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Why Won’t My Furnace Turn Off?



In the midst of a prolonged arctic blast, the problem of a furnace that will not turn off is certainly preferable to the specter of a furnace that will not turn on. However, a furnace that doesn’t stop-and-start, or cycle, is problematic. While it’s certainly plausible that your furnace is perpetually running because it’s simply not able to supply the heat demanded by the thermostat, continuous operation of a furnace over a protracted period causes undue strain on the machinery, and it causes undue strain on your wallet. Being able to troubleshoot and correct some of the common malfunction causes is not brain surgery, although some fixes will require a professional’s touch. Here’s a rundown of the usual suspects that keep a furnace running constantly:

First, make sure that your thermostat is set to “Auto” rather than “On.” If in the “On” position, the blower will run non-stop even if heat isn’t being expelled from the vents. The “Auto” setting ensures that the furnace is cycling, and when it reaches the desired temperature the machine will settle down until the thermostat “tells” it to go back to work. In addition, make sure the thermostat is set to “Heat” rather than “Cool” - that may sound obvious, but this flub actually occurs more often than people want to admit.

If the thermostat is set to “Auto,” and furnace operation will still not cease, set the thermostat to a temperature that is lower than the present room temperature. Wait about five minutes for the blower fan to stop as the furnace may have been in the middle of a heating cycle. If the furnace fan came to a halt it’s more than likely that you’ve set the desired temperature too high, inciting your furnace to operate unceasingly as it strives to reach a temperature beyond its grasp.

If the problem still persists you’re going to have to give the unit a visual once-over and determine whether the Fan Limit Switch is the culprit. This Fan Limit Switch dictates when the blower turns on and off by using a probe to monitor the temperature and regulate blower operation. If the Fan Limit Switch has been set to “manual override,” which is another way of saying “nonstop fan operation mode,” the blower will not stop. You’ll be able to decipher if it’s in “manual override” because the button will be pushed in. By pulling out the button you’re resetting the fan to “Auto.” This ensures the fan automatically engages when the furnace is heating, and also causes it to stop once the request for heat halts. If the Fan Limit Switch was already on “Auto” and the blower kept running there’s a chance the switch is faulty and needs to be replaced or re-wired.

Should the furnace keep running like the Energizer Bunny there are other possibilities to research. Your home may not be adequately “winterized” - doors and windows may be severely cracked or unsealed, letting hot air escape easily. The insulation could be inferior, or non-existent. Ducts may be leaking like a sieve. And, if the furnace filter is dirty and clogged that will drastically inhibit hot air flow, causing the furnace to work harder, and work longer. Your best option is to seek the counsel of a reputable professional HVAC tech who will diagnose and resolve your furnace problems before they worsen.

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