Furnace pilot lite is on but when i slide t-stat over to about 75 or 80 , pilot lite goes out. Millivolt gas valve. Changed out thermopile and gas valve and still doing the same thing. What could be wrong? Also ohm the limit switch on top of unit and is giving me a reading? HELP
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I've worked on MANY floor furnaces, and the previous answer is COMPLETELY opposite of what I've experienced. If you've cleaned or replaced the thermo pile, have a decent flame, and allowed about 3-5 mins for the pile to generate a millivolt signal, it should have enough to power the line to the Tatar, then to the valve. You can jump by the wires to the Tatar to check that circuit either at the valve of the Tstat, as long as you're experienced and safe. Now if you hold down the pilot knob and it don't release, you have a damaged valve that should be replaced.
So are you saying the pilot stays lit, but the main burner won't always fire. If this is a millivolt system you could have a weak pilot flame, not getting the pilot generator hot enough or you could have a weak pilot generator. If you are handy you can try cleaning the pilot. It may be better for a qualified service tech.
Check the new thermopile with a millivolt meter and make sure it has the correct output not connected to the gas valve.If it does,then the gas valve is defective.Make sure to test ONLY the thermopile.If it does not generate enough millivolts the gas valve will not open.If the piliot is dirty it will not generate enough heat for the thermopile.Make sure you have a good,strong,blue flame at the pilot.You said the piliot will not stay lit?If not then start by blowing air back through the piliot where the flame comes out with compressed air.hopefully that will clean it well enough.If not you may have to take the pilot tubing loose from the valve and use compressed air to clean it.Make sure when you reconnect the tubing to the valve USE FINGERS ONLY to start tyubing or you may crossthread the tubing....and thats NOT a good thing.Good Luck.
You could have a weak pilot or thermopile. If the thermopile is not heated correctly by the pilot, it cannot generate enough millivolts to open the gas valve. The same is true if the thermopile is faulty. Thermopiles consist of several junctions of which only some may go bad causing low millivolt output. Millivolt output with the thermostat off and pilot lit should be around 600 millivolt, you can check with a DC multimeter set on MV across the two wires coming from the thermopile. Remember, the thermopile has to be heated by the pilot thoroughly before you will get the full millivolt reading and the thermostat must be off. Cleaning the pilot assembly is often helpful, thermopiles are inexpensive and relatively easy to change. Of course, other problems can cause this issue as well, but the above are most common.
This heater has 2 type gas valves one valve has the thermostat built into it. No wiring needed for that puppy. Looks like this:
So I am guessing that one ain't you. Your heater must use a millivolt gas valve with the 2 wire wall thermostat. The pilot generator does 2 things. It holds the pilot on and it produces voltages that operate the solenoid and open and close the valve when the room thermostat makes contacts. So what goes wrong with these? The pilot generator will sometimes hold the pilot flame but whn the 2 wires are touched together or you tun on the wall thermostat the whole thang just dies. That my friend is a weak thermopile generator. The other thang that goes wrong is the solenoid on the gas valve is open and therefore the gas won't come on. Their is also a fuse mounted on there that melts and kills the magnet that the generator holds in for the pilot.
Probably the thermopile is not receiving enough heat from the pilot flame to generate 500 to 750 millivolt to hold pilot flame. The thermopile may have soot or dirt on it so try to clean or the pilot flame itself does not give enough heat, try adjusting pilot flame. Or the thermopile could not generate the right amount of millivolt even if it is clean and receiving enough heat from pilot flame, so better replace the thermopile.
Check also the hi limit, it must have continuity across its terminal.
Has the control knob been turned from the PILOT position to the ON position? If that has been done, check the wiring from the thermostat to the Gas Control Valve in the heater. Make sure the wires are connected in both places (T-Stat & Control Valve). If all of that appears to be correct, there's a good chance that the Thermopile (I think Williams calls it a Pilot Generator) has failed. This part generates millivolts of electricity to open and close the gas valve when the Thermostat calls for heat. But, don't confuse the Thermopile with the Thermocouple. Your unit has both. And since the pilot is burning, that tells me it's not a problem with the Thermocouple.
Anyway, the only way to test the thermopile is with a millivolt multimeter. You will have to consult your Owners Manual for the proper millivolt output for your heater.
Hope this helps head you in right direction in solving this problem.