Furnace blower cycling

[needhelp1]

It is summertime so there is no demand for heat. The temperature is cool s0 there is no demand for cooling. The blower on my furnace is cycling on and off. I turned off the thermostat and this had no effect. Still cycles. I disconnected the green wire from the thermostat to the furnace to eliminate a thermostat problem. The blower still cycles on and off. If I turn the main power off to the furnace and leave it off a few hours and then turn it back on the blower may not cycle again for 1/2 a day or a day but will eventually cycle. There does not seem to be any pattern to the cycling. Any ideas on what to may be the problem and what to check?

 

[Blue Jay]

It would help if we knew what kind of furnace, if it is gas with a pilot light I have seen them warm up the bonett enough to trigger the limit switch.

 

[needhelp1]

The furnace is a Janitrol gas fired furnace with a glow plug,. No pilot light.

 

[user 4883]

Weak relay to the fan circuit?
Bad limit switch?

 

[needhelp1]

If the limit switch was bad wouldn,t the circuit board light flash to indicate a bad limit?

 

[user 4883]

Yes, it would. Maybe it's getting lazy and engaging prematurely.
Does the fan run at full speed when it comes on when it's not suppose to?

 

[needhelp1]

Yes the fan runs at full speed. It is not a two speed fan. The air conditioner has been switched on now due to the warm days we are having and the air conditioning system works as it should.
Periodically though the vent fan and the blower turn on for no reason. No demand for air was given by the thermostat. Most times it will run for awhile and then cycle off. Other times I turn the main power off to the furnace and leave it off for a few minutes. When I turn the power back on whatever the caused the fan to turn on must of reset because the fan does not come back on.

 

[user 4883]

Reconnect the green wire and disconnect the (W) white wire from the back of the thermostat and see if the problem continues. If not, it may be a bad t-stat.

 

[needhelp1]

Disconnecting the white wire had no effect. This morning the blower is running and does not shut off. Circuit board light is flashing five times. Ended up being a component on the circuit board is starting to fail. Replace the board and all is well. Thans to everyone for your help.