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.