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I have a moncrief furnace and replaced the thermostat but the furnace will not cut off once it reaches the desired temperature. What could be wrong?

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    so it just keeps heating and the temp keeps increasing? – Steven Nov 26 '12 at 21:07
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    How many degrees over the thermostat setpoint does it get up to? – littleturtle Nov 26 '12 at 21:19
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    In addition to the questions above: Did the problem start happening after replacing the thermostat, or did you replace the thermostat in an attempt to fix? What kind of thermostat was there before and what did you replace it with? Were the terminals/wires labelled and are you sure you reconnected them properly? (This may not be relevant, but:) Does the problem go away if you put the old thermostat back in? – gregmac Nov 26 '12 at 21:40
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    I'm downvoting until more detail is added to this question. In the current state, it qualifies as "not a real question" since we don't have enough information to provide a proper answer. – BMitch Nov 27 '12 at 12:55
  • When I removed and replaced the Honeywell thermostat from my boiler to fix the hot water coil I had the same problem. I found that when I pushed the copper probe into the crevice in the boiler it was not in all the way, and did not register the water temp. Once I made sure it was seated in the whole way it shut off at the correct threshold. – Bob A. Nov 28 '12 at 18:29
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Home thermostats typically operate with a "dead band", a range above and below the set point that the furnace operates over. For example, if the setpoint is 68°F the furnace might turn on at 62°F and off at 71°F.

  • Hysteresis is a useful term to look up when it comes to thermostats acting this way. There is usually an adjustment somewhere. – Wayfaring Stranger Jan 6 '18 at 5:19
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Bottom line the thermostat controls the off and on of your heat system thermostat is nothing but a switch that shuts your furnace on or off when the desired heat level is reached, you can actually run a heat system manually with a simple toggle switch and get the same results as a thermostat, so to simplify your problem first make sure your power supply to the furnace is correct if so then remove the cover from the thermostat and place a jumper wire from the red terminal to the white terminal or green is your fan yellow is your air condition if nothing starts when you touch all three terminals then you need to check the transformer voltage it should read 24 to 26 volts, if this checks out and still no function then you have other problems and it may require a trained tech.to trouble shoot as it could be a combination of many things, there is no simple one solution to any heat system every problem must be considered, in plain English no one can fix your furnace with one simple answer can only offer a few quick tests that you can preform yourself before calling in the experts, (NOTE) no person should attempt to probe around in anything electrical unless your experienced in heating & air systems you could get electrocuted in short order burn your house down or severely injured, so my advice call in a tech that knows what he's doing

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