I have an old gas free air carrier furnace. Two days ago the thermostat stopes working. I opened up the furnace and this is what I know. The pilot light is on, the blower turns on and operates, there is power to the furnace, there is no power to the gas regulator or the thermostat from the furnace, I put a line from R to W and the gas still will not come on. Tried the breaker and wall switch. I am guessing there is a switch somewhere I am not seeing. Any suggestions???

 

Sounds like you have the classic Carrier three wire pilot burner intermittent ignition system.

When the thermostat calls for heat, the circuit board turns on the gas to the pilot burner and the spark to light the pilot. After the pilot lights, the pilot burner flame licks up against a single pole double throw switch which changes the state of the switch when the temperature sensitive switch gets hot enough.

When the switch begins to move it shuts off the spark. When it gets hot enough, the power is switched on to turn on the main burner gas.

Your switch isn't getting hot enough to turn on the main burner gas. That is VERY commonly caused by a dirty pilot orifice.

You need to take out the two 1/4" nuts which hold the pilot switch to the furnace chassis.

Gently bend the pilot tubing so that you can work of the pilot switch. Use a wrench to loosen the brass bezzle that holds the pilot tubing in to the pilot burner.

Remove the bezzle from the bottom of the pilot burner and the pilot orifice will usually drop out into your hand.

Use a wire from a wire brush to clean out the pilot orifice, and blow any dust and dirt from the pilot burner.

Reverse this procedure to reinstall the pilot burner and you should be back in business.

Clean the pilot burner once/year as annual maintenance to avoid having repeat problems with this in the future.