Hiya...

Are you sure the gas burner is actually firing for those few seconds, or just the blower ?

I'm NOT a pro. Yet I had the same "issue" with my oil furnace. In my case, the blower thermostat needed adjustment. It's located within the furnace right over the heat exchanger.

This thermostat has three settings...

Kick in - Temperature at which the blower fan starts.

Kick out - Temperature at which the blower fan stops.

High limit - Temperature at which the burner is forced off due to overheating.
(the blower remains on in this mode to cool it down)

In my case, the kick in/kick out temperatures were too closely set. After the burner had shut down and the blower kicked out, the temperature would rise just enough for the blower to kick in again. Just like you, the thermostat was not calling for heat at this point. The burner wasn't firing either. The blower would stay on for only half a minute before falling below the kick out point for good. Of course, the blower would kick on again after the thermostat called for heat and fired the furnace.

Mine was a new installation. So it would be expected that the my blower thermostat would need some adjustment.

However, you've had a trouble-free system for 15 years. So for this to be a simple blower thermostat adjustment is highly unlikely.

You could have a failing blower thermostat. However, a partially clogged heat exchanger, exhaust flue, or chimney could cause what you are seeing. Have you had your furnace cleaned or tuned recently ?

Draft issues might cause an issue, as well. Have you any stuck dampers ?

In any case...I recommend you do these two things before you do ANYTHING to your furnace...

- Wait for one of the pros on this forum to chime in on your problem. I'm not one.

- Have your furnace cleaned and checked by a pro. Have him/her? look it over, as well.

Good luck !

Marco

Marco,
Thanks for your thoughts. Yes, I am sure the furnace is firing, however briefly. The air coming out of the registers is warm.
Its been a few years since the furnace was cleaned. I guess that's the logical place to start.

Hello,
Clean the furnace and burner rack first. Clean the cavities with a vacuum. If you have a thermocouple, make sure it is in the middle of the pilot flame. When the burners light, make sure they are blue if gas. I don't know what color for oil? Normally the operating temperature and the overtemp are kinda close, and come set from the part manufacturer. If you have to adjust very much, check for airflow problems. High static pressure can cause the temp to rise after the unit is off. Normal operations will not cause this problem. If you have a problem with burner firing, check the exhaust pipe for something inside.

I agree with daft punk. A furnace does not produce heat for delivery to the registers in only a few seconds of firing. Modern furnaces may not be exactly as I describe, but this is the way that many work. The wall thermostat clicks on, "calling" for heat. The wall thermostat controls only the burner, nothing else. The burner starts and begins to heat the combustion chamber and the heat exchanger. When the heat exchanger reaches the preset "ON" temperature (120 degs. for example) for the fan control, that device sends power to the fan. When the room thermostat is "satisfied", it shuts off the burner. The fan continues to run, extracting some of the residual heat from the heat exchanger. This boosts furnace efficiency. The fan control also has an "OFF" setting (85 deg. for example) and when the temperature of heat exchanger drops to that level, the switch opens and the fan stops. After the fan stops, the residual heat from the combustion chamber (the hottest part of the furnace, if it is an oil furnace) rises and the fan switch senses the rise in temperature and the fan runs briefly again. The fan control, if adjustable, can be adjusted to stop the extra brief fan operation. The differential between the ON and OFF can be increased to that end. But that does not help with efficiency, except to decrease it somewhat, perhaps slightly. My trade is not HVAC, but I had a case once where I was the electrician and plumber for a home addition and was asked to extend the ductwork to the addition. I have my late father's sheet metal tools, but only some of his skills. The extended ducts did not adequately serve the addition and starting over was not in the budget. So I used booster fans in the ductwork near the registers. Good results for temperature, bad for noise. I told them that for their budget, the choice was the noise of the fans or the noise of chattering teeth. So you now need to make a choice about your system. My two typing fingers are tired now.

karenrunge, Marco and bus driver are right, just because you feel warm air coming through the registers doesn't mean the burner has fired. Like bus driver said, the furnace is probably building up residual heat after it shuts down and its causing the blower to come back on to send that heat up into the space. The control is called a fan/limit switch. The fan on tempeature is usually 140 degrees and the fan off temp is normally 100 with the limit setting normally at 180. What the limit setting does is shut off the burner in case of an overheat condition. Such would be the case if the blower motor would cease to operate or the furnace has a plugged filter for instance. Since its highly unlikely you've been playing around with the settings on the fan/limit switch its possible something has changed. For starters, is the filter partially plugged causing the furnace to run hotter then normal. Are all the returns open and not blocked by rugs, kids toys of maybe fido? Same goes for the supply registers,, any closd off or blocked, is the blower fan plugged with dirt? Once the room has come up to temperature and the thermostat shuts down the burner if you want to run down to where the furnace is there's a pretty good chance that you'll notice that the burner is off but the fan may still be running. Once it stops, sit tight and give it a few minutes. More than likely you will probably hear the blower come back on before long. More then likely that is whats going on with your system. Now you just have to figure out what has changed to cause it. Good luck and give another shout if you need more help.