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.
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?
Suggest you get some one to look at it or phone someone to find out
about part as most are the same size and are hence marked universel on
package. You sound to have more wrong than what you think suggest a
pro or at least some one with more experience take a look for you it
is gas after all. Strongly look at a new furnace if you can.
Just because you are ignorant and insecure doesn't mean that everyone
else is. If you don't have anything useful to suggest the than
calling a tradesman I suggest you keep your non advice to yourself.
Tony Hwang did remind me of a simple voltage test. Its there at 28
Vac. Thanks Tony. I don't do this often enough to have missed a
simple test llike that. But it was midnight, the light was poor and I
was tired. My next step is to take out the current thermocouple and
get a match. Its a $8 item and gets me out of the house. If it
doesn't fix the problem I leave it there anyway as a new one saves me
a future failure. Then I'll troubleshoot something else. The kind of
repairs require only a module change, no fancy parts repairs. That's
how things are done these days be it a module or a whole PCB for even
the most complex equipment. I have more more than 20 years of fixing
oilfield and industrial equipment.
Calling in one of your fellow tradesmen will easily set me back more
than $100 plus the aggravation of hearing him cry death and
destruction if I don't get him to replace the whole furnace
immediately. I do my own maintenance and my furnace looks as new as
the day it was installed. I had the heat exchanger tubes inspected by
the gas company two years ago and there is a lot of life left in them.
I asked and he showed me what to look for. When there comes a day for
them to be replaced I'll do it myself and I can do a better job than
many a tradesman.
Just a shot in the dark -- but it saved me years ago when I returned from
Florida late at night in the dead of winter to find my gas furnace had shut
If your water heater was installed at the same time as your furnace --
assuming it was also gas fired -- it may well have the same thermocouple.
Mine did so I pirated it over to the furnace and headed off to bed. I
bought a new one the next day. But you situation may well be different.
Best of luck!
Actually that swap idea did occur to me. But its not that cold yet
(58 deg F in the house) where I have a problem - frozen house plants.
I can tolerate cold quite well. Anyway its off to the parts store for
Went to two more places and they both have only the new "slim"
thermocouples. I'll be darned if I am going to drive across town in
the hope of finding an antique at probably double the price. So went
home. Cut out a rectangle from a food can. Drilled the appropriate
sized hole for the replacement thermocouple. Clipped that over the
existing holder and secured the clip with a small screw. Works like
Should have thought of this originally but I am not that enthusiastic
about duct tape and bailing wire stuff anymore. Its a lot easier to
just buy the right stuff and replace.
To the Jeremiah's. The majority of house equipment problems are
simple problems requiring simple fixes. Use a bit of common sense to
get them running again. You don't have to go to get a university
degree to learn how to design and manufacture the darn things in order
to fix them. And when things go wrong don't tear apart the house to
find the problem. I have fixed everything from fine wind up watches
to that hernia box of a TV set, the electric organ, my car, the lawn
mower, plus much more. The fun is in figuring out how things work as
you go along and fix them. The worse that can happen is to toss them
On Thu, 11 Oct 2007 05:21:58 +0000, PaPaPeng wrote:
Thermocouple is ok if it holds the pilot valve open. Need to verify that
there is a call for heat via the thermostat and the low voltage supply is
available. Check for 18 - 24 volts AC at the gas valve. If so then the
vavle is stuck or the coil is open. If not then check your low voltage
transformer and wiring. BTW the thermocouple should put out maybe a tenth
of a volt or so.
On Thu, 11 Oct 2007 18:14:42 -0500, Moe Jones wrote:
Well he posted again and said he modified the way the new slim
thermocouple mounted and now all is well. I suppose if the gas valve was
stuck shut that all the vibration from drilling etc.. could have unstuck
it. It's been years and I don't recall if I've run into a situation where
a thermocouple could have just enough output to hold the pilot open but
not enough for the safety on the main valve or even if if the older valves
were designed in a way that this could be possible. I have my doubts that
this is the end of his problems. As far as a short taking out the gas
valve on a 20 year old system, that's not likely to happen as a short
would remove the voltage path to the gas valve and possible take something
else out instead.
Some of the old gas valves supplied gas to the pilot independent of the
thermocouple. If the pilot went out or the thermocouple went bad, the pilot
gas was not cut off. Some systems even had a small separate valve which
supplied gas to the pilot, totally independent of the automatic main gas
On Thu, 11 Oct 2007 22:16:55 -0500, Don Young wrote:
Seems very hazardous to allow gas to the pilot if there isn't a flame to
burn it doesn't it? I would hope this wasn't the case in valves
manufactured within the past 50 years or so during which heating migrated
from coal to gas.
Yes, there was a hazard which is why that type is no longer used. I think
the idea was that such a small amount of gas was not likely to be a problem
as long as the venting was good. I do not know when they went out of use but
it may have been when LPG became common since LPG is heaver than air and
does not dissipate as well. Cookstove pilots used to be that way too and I
do not know if any are still.
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