[LazyPup]

Perhaps a little bit of HVAC theory will help everyone understand the question a bit better.

Many people erroneously feel the discharge air from a duct expecting the air temp to be whatever the thermostat setting is. This simply is not the case.

Both a furnace and an AC have a differential temperature, which is the amount of temperature change that will occur as the air passes through the system.

Let us assume for the moment that the designed differntial temperature for your furnace is 18degF. (A differential of 12 to 14 degF is typical for AC or heat pumps)

Now let us assume your system has been off and the house has cooled to 40degF.

When you turn the furnace on, the return air will go into the heat exchanger where it is heated 18degF, which means the discharge air temp in now 58deg.F which would still feel quite cool if you were to put your hand near a supply duct.

The main fan and air ducting system is designed to effect three or four complete air changes per hour. Thus for the first 15 minutes or so the dischare air would be 58degF.

By then end of the first 15or 20 minutes the air in the room should now be 58degF which means it would then be heated another 18 degF on the second pass, thus the discharge air will now be 76 degF. This cycle continues until the air temp in the room is equal to the temperature set on the thermostat. When the calling set temp is achieved the thermostate then turns the system off until the room air cools and the cycle begins again.

The furnace or AC runs at a constant differential temperature regardless of what temperature is set on the thermostat. The only difference being: if the calling temp were higher the system would have to run longer to meet the demand.

When diagnosing a system an HVAC tech will often measure the temp of the return air, and the discharge side of the plenum to determine the differential temperature. A radical difference in differential is a primary indicator of a blocked air flow. Most common cause, a dirty air filter.