Tractorman44 thanks but misunderdstood husband! When thermostat turned up the fan comes on before flames ignites. Plenum heats and furnace runs.
If the fan comes on first, then the flame ignites, it USUALLY indicates there is a problem in the ignition process of the furnace. Most new manufacturers have the adjustable 'time off' sequence for the blower, and most initiate a call for the blower to run within a pre-determined amount of time AFTER a call for heat has been established. ALSO, and this is where I think the problem is, the ignition sequence starts immediately upon a call for heat. If there is a heat ignition failure, it will attempt a restart. USUALLY, they allow 5 restarts within a certain timeframe then the circuit board locks out the heat sequence until it is reset by cycling the power or cycling the thermostat manually.
What I think is happening with yours it that your ignition sequence is experiencing a 'flame failure' upon a call for heat....then it re-establishes the ignition sequence, which takes another 30 to 45 seconds. During this time, the blower automatically starts (because the allotted time has expired) and blows COLD air because of the initial flame failure. In the meantime the ignition sequence establishes a flame on the second or possibly the third try and then you feel the warm air.
IF this is the case, ultimately what will happen is that the furnace will continue to experience 'flame failure' and eventually fail to the point of 'flame failure lockout' and you will have no heat, but the fan could possibly run continuously.
At any rate, without being able to be there to troubleshoot and test components physically, this is just a calculated and reasonably reasonable guess. What it is telling you is that you probably need to contact a reputable, reliable service organization to give your furnace a good going over before it lets you down on a good cooooold night!!!
I hope this helps you understand the basics of what happens within the system and bear in mind, this is a 'generic' sequence and possibly NOT the exact sequence of your particular furnace.
Oct 01, 2015 |