Gas and Oil Home Heating Furnaces - Furnace trouble

I have a older Bryant Furnace in my home. I am having trouble with getting to run correctly. Let me explain: I can set the thermostat and it will not kick on the gas valve. I have a pilot light. Now, I can turn the burners on manually and they light just fine. After about two minutes the coil on the gas valve activates and then I can turn it back to automatic and it runs fine. I have rang out the High limit sitch and it is fine. I have also replaced the transformer, completly looked over the electrical portion of the gas valve. What else am I missing? Also, I noticed there is a 2 wire type switch coming out of the pilot light area. Is this what they call a Thermo-pile? Any advise would be helpful.

Thanks,
J Hansen

I noticed there is a 2 wire type switch coming out of the pilot light area. Is this what they call a Thermo-pile?

I'm not familiar with your unit.

Can you provide a close-up picture or two?

Because you have a manual control, it is somewhat possible that your gas valve is operated by a thermopile and that it is corroded such that only with the added temperature from the burner that it makes enough current to energize the burner gas valve. Does your furnace operate normally when the electricity is off?

Does your furnace have a transformer?
Can you test for voltage on the gas valve (or perhaps in the thermostat)?

Otherwise, if you have a single-stage pilot, all that thermopile does is to keep the pilot (main) valve open and is not the problem.