Heating Help

spark ignition

I have very little experience with sparkies. Customer has pilot on but main burner wont fire. I jumpered the thermostat and pilot goes out. Tried a few times with same result. There are 24v at gas valve without jumpering. Am I doing something wrong? Why would jumping thermostat shut down pilot?

spark

does the gas valve have a little black box with 3 or 4 round prongs pluged into it?
The little black box have a thin rigid wire coming out of it to what looks like a thermocouple?

yes

Little thin wire leading to thermocouple .white rodgers 3098-156 flame sensor.

mercury switch

Either the sensor is bad or the pressure switch built into the gas valve is bad.
They used to make a resistor plate that went between the valve and the plug IF the pressure switch was bad.
More than likely the mercury switch has gone bad. Try jumpering two of the prongs while the block is partially unpluged from the gas valve.
If the mercury switch is bad when you short the right pair the gas valve will open.
Replace the switch - they are pricey.

confused

I get why the main burners wont fire. What I am not getting is why the pilot shuts off when I jump the thermostat.

mercury switch

By the way - If you ever get one of these valve / mercury switch combos where the pilot lights for about 15 to 30 seconds and then goes out and after the same amount of time the pilot lights again. The pressure switch built into the gas valve is bad. See if you can find one of the resistor plates.

Time

I think it may be what I just discribed.
leave it alone and see if the pilot cycles every 30 seconds or so.
If so the only solution is the resistor or replace the gas valve.

standing pilot

Think I will do myself a favor and replace with standing pilot or suggest that customer find someone else. Need to do more homework b/4 I feel comfi working with these things. Thanks Steve for the info. When things quite down a bit I will be doing my homework.