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I have a Rheem Imperial 90 Plus furnance.When the thermostat calls for a rise of around 5 degrees in temperature the 135 degree limit switch on the inducer blower housing kicks out. Everything is fine if the thermostat just calls for around a 2 degrees increase. I checked the limit switch and it kicks out around 140 degrees. I also notice there seems to be lots of moisture coming out of the vent pipe. The inducer blower motor is very hot after it kicks out. I don't know whether the inducer motor is going bad or if something else is going bad. The blower that circulates air through the house seems to be working fine. Any suggestions would be greatly appreciated. Thank you for your help. Donnie
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Although in commercial, and some very high end residential furnaces, there are "High Fire" and "Low Fire" burners, for the most part the burners used in residential heating systems are a constant fire, or more precisely a constant flame height irrespective of whether the thermostat is calling for one degree or a hundred.
Once the thermostat calls for heat the system begins heating and circulting the hot air until it has had sufficient time to raise the air temp in the demand area enough to satisfy the thermostat setting.
In turn, the induced draft fan is normally a single speed motor/fan that simply induces a sufficient supply of air to maintain complete combustion, and given that the combustion takes place in a sealed cumbustion chamber, the force of induced air coming in then forces the exhaust gases out through the exhaust flue.
When all is well a fan motor should be able to perform its function on a continuous duty cycle without any significant increase in temperature.
If a fan motor is overheating, that is indicating there is some adverse condition effecting its operation, which is normally one of three problems.
1. A restriction in the air flow.
2. Bad bearings in the motor
3. Improper voltage to the motor windings. (may result from a bad capacitor)
While the accumulation of excess moisture on the discharge could be a result of colder outside ambient air temps and higher atmospheric humidity, it also can indicate a noticable decrease in air flow.
It sounds as if there is some condition causing your motor to overheat. The increase in temperature is gradual at this time, therefore when the system is only running long enough to satisfy a couple degrees there is not sufficient heat buildup to cause the motor to begin overheating and slowing down. When the thermostate is calling for a higher demand the system in turn runs longer and the heat buildup in the motor is now sufficient to slow it down and cause the limit switch to trip.
I would begin by examining both the air intake and exhaust to insure there is not a birds nest, insect nest or debris blocking the air flow.
Next, check the motor to see if it has "Oil" ports on the end bells over the bearings. They will appear like little plastic mushroom shaped caps on top of a tube to the bearing near where the motor shaft enters the motor housing. If you have oil ports, remove the caps and squirt a few drops of light machine oil into the tubes. (3 in one oil will work fine)
To get a true test of the motor performance you would need to first check the data plate on the motor to determine its proper amp draw, then use a snap on ampmeter to measure the actual amp draw of the motor. If the actual amp draw is more than 5-8% higher than the rated amperage it is indicating the motor is operating under severe stress, usually resulting from bad bearings. (If the motor has a capacitor the capacitor could also be the problem. Look in the illustrations forum for information on how to test capacitors.)
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Richard never ceases to amaze me with such knowledgeable replies in so many different categories.
I would just like to add that you might check any drains coming off of the furnace to make sure there isn't one plugged up.
The fact that the inducer will keep running for the duration of the shorter cycles but is effected by having to run longer should be a clue.
Is this a back draft switch that is tripping out?
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