# HVAC Blower issue **URL:** \ **Category:** Factual Questions\ **Created:** [December 10, 2011, 12:17am UTC](https://boards.straightdope.com/t/hvac-blower-issue/605682 "2011-12-10T00:17:11Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![cmart](https://avatars.discourse-cdn.com/v4/letter/c/977dab/32.png) [@cmart](https://boards.straightdope.com/u/cmart)\ **Post date:** [December 10, 2011, 12:17am UTC](https://boards.straightdope.com/t/hvac-blower-issue/605682/1 "2011-12-10T00:17:11Z")
I have a Comfort Master about 13 y.o. and I realized that the indoor fan unit was continuously on after I turned off the thermostat due to good weather. When I turn the heat or a/c on it blows out the correct air through out the house but when it turns off after reaching its specified temperature or whether I turn the thermostat off, the fan keeps going non-stop unless I pull the fuse jumper in the attic. Please advise. Thank you cmart ---
**Author:** ![Skywatcher](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/skywatcher/32/254_2.png) [@Skywatcher](https://boards.straightdope.com/u/Skywatcher)\ **Post date:** [December 10, 2011, 12:20am UTC](https://boards.straightdope.com/t/hvac-blower-issue/605682/2 "2011-12-10T00:20:27Z")
Reported for forum change. ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [December 10, 2011, 12:34am UTC](https://boards.straightdope.com/t/hvac-blower-issue/605682/3 "2011-12-10T00:34:55Z")
Did you recently put in a new thermostat? Is the furnace natural gas? ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [December 10, 2011, 12:45am UTC](https://boards.straightdope.com/t/hvac-blower-issue/605682/4 "2011-12-10T00:45:10Z")
To me…it sounds like a bad limit switch. A quick and dirty way to check it is to wait until well after the furnace has finished firing (say, 5 minutes after the burners have turned off) and find the part that looks like [this](http://www.inspectapedia.com/heat/Fan_Limit_Switch576-DFs.jpg) and give it a good whack\* to see if the blower shuts off. If it does, that’s the culprit. I can’t say that’ll always diagnose a bad limit switch, but I’ve had it work it the past. \*By ‘good whack’ I mean a firm tap on the side or front with a screwdriver handle or something else heavy enough to jostle the contacts but not something that’s going to break it. Also, don’t hit the dial. ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [December 10, 2011, 2:00am UTC](https://boards.straightdope.com/t/hvac-blower-issue/605682/5 "2011-12-10T02:00:11Z")
More likely, it will look like [this](http://www.google.com/products/catalog?q=5d51-90&rls=com.microsoft:en-us&oe=UTF-8&startIndex=&startPage=1&um=1&ie=UTF-8&tbm=shop&cid=15121002190499864811&sa=X&ei=zbviTqTIENGrsALljKGwBg&ved=0CFgQ8wIwAg#), unless someone lost the cover. It’ll be about a 3x6" box, and a foot or so over the gas burners. You’ll probably need to remove an access panel to see it. I’ve had to replace one in the past, for the very same reason - the blower would not shut off. ---
**Author:** ![dzero](https://avatars.discourse-cdn.com/v4/letter/d/5f9b8f/32.png) [@dzero](https://boards.straightdope.com/u/dzero)\ **Post date:** [December 10, 2011, 4:51am UTC](https://boards.straightdope.com/t/hvac-blower-issue/605682/6 "2011-12-10T04:51:08Z")
I had this happen to me recently when the igniter on my oil burner went bad. I guess the thermostat had sent a signal to run and even with the thermostat (electronic) set to “off”, the furnace didn’t get or ‘understand’ the message. So I pulled it off the wall. It’s made to mount to a bracket with pins so it wasn’t out of anger. 🙂 That seemed to work. when I put it back, the fan was off. Not sure this will help, but sometimes it can be something simple but non-obvious.