How do I know if furnace thermocouple is bad?


My heat went out last year, and the HVAC company came and replaced the thermocouple. It was cracked.
I noticed this morning that the furnace wasn't working. I don't get that "click" when I raise the thermostat.
I checked the thermocouple. There's no obvious visible problem.
What else can I check? I really don't want to pay for a service call.
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BTW, the furnace is an Amana Air Command 95. I have a digital thermostat and replaced the batteries. No effect. No breakers are thrown.
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wrote:

Is it gas, oil, or something else?
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" snipped-for-privacy@crud.crap" wrote:

Click from where, the furnace or the thermostat? Could be a lot of things like a bad thermostat, a bad transformer, broken wire, bad relay, etc.
Pay for a service call or pay for a class or two at the local tech school to learn how to troubleshoot all the possibilities and the basic tools needed for that troubleshooting.
Pete C.
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I don't get the click from the thermostat.
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" snipped-for-privacy@crud.crap" wrote:

So take your meter and see if you have control voltage from the furnace wires to the thermostat.
On a digital thermostat the batteries only serve to run the thermostat electronics and maintain it's program memory, the control voltage provided from the furnace is what operates the relay in the thermostat.
The control transformers on furnaces sometimes fail, sometimes just the wiring from the furnace to the thermostat fails, sometimes the thermostat itself fails. The fact that the display on a digital thermostat seems to be working ok is no guarantee that the thermostat has not failed.
If you get control voltage on the wiring to the thermostat (you should be able to measure on the sub plate with the thermostat removed) it is possible to manually short the appropriate connections to test the furnace since this is all the thermostat does.
The wiring colors can be all over the place so if you have the manual for the thermostat you should be able to find a diagram showing it's connections with more descriptive language. In a simple heat only setup there might be only two wires, a control voltage common and a heat wire. For a heat / cool forced air setup a control voltage common, a fan wire, a heat wire and a cool wire would be expected. It doesn't get complicated unless you have a heat pump with reversing valves or similar.
This link: http://home.howstuffworks.com/home-thermostat.htm has a pretty good description on how a thermostat works and includes the following description of the normal connections to a thermostat:
RH - This wire comes from the 24VAC transformer on the heating system. RC - This wire comes from the 24VAC transformer on the air-conditioning system. W - This wire comes from the relay that turns on the heating system. Y - This wire comes from the relay that turns on the cooling system. G - This wire comes from the relay that turns on the fan.
If you measure between say RH and W with a meter you should read 24VAC or close to it. If you jumper from RH to G the fan should come on, jumper from RH to W and the furnace should come on (you should have the fan on first). If these tests work the problem is in the thermostat, if not the problem is further down the line.
Pete C.
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Buss fuse. Woohoo!
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" snipped-for-privacy@crud.crap" wrote:

Fuses rarely fail on their own, they are usually an indicator of a problem elsewhere.
Pete C.
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We had some warm weather over the weekend, so I turned the thermostat off. I asked my son to turn it back on, and I noticed that the fan switch was set to "On" instead of "Auto."
Would the blower running continuously for a day or two be reason enough to blow the fuse?
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" snipped-for-privacy@crud.crap" wrote:

Even without knowing which fuse it was, it's highly unlikely. Any fuse in the system should be sized for continuous operation.
Pete C.
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wrote:

Are we talking about a fuse in the main fuse box or somewhere else?
A fuse that is a glass cylinder 3/4" long and less than 1/4" in diameter. Or one that screws into a socket almost as big as a lightbulb socket?
What size failed? How many amps? What size have you been using since you moved in. Replace the fuse with a 15 amp fuse, unless you know for sure it was supposed to use bigger and had used bigger, and see what happens.
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mm wrote:

I'm assuming from the reference to circuit breakers earlier and then this reference to a "buss fuse" that he means a small glass 3AG or 5x20 fuse in the thermostat itself possibly located when removing the thermostat from the sub base to check for control power. It would help if he posted more complete details though.
Pete C.
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The fuse is a 25A "Edison base" fuse in the switchbox on the furnace itself.
There's no reference to the fuse anywhere in the manual.
I've lived here for 7 years, so the fuse is at least that old. The house is 12 years old.
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" snipped-for-privacy@crud.crap" wrote:

Sounds like the builder recycled an old fusible disconnect since they haven't used edison base fuses for quite some time. New ones use cartridge fuses, or they use small circuit breaker panels.
At any rate a problem might still exist since fuses usually blow as a result of a problem, not old age. 25A is also a pretty large fuse and an oddball size as well so it may not be the correct one, 20A or 30A would be expected.
You haven't indicated what type of furnace this is yet (oil, gas, etc.) but most would be expected to be on a 20A circuit since burners and blowers don't take that much power. Possibly the blower bearing are going or it's clogged up and is drawing more power than normal?
Possibly a pre-existing condition where someone overfused to 25A to mask the problem? What size breaker is feeding the furnace from the main panel? What gauge wire is feeding it if you can see? 12ga would be good for 20A, 10ga for 30A.
Pete C.
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FWIW, the function of a thermocouple is to keep the safety portion of the gas valve open on systems with standing pilots. Does your unit have a pilot? There were some units that were available with standing pilots 12 years ago, but they were already getting pretty rare even then. If you do not have a pilot, I kinda doubt they replaced the thermocouple. Larry
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wrote:

So did you replace it, like I said to? What happened?

This is the wire going TO the furnace, right? I recently learned in a thread here that the wire IN the appliance might have better insulation and maybe doesnt' have to be as thick a gauge as the wire from the main box to the appliance.
25 is unusual, and I too would be very reluctant to go to 30. But if he has been using 25 for 7 years, I'd find myself some 25s. If it normally uses 22 and once used 27 and blew the fuse, he'll never find why it once used 27. If it blows the new one, then he may have / probably has a findable problem.
Of course I agree that a blown fuse represents a bigger problem at least half the time. Similar to this, a couple days ago, my hot water limit switch tripped, and I reset it by turning off the breaker, taking off the cover, pushing the red button, and turning the breaker on again. It tripped again not long afterwards (don't know when.)
OTOH, in the trash I found a Black and Decker air pump (they call it), and on the cigarette lighter plug, it said "No user serviceable parts inside", but they lied. Inside was a glass fuse, and after I drilled out the rivet, replaced the fuse, and somehow put the plug back together, the pump has worked fine for a long time. I wonder why it blew.

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wrote:

Caveat first: I'm not a tech. I don't keep up with the latest developments.
That said, the themostats that I have used essentially connect the two wires together that come from the boiler in order to turn the boiler on (i.e. "call for heat"). If his happened in my house I would disconnect the thermostat and connect those two wires to each other directly. If the boiler came on properly then I would suspect that the thermostat was bad or improperly set. If the boiler didn't come on I would know the thermostat was NOT the problem. I might then check some of the obvious things like the water feed before I called a serviceman. I don't fix gas appliances.
Greg Guarino
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