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PROPERLY SIZED EQUIPMENT
Properly sized
equipment will run nearly
continuously on the coldest and
hottest days of the year.
Oversized equipment runs for shorter
periods at higher intensity,
compromising comfort, performance
and equipment lifetime. It
costs more up front. Higher
capacity equipment also requires
higher air flow, meaning larger
ductwork and larger blowers.
Excessive oversizing
was observed for 70% of study home
furnaces and every study home air
conditioner. Furnaces were
sized an average of 158% of the
minimum required size (maximum 238%)
versus an industry recommended
maximum of 140%. Furnace
sizing practices did not appear to
reflect the reduced heating loads
due to insulated basements.
Air conditioners were even more
oversized, at an average of 208% of
the minimum required size (maximum
322%) versus an industry recommended
maximum of 115%. These data
raised questions about sizing
procedures and excessive safety
margins.
With filters removed
for testing, about one-quarter of
study home furnaces operated near
the limits or outside of
manufacturer's specifications for
external static pressure and/or heat
rise. In about half of the
homes with air conditioning, air
flows across indoor coils deviated
considerably from the typical
specification of 400 cfm/ton.
Fourteen percent of air conditioners
had air flows below 350 cfm/ton,
while a third of the sample had
flows exceeding 500 cfm/ton (with
filters in place, external static
pressure would increase somewhat, in
turn decreasing air flows and
increasing furnace heat rise).
*
Oversized equipment was found in
many study homes. Almost half
of the furnaces exceeded the maximum
recommended size. All of the
air conditioners were excessively
sized; the average unit was about
twice as large as needed.
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