I replaced my central A/C unit about 6 years ago. I think it is 2-ton capacity and the house is about 1,900 sq ft and we live in south Florida. I installed a digital thermometer with a metal probe through the ductwork just about the evaporator in the air handler. I have read that an efficient unit will cool the air about 20-22F. The thermostat is directly above the 24” X 24” air return. When the thermostat reads 78F, the air temperature as it passes through the evaporator is consistently 56-58F since the unit was installed. I change the air filter regularly and once every six months I clean the coils in the condenser. Therefore, I think the A/C unit is working about as efficiently as can be expected.
The ducting is primarily a round flexible tube wrapped in insulation and a vapor barrier. The attic has blown insulation and the ductwork is on top of the insulation. Therefore, the ductwork is exposed to the full heat of the attic. About half of the ductwork is covered in a foil and that is holding up well given the house is 24 years old. The other half was covered in a grey plastic that just disintegrated over time. At one point, I thought I had a roof leak because of a large water stain on the ceiling. It was actually condensation forming where the plastic had broken down and the insulation had fallen away from the duct. I replaced all of the damaged ductwork a few years ago.
Last night I decided to measure the air temperature coming out of the more easily accessible vents as a possible indication of the integrity of the ducts since it is too *&^%$ hot to go up in the attic. The air temperature coming from the vent in the master bedroom was 3 degrees warmer than the temperature in the air handler (56F vs. 59F). This vent is near the end of the ductwork and probably at least 50-60 ft from the air handler. Another bedroom vent that is a little closer (maybe 30 ft of duct) was 2 degrees warmer. This was better than I expected, but I really do not know what to expect. This is telling me that the insulation around the ducts must be in good condition and there really isn’t anything I can do to improve the efficiency in a cost-effective way. Am I overlooking something?