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I happened to be down in the basement a short while ago and heard the furnace turn on and off after just a few seconds. About a minute later it came on and this time it stayed on for about 15 seconds and then it shut off. This has happened before due to a dirty flame sensor, so I assumed it is probably due for a cleaning again. But, I went upstairs and checked the thermostat and it's set to heat at 73 degrees, and it's showing the air temp is 74 degrees! I'm pretty sure it normally has to drop down to 71 before the heat comes on, yet something triggered it somehow. Could this be a faulty thermostat, and if so could this explain the start and quick stop too?

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    It could be -- I'd wonder if something is intermittently shorting between the R and W leads to the 'stat. – ThreePhaseEel Feb 24 '17 at 4:59
  • What's the make and model of the furnace? When you say "turn on", what do you mean? Did the blower turn on, the inducer, the burners, etc.? – Tester101 Feb 24 '17 at 12:48
  • @Tester101 -I think the furnace make is Lenox, I'll have to check the model later. I leave the blower on all the time to circulate air. The burners came on briefly- I could see the flame. – TTT Feb 24 '17 at 14:34
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You probably have a boiler that heats water (or steam) and then pumps it through the house to radiators or baseboards. The boiler needs to keep the water heated to a certain temperature so that when the thermostat calls for heat, the boiler can pump the water (at the design temperature) through the house.

So, the boiler will "kick on" from time to time to heat the water.

Also, some boilers can heat your potable hot water too (through a heat exchanger). It could be "kicking on" to heat your hot water.

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    If that were the case, wouldn't it stay on longer than a few seconds? – Tester101 Feb 24 '17 at 12:46
  • It is central heating and there's a separate water heater... – TTT Feb 24 '17 at 14:37
  • They did not say it is a boiler they said furnace, and a boiler does not need to keep the water heated to a certain temperature; that is not how they work. – d.george Feb 24 '17 at 16:02
  • That's how mine works! Re: "it's a furnace not a boiler" that's what we call it. To-ma-to, To-mah-to. My father-in-law installed and repaired "furnaces" for 40 years. He showed me how ours worked when we bought this house. AND yes, it heats water and sometimes it takes a few minutes depending on other hot water uses going on in the house...If you want to see furnaces with hot water system, Google Weil-McLain. – Lee Sam Feb 25 '17 at 4:27
  • Hmmm...Are you thinking of an "air handler"? Some call an air handler (without the heat-pump side) a furnace. I think of them as electric furnaces. I described an oil or gas fired boiler/furnace. – Lee Sam Feb 25 '17 at 9:08

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