PaPaPeng wrote:

My gas furnace has a thermocouple problem. Stronmg winds blew out the
pilot light. I can relight the pilot light, but no heat. Given that
the thermocouple worked without trouble for some 20 years it has more
than paid for itself. This was the only replacement in a 30 year old
furnace. The trouble is that the replacement thermocouple I bought
from Canadian Tire doesn't fit. The barrel at the base is undersized
and even the adaptor ring doesn't bring it up to size. That's the
only model (other than the copper tube conductor length) they have.
The trip to a appliance parts store is quite a drive. To cut a long
story short is there a way to test it with a multimeter to see if the
original thermocouple is still good? I had already cleaned it and the
circuit is good enough to keep the pilot light on but not enough to
fire up the furnace.

I don't think it is a thermostat problem since it is one of those old
style mercury switch types that doesn't have any fancy electronic
circuitry to go bust. I do have an electronic one that I had
disconnected long ago. I can never remember how to program it. Its
still on the wall and used as a thermometer.

This thermocouple was in the plumbing (hot water tank) section and is
the only thermocouple available. I would think its the same one used
for the furnace as all is does is to sense the pilot light ON so that
the gas can be fed to fuel the flame.
Hi,
Thermocouple's function is to provide small amount of gas for pilot
light. Main gas valve is actuated when thermostat sends 24V signal(W
termnal from the 'stat) What makes you think thermocouple is your
problem? 30 year old furnace? Isn't it time to replace it? Do you see
24V AC on the gas valve terminals when heat i called for?