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A common problem with a furnance is the furnance will attempt to ignite, several times, you will notice that the house is not heating properly. You will hear the furnance come on and you will assume the heat is on, and the house will continue to be cool. You wiil check the furnance and you will hear it start up and the blower will come on, however a few seconds later after you have walked away the blower will shut down and there is no heat on at this time. You will again go check the furnance and after the furnance start and go through the cycle the furnance will shut down you will notice that the furnance has indeed shut down. This is the common problem of the flame sensor, you will have to remove the flame sensor, get a small piece of fine grade sand paper, and remove all the black colored carbon from the flame sensor until is is shinning bright, replace the flame sensor and attempt to start the furnance. You will notice that apatment or house have reach the set temperture, that the furnance has went through its cycle and the furnance will ignite and stay on continueously until the house obtain the preset temperture..
check the fan setting on your thermostat. On means always on, auto/int means only comes on during heating/cooling. if the thermostat is set to auto / int and the fan keeps running? you may have a bad fan relay switch. It may be integral on the primary control board of your unit.
if you turn Tstat off and it still runs then there must be a wire touching another either in the Tstat on wall or in the furnace. i would suggest you look close at wires to R and W to make sure they are not touching.
Check the unit for a high temperature limit switch, usually a round bimetal disc or a square plate that
would insert into the unit. If the switch is bad, it opens up and the fan will run no matter what. Complete the circuit with jumper wire, if fan shuts off
replace switch
What type of heater are we talking about? if its the home furnance, it sounds like the ignitor is coming on and shutting back down, the heat comes on ignite and the blower comes on like normal, then after a few minutes it will shut down, you will have to remove the ignitor, clean it with a little piece of sand paper to remove the carbon build-up, replace the ignitor and turn the furnance back on, this should correct the problem.
It is rare that a limit switch goes bad. It is a safety control. If it is shutting off your furnace there is a problem that it is protecting you from.
If the limit switch is shutting the unit down your furnace is overheating. Check the filters. Check for any obstruction in the blower assembly. Check to see if you can raise the blower speed. Make sure all registers are open in the house. Your evaporator coil could be obstructed if you do not take good care of your filters.
Try to locate the red and white wires either behind your thermostat or on the furnace itself. Jump them out and see if the furnace stays on. If you do it at the furnace you will need to hold the door switch down. If it runs fine, you have found your problem (the thermostat) if it goes out again, the furnace will display a number of blinks on the circuit board. Look on the reverse side of the furnace access door and there should be an Error Code chart to get you started figuring where to look next.
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