How do I know if furnace thermocouple is bad?

Discussion in 'Home Repair' started by Mitch@crud.crap, Oct 31, 2006.

  1. Guest

    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.
     
    , Oct 31, 2006
    #1
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  2. Guest

    BTW, the furnace is an Amana Air Command 95. I have a digital
    thermostat and replaced the batteries. No effect. No breakers are
    thrown.
     
    , Oct 31, 2006
    #2
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  3. Pete C. Guest

    "" wrote:
    >
    > 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.


    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.
     
    Pete C., Oct 31, 2006
    #3
  4. Guest


    >Click from where, the furnace or the thermostat?


    I don't get the click from the thermostat.
     
    , Oct 31, 2006
    #4
  5. Greg Guarino Guest

    On Tue, 31 Oct 2006 16:37:27 GMT, "" <>
    wrote:

    >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.

    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
     
    Greg Guarino, Oct 31, 2006
    #5
  6. Pete C. Guest

    "" wrote:
    >
    > >Click from where, the furnace or the thermostat?

    >
    > I don't get the click from the thermostat.


    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.
     
    Pete C., Oct 31, 2006
    #6
  7. Guest

    Buss fuse. Woohoo!
     
    , Oct 31, 2006
    #7
  8. Pete C. Guest

    "" wrote:
    >
    > Buss fuse. Woohoo!


    Fuses rarely fail on their own, they are usually an indicator of a
    problem elsewhere.

    Pete C.
     
    Pete C., Oct 31, 2006
    #8
  9. Guest


    >Fuses rarely fail on their own, they are usually an indicator of a
    >problem elsewhere.


    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?
     
    , Oct 31, 2006
    #9
  10. Pete C. Guest

    "" wrote:
    >
    > >Fuses rarely fail on their own, they are usually an indicator of a
    > >problem elsewhere.

    >
    > 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?


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

    Pete C.
     
    Pete C., Oct 31, 2006
    #10
  11. mm Guest

    On Tue, 31 Oct 2006 17:02:27 GMT, "" <>
    wrote:

    >BTW, the furnace is an Amana Air Command 95. I have a digital
    >thermostat and replaced the batteries. No effect. No breakers are
    >thrown.


    Is it gas, oil, or something else?
     
    mm, Nov 1, 2006
    #11
  12. mm Guest

    On Tue, 31 Oct 2006 20:10:28 GMT, "" <>
    wrote:

    >
    >>Fuses rarely fail on their own, they are usually an indicator of a
    >>problem elsewhere.

    >
    >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?


    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.
     
    mm, Nov 1, 2006
    #12
  13. Pete C. Guest

    mm wrote:
    >
    > On Tue, 31 Oct 2006 20:10:28 GMT, "" <>
    > wrote:
    >
    > >
    > >>Fuses rarely fail on their own, they are usually an indicator of a
    > >>problem elsewhere.

    > >
    > >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?

    >
    > 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.


    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.
     
    Pete C., Nov 1, 2006
    #13
  14. Guest

    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.
     
    , Nov 1, 2006
    #14
  15. Pete C. Guest

    "" wrote:
    >
    > 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.


    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.
     
    Pete C., Nov 1, 2006
    #15
  16. lp13-30 Guest

    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
     
    lp13-30, Nov 2, 2006
    #16
  17. mm Guest

    On Wed, 01 Nov 2006 14:52:57 GMT, "Pete C." <>
    wrote:

    >"" wrote:
    >>
    >> 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.


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

    >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.


    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.
    >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.
     
    mm, Nov 2, 2006
    #17
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