My furnace kicks in and tuns off by itself?

Question

I turn off my white rodgers thermostat ( no heat no air conditioner), the switch fan is in auto, but once a while the furnace turn ON by itself, ( just the motor no blower) for couple second or may be a minut max then it turn off. I was wondering why. I turn off the flam for the summer is that may be the problem?

Answer 1

The motor you're referring to is probably the 'pre-ignition' blower which is used to clear the exhaust of any fumes prior to lighting the flame. With furnace operation that usually runs for a minute or so then the flame comes on and the fan starts up. Mine uses a solid state ignitor (no pilot) so there's a slight difference. Maybe it IS something with the pilot being turned off or perhaps it's some fault on the furnace control.

Answer 2

I'd say you have a faulty t-stat because something is calling for heat.  Given that nothings really changed since last summer and it never did this in previous summers when you'd put the pilot out that's my best guess.  There should be a switch on the side of the furnace that'll kill the power.  Shut it off for now and test your heat in the fall.  Its using a little gas every time it tries to start.

Answer 3

If you have an electronic thermostat it may be a power stealing type that may not be quite compatible with you furnace control circuit.

These thermostats can start the heating circuit because of the small trickle of voltage used to power the stat.

The fix for these is to add a resistor or an isolation relay or simply use a different stat that has a common terminal.