We just got a new system installed. It is all electric. When the furnace runs you can hear the thermostat click to end the cycle. It continues to run to force all the warm air out of the ducts. Good. The problem is, using incorrect but understandable terms, there is 1 gallon of warm air in the ducts. The fan blows long enough to push 5 gallons of air out. Those last 4 gallons of air are cold air from the attic, as if the a/c was running. Is it possible, and if so how, to adjust the timer controlling fan shutoff after the heating element is shut off?