said by KrK:
The thermostat calls for heat, then a blower starts up. But it's not the blower that blows air into the house? Or maybe it spins at a slow speed or in reverse or something... I'm not sure but it's definitely some type of blowing/venting BUT is not coming into the house yet.
If it is anything like the furnace in our house (Most operate in fairly similar fashions as far as procedural steps to normal operation), that would be the blower that pulls in combustion air for the gas.
Essentially it should operate like this:
1) As the thermostat calls for heat, the furance kicks on the combustion air intake blower.
2) After a given period that #1 runs, the gas valve opens and the burners ignite (obviously sometime around here the igniter lights to accomplish this)
3) After the burners run for a sufficient period to heat up the heat exchanger, the main forced air blower kicks in.
-Now during this period the intake blower will continue to run as long as the burners are lit-
4) After the thermostat is satisfied and ends the call for heat, the gas valve shuts, burners go out, and the intake blower cuts out.
5) After a sufficient amount of time for the heat exchanger to cool down (some furnaces use a temp sensor, others go by a set time delay. Mine is the latter and the delay can be set via jumper on the control board), the main forced air blower shuts off and that is that until the next time a call for heat is made.
To the OP, I will concurs with the others here on the flame sensor. That seems to be the most common culprit of all especially of the furnace has not had a checkup or maintenance in a while. And it is the easiest and cheapest to check. Take it out and clean it yourself. If that does not solve it, then you can go down the next options until something sticks.--
I swear, some people should have pace-makers installed to free up the resources. Breathing and heart beat taxes their whole system, all of their brain cells wasted on life support.-two bit brains, and the second bit is wasted on parity! ~head_spaz