# Central Heater has gone mildly crazy **URL:** \ **Category:** Factual Questions\ **Created:** [November 23, 2011, 4:38pm UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881 "2011-11-23T16:38:36Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![amaguri](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@amaguri](https://boards.straightdope.com/u/amaguri)\ **Post date:** [November 23, 2011, 4:38pm UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/1 "2011-11-23T16:38:36Z")
It’ll run a normal cycle, then shut off. Then the fan turns on again for one second, then shuts off. Then (sometimes) the fan goes on again for a second and shuts off. It seems to do this after every cycle now, sometimes one “bonus” second, sometimes two, but never more than two… Is this something to be concerned about? ---
**Author:** ![Dereknocue67](https://avatars.discourse-cdn.com/v4/letter/d/6de8d8/32.png) [@Dereknocue67](https://boards.straightdope.com/u/Dereknocue67)\ **Post date:** [November 24, 2011, 8:19am UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/2 "2011-11-24T08:19:05Z")
My understanding of a forced air heating system is that the fan is controlled by a thermostat contained within the furnace that monitors the upper and lower limits of heat. If memory serves, the upper limit controls when the fan turns on and lower limit controls fan shut off. When your furnace first fires, there’s a delay between that initial firing and when the internal furnace temperature rises enough to turn on and fan and circulate warm air. When the furnace cycles off, the fan continues to run until the lower limit air temp is reached and then the fan turns off. If you look inside your furnace, you may see a “fan limit control” switch allowing you to change these settings slightly. It may be that this switch is faulty or it could be that once the fan cycles off, the internal furnace thermostat senses some residual heat from the furnace and that tricks it into thinking the furnace has fired up again so it tells the fan switch to turn on. The fan runs for a minute but quickly realizes the circulating air temp is too low and shuts off. I think it’s something to repair/replace. If this fan limit switch fails completely, there is a risk of fire caused by furnace overheating. Even though the unit has a built-in fail safe that should shut the unit down in the event of overheating, that fail safe is your last line of defense. ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [November 24, 2011, 8:49am UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/3 "2011-11-24T08:49:05Z")
I had a similar issue with my central air a few months after it was installed. They replaced a faulty circuit board under warranty. Mine was cycling on/off every three or four minutes. I knew the house hadn’t heated back up that quickly. So I called the place that installed it. I’m pretty sure it would have don the same thing in winter with heat. ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [November 24, 2011, 2:56pm UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/4 "2011-11-24T14:56:25Z")
Do you know how old your furnace is? (for example, does it have a standing pilot light?) ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [November 24, 2011, 3:00pm UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/5 "2011-11-24T15:00:01Z")
When you look inside the furnace does it have a component that looks like [this?](http://www.rockford-industrial.com/Packing-and-Shipping-Supplys-/Rockford-/New-honeywell-combination-fan-limit-control-L4064A.HTML) ---
**Author:** ![thelabdude](https://avatars.discourse-cdn.com/v4/letter/t/dc4da7/32.png) [@thelabdude](https://boards.straightdope.com/u/thelabdude)\ **Post date:** [November 24, 2011, 6:31pm UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/6 "2011-11-24T18:31:24Z")
Newer furnaces the normal control for the fan is timing controlled by the circuit board. After the furnace senses ignition, the fan comes on a certain amount of time later, and continues a certain period after the controls tell the gas valve to close. In case something goes wrong, there are still thermal sensors to turn the fan back on. They are much smaller than the older ones in raindog’s link. I would clean any I find and if that doesn’t help, replace it. ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [November 24, 2011, 7:08pm UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/7 "2011-11-24T19:08:26Z")
> [@thelabdude](#): > > Newer furnaces the normal control for the fan is timing controlled by the circuit board. After the furnace senses ignition, the fan comes on a certain amount of time later, and continues a certain period after the controls tell the gas valve to close. In case something goes wrong, there are still thermal sensors to turn the fan back on. They are much smaller than the older ones in raindog’s link. I would clean any I find and if that doesn’t help, replace it. Generally those devices are in series with the gas valve, and would disable the gas valve, rather than turning on the fan. While some manufacturers use a high limit to turn on the fan (as well as others to disable the gas valve) they are comparatively rare[er]. In any event, those limit safeties aren’t prone to carbon build up, (like flame sensors) and so they don’t need to be cleaned. What the OP is describing is a fairly common thing, and while we haven’t been given enough information, it’s likely caused by what **Dereknocue67** describes. ---
**Author:** ![thelabdude](https://avatars.discourse-cdn.com/v4/letter/t/dc4da7/32.png) [@thelabdude](https://boards.straightdope.com/u/thelabdude)\ **Post date:** [November 25, 2011, 6:00am UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/8 "2011-11-25T06:00:07Z")
Sorry raindog, the fan will come on when over heated. ---
**Author:** ![VOW](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/vow/32/3043_2.png) [@VOW](https://boards.straightdope.com/u/VOW)\ **Post date:** [November 25, 2011, 6:09am UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/9 "2011-11-25T06:09:18Z")
The FIRST thing I’d do is change the filter and vacuum the intake vent! Then I’d change the batteries in the thermostat. ~VOW ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [November 25, 2011, 6:27am UTC](https://boards.straightdope.com/t/central-heater-has-gone-mildly-crazy/603881/10 "2011-11-25T06:27:46Z")
> [@thelabdude](#): > > Sorry raindog, the fan will come on when over heated. On some they will, but most will not. And, if the furnace uses a timer on the circuit board (versus a temperature sensitive switch) it will not produce what the OP describes.