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Laporte, IN on Nov 06, 2011

I noticed that my furnance was running constantly.

I even shut it off and it continues to run. It sounds like it is just the fan running. What could be the problem? The temp in the house seems fine, and it doesn't go over the programed setting but the furnance keeps running.

5 Comments

KMS Woodworks Nederland, CO

Have you been "inside" it lately? I have fixed a few of these for some clients, who had pulled the cover as part of a filter install...when inside they accidentally bumped the fan override button on the furnaces temperature sensor ( the sensor is a 3" x 4" box shaped controller with some wires on it...it is located above the gas burners on the side of the main plenum) On hers it was a yellow plastic "push pull" rod about 1/4" in diameter.....simply pull that back out. the fan should shut off.

on Nov 06, 2011 · Like 0
Nichter's Home Services C... Islesboro, ME

Most furnaces have a "fan only" button that will keep the fan running to recirculate air. It probably got set/. This is usually on or very near the furnace itself

on Nov 06, 2011 · Like 0
Woodbridge Environmental ... Colonia, NJ

Nichter's could be correct on this. It sounds as though your fan switch is on the on position.
Still another reason why this happens. And its dependent on the type of control that operates the fan is that the fan switch that turns on the blower once the furnace comes up to temp is sticking and not turning off.
Older furnaces have a control that is operated by heat. It has a metal spring device that is ...»
extended into the furnace heating area. When it gets warm it twists the switch causing it to turn on the blower fan. When it cools down after the heat is turned off to the unit it continues to blow air over the heat exchanger until a pre-set temp is reached. Sometimes the older spring that twists the switch from on to off gets stuck and it does not have enough power to turn off the fan. When this happens this device needs to be replaced or it is an indication that its aging and will soon need to be changed.
You will find this switch in the front of the furnace above the gas valve. Sometimes it has a silver cover over it. its about 2 inches wide and four inches high. Some of these have a pull lever that overrides the control and keeps the fan on all the time.
If you remove this cover, by squeezing it on the sides and pulling it off, Do so only when the power is turned off to the furnace. Then Look at the dial that faces you. There are little sliding devices on this some with screws sometimes without. Much like a timer it has three settings. One for limit, Meaning it turns off the gas while allowing the fan to run. This is to prevent the furnace from overheating for various reason. The second is the fan on setting, which is about 120 degrees or so. Then a fan off setting which is around 95 or so. If its set to low the fan can come on during the summer when the house warms up so this setting needs to be left where its at.
In any case, carefully twist this about 1/8 of a turn. No more. Then let go and let it spring back. Then turn on furnace to see if its still running. This should have stopped the fan if indeed that was the issue.
Some newer furnaces do not have these controls. If you do not see this little silver box or a box with a round dial in the center, you have a different type of fan control and you need to check with a service person for the repair.
on Nov 06, 2011 · Like 1
LandlightS Roswell, GA

you most likely need a replacement of the fan relay. Best to call your HVAC service company.

on Nov 06, 2011 · Like 0
KMS Woodworks Nederland, CO

The light colored button on the lower left of woodbridge's picture is the override button I refereed to in my first post...check that an let us know what you found

on Nov 07, 2011 · Like 0