Active solar heating:  Mechanical or electrical systems that collect and absorb solar radiation, then transfer the solar heat to the interior space or to a storage system, from which the heat is distributed in the home.

Advanced framing:  see optimum value engineering

Aerator:   The screw-on tip of the faucet that determines the flow rate. Aerators are inexpensive to replace and they can be one of the most cost-effective household water conservation measures. 

Annual fuel utilization efficiency (AFUE):   The measure of seasonal or annual efficiency of a residential heating furnace or boiler. Specifically, it is the ratio of heat output of the furnace or boiler compared to the total energy consumed by a furnace or boiler. An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes up the chimney and elsewhere.