We live in a 70's home, with a gas heater, and the heat had been turning on (delayed) and then turned off inconsistently last winter. This winter, we would turn up the heat, and sometimes it would turn on and sometimes not. So, we replaced the heater thermostat with a LUX digital thermostat, and it turned on once, hours after we turned up the heat, and since has not turned on again. So, it doesn't appear that the termostat was the problem, but what could it be? Starting to get cold in Calif...

So what does happen? If it is a older furnace, the thermostats will switch power from the red wire to the white wire, which opens the gas valve. After it warms up a limit switch, the blower relay starts the blower. If the blower doesn't come on, another limit switch shuts the gas valve off.

The gas valve won't come if the pilot light isn't on. You must also have 24 volts AC from a transformer in the furnace. It may have an inline fuse.

Newer furnaces work a little differently.

If you can't find the problem, post back. If the furnace has a little circuit board inside, maybe near the blower and filters, it is a newer furnace. It would help the better people here to give the brand and model.

Sounds like the coil in the gas valve may be getting weak. Make sure when it calls for heat that you are getting 24 volts across the valve. If so and it does not light, the gas valve is bad...

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Originally Posted by maricela

We live in a 70's home, with a gas heater, and the heat had been turning on (delayed) and then turned off inconsistently last winter. This winter, we would turn up the heat, and sometimes it would turn on and sometimes not. So, we replaced the heater thermostat with a LUX digital thermostat, and it turned on once, hours after we turned up the heat, and since has not turned on again. So, it doesn't appear that the termostat was the problem, but what could it be? Starting to get cold in Calif.....

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First check the filters .

Poor airflow will trip a safety that cycles the burner,, I would inspect the ac coil and the blower as well as check to see if the return and supply ducts are adequate