If it is just the blower coming on:
Warm air furnaces have limit switches that have preset high limits, which shut off the burner when the heat exchanger high temp is reached. The low limit shuts of the blower when the heat exchanger cools down. Typically, there will be residual heat radiating from the burner chamber. Once that rises to the fan limit, it senses the heat and briefly turns the blower back on, distributing remaining heat.
If the burner is turning on:
It could be the t-stat itself, batteries, wiring at either end or even in between (hungry critters in the wall) drafty area. Possible limit switch. It is rare that the anticipator in the t-stat is a problem unless it is not set for the correct heating system. If it is an old school mercury no one would touch it unless heating appliance has been upgraded.
In any case, no quality technician should diagnose unless they are in front of the appliance. I would consult a licensed qualified technician. Hope this helps.
The furnace could be shutting off on High Limit ...means your furnace is running too hot...check your filter and make sure return air vents are clean and clear....also it can be the Heat anticipator in your thermostat...which controls how many times your furnace cycles...basically your furnace should come on and off 5-6 times/hr ....need more info on your type of furnace and type of thermostat
1.
Thermostat calls for heat.
2.
Draft inducer motor starts.
3.
Pressure switch attached by a small plastic or rubber tube senses the
negative pressure produced by the draft inducer and
closes.
4.
Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small
metal probe about 1/8" in diameter, with a white
porcelain base) does not sense the flame, so after 8
to 10 seconds the hissing sounds stops with no
ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back
off and on again. Then the sequence starts all over
again with no ignition of the gas.
Solution:You probably need to
purchase and install a new ignitor. I would suggest that you inspect your
ignitor closely for cracks. Make sure you do not
touch the ignitor with your bare hands. If you do not
visually see a crack, then you could have a furnace control board problem or a
limit, rollout switch problem. The furnace's control board
might not be supplying the voltage to the ignitor.If your furnace
lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then
you need to clean your flame sensor with light sand paper or steel wool. You
might need a new flame sensor, but most of the time they can be cleaned an will
work well after cleaning.
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