| Thousands
of tests have shown that most duct systems, including new ones,
leak over 25%. The cost of testing is a tiny fraction
of the cost of building a house and worth every penny for you
to find out if you're getting the system performance you intend.
A direct duct-pressure test must be performed by a GBP-approved
testing company; air leakage may be no greater than 10% (not
required if all ductwork is located within the thermal envelope).
This test tells you how much conditioned air is being lost (or
unwanted unconditioned air gained) through leaks in the duct
system.
It should be stated in the specifications and/or agreement between
the general and mechanical contractors that this test will be
conducted by a third-party testing contractor and that duct
system air-tightness must meet the test requirement (allowable
air leakage no greater than 10%). We suggest that a blower-door
test for envelope leakage, room-by-room air-flow test and a
safety backdraft test be done at the same time. This is very
economical, once the testing contractor is on
the site.
It is a code requirement in the State of Texas that residential
cooling equipment be sized by a Manual J calculation, based
on actual house design, specifications, orientation, correct
climate-zone and other standard design criteria. New
construction, built to current codes, should result in 600 square
feet or more of heated and cooled space per ton of cooling,
if the design is appropriate for a hot climate.
The change from uncoated window glass to southern low-e alone
typically allows equipment to be downsized by approximately
one third. If the Manual J calculation results in a substantially
lower number than 600 square feet per ton, find out why. When
equipment is too large for the cooling load, it never runs long
enough to dehumidify properly or meet its rated efficiency.
For example, a 12.00 SEER unit may never run more efficiently
than 8.00 SEER. If a correctly sized unit does not cool properly
due to loss of conditioned air from duct leaks, fix the leaks.
New federal minimum efficiency standard for residential cooling
equipment in
January, 2006: change to 13.00 SEER.
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