor, thereby requiring high returns.
The Odd Couple
HVAC Systems
And Windows
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
HVAC systems and windows have almost as interesting a relationship as the " Odd
Couple " -Oscar and Felix-did in the '70s.* However, unlike Oscar and Felix's relationship,
the relationship between HVAC systems and windows has changed dramatically
over time. How they now live together is interesting and changing.
Way back in the day, we had lousy windows. They
were thermal nightmares. Insanely uncomfortable in
the winter and ugly uncomfortable in the summer. You
couldn't be near them when it was cold or when it was
hot and sunny. Thank God for HVAC systems. Good
HVAC systems saved lousy windows. We had to blow
warm air against them in winter to raise the mean radiant
temperature and blow cold air against them in the
summer to lower the mean radiant temperature. Laying
out ductwork was a big deal.
In cold climates warm air would be blown up against
the cold glass from floor registers located under the
*The Odd Couple was a movie starring Jack Lemmon and Walter
Matthau that turned into a TV series with Tony Randall and Jack
Klugman. The premise was about two separated men who have to
live together whose " ideas of housekeeping and lifestyles " were
" as different as night and day. "
28
ASHRAE JOURNAL ashrae.o rg
A U G UST 2022
windows. The " blown " air would rise to the ceiling, and
the assumption was that the air would flow across the
ceiling, be cooled, fall downward and be picked up at
the bottom of interior walls by a return register (Figure 1).
In hot climates cold air would be blown down against
the warm glass from ceiling registers located above the
windows. The " blown " air would fall to the floor level,
and the assumption was that the air would flow across
the floor, be heated, rise upward and be picked up at the
top of interior walls by a return register (Figure 2).
As buildings became better insulated, it turned out
that the air at ceilings would no longer be cooled in cold
climates and fall to floor. The simple fix to this issue was
to locate returns at the top of interior walls (Figure 3).
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.
FIGURE 1 (left) Cold Climates. Warm air would be blown up against the cold glass from floor registers located under windows. The " blown " air would rise to the ceiling and flow
IGURE 2 (center) Hot Climates. Cold air would be blown down
(center) Hot Climates. Cold air would be blown down
against the warm glass from ceiling registers located above windows. The " blown " air would fall to the floor level, and aihe " blown " air would fall to the floor level, and air would flr would flow across the floor, be heated, row across the floor, be heated, rise upward andise upward and
IGURE 3 (right) High Returns. As buildings became better insulated, air at ceilings would no longer be cooled in cold
(right) High Returns. As buildings became better insulated, air at ceilings would no longer be cooled in cold
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ASHRAE Journal - August 2022

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