Because the evap coolers are so much less to operate, many homeowners figure that they it's cheaper to run the evap cooler in May and June and then switch to central air for July and August. How much does that method really save though? As energy auditors and home performance contractors we have seen dozens of homeowners faced with this situation. The short answer is that homeowners will save more money (and be more comfortable) by capping and sealing the evap duct and just using their central AC in the summer if the ducts are sealed, the home is sealed from the attic and insulation is up to R-38.
Surprised? When our energy auditors measure the duct leakage on a home with an existing evap cooler connection, that leakage is sky high! The dampers on evap duct systems are ineffective and a major source of leakage. The damper will cause a 4 ton AC unit to put out only 3.5 tons of air. Add to that the air leakage and heat gain from the upducts located in each room and you have a recipe for high energy bills. There are replacement dampers for evap ducts and some are actually pretty good. The problem is finding an AC contractor who can source and install them. If duct sealing is done with the evap connections still in place for future use, the major source of leakage is still present after the work is done, which is why energy bills will not do down that much. When a home is properly sealed, the ductwork is tight and a thick insulation blanket is keeping all the conditioned air inside the home... bills go down and the cost savings begins!