Williams 450FX-R vent safety switch

CALNNC

Bringing an older Williams propane gas furnace back to life. It is in excellent shape, needed a new thermocouple, but the proper thermocouple with the terminal block in the middle of the copper line for the vent safety switch wiring was not available. I installed and tested the new thermocouple and the furnace works fine. I cut the line in the right spot, I removed the bad thermocouple end from the in line junction box, soldered the new one in, and when you jumper out the connections, the furnace works fine. But, when hook up the wires to the the thermocouple junction box to the vent safety switch, the pilot will not stay lit. I have good continuity, a few tenths of an ohm through the wiring to the vent swtich, but the only way it will light is if I jumper out the connection on the thermocouple line junction box directly. Am I losing to much of the millivolt signal by going up the the vent switch and the this unit needs a 750mv thermocouple to make up the loss through wiring and switch? Williams as a furnace making company has no customer service worth a darn all they say is that it is over 10 years old and buy a new one. If you are in an area that has no ice or snow buildup, is the vent safety switch really necessary?

I attempted to move this to Thermostats and Controls but now not sure where it is.

EBEBRATT-Ed

It is not a thermocouple it is a power pile. Floating around on my computer are a set of instructions for testing but I have to search for it. You have to test it by measuring the voltage across the powerpile when the pilot flame is burning

POWERPILE INFORMATION.pdf

`Hope this helps

CALNNC

I get about 23mv. The one originally in it looked like a standard thermocouple as does the replacement, and the replacement did fit fine and screw in without issue. I did read that 750mv units were called power piles, so guessing that is not what was used in this unit. So the mystery remains as to why with the good vent switch circuit making the connection, it does not work.

EdTheHeaterMan

@CALNNC indicated this: " I installed and tested the new thermocouple and the furnace works fine."

Then use a new thermocouple and allow the system to operate "Fine". You can not modify a thermocouple and expect it to work "Fine". You may want to try one of these things: Thermocouple Adaptor that will fit between the thermocouple and the gas valve.

CALNNC

Isn't that device you show for testing a thermocouple output with a meter? I need one that opens the center conductor in the line which is then completed by an external switch. The question is why modifying one exactly like is done by someone else won't work? All it is an enameled copper wire in a copper tube outer shield, much like a coax, cut the tube in two, discard the gas valve end, cut off a bit more on the thermocouple side, to expose the enameled center conductor and then it is just simple solder work. If you use a tubing cutter, you have to just score the tube a bit and then bend and break or if you go too far closing the cutter, the cutter will make a sharp edge on the inside of the tube which will short the center conductor making it useless.

mattmia2

baso makes the terminal adapter you need