Gas and Oil Home Heating Furnaces - short cycling furnace

when my furnace gets a call for heat from the thermostat, it goes through a normal cycle except the blower compartment hi temp limit is reached and shuts down the gas valve prior to the thermostat being satisfied. I have a furnace with a fan/limit switch (round dial type with three setpoints). The three setpoints appear to be 100, 150, and 210. 210 is the highest the device will go, and I believe that is the hi temp limit, but what do the other sepoints do? I already have the blower set to its highest speed to remove maximum heat from the burners. Should I put in a smaller gas valve? Is my 100,000 btu furnace too large for my 2200 sq ft house? I estimate an 80% efficiency rating.

100 is fan off and 150 is fan on. 210 is the limit and will shut down the gas valve if that temp. is reached. The settings are normal enough. You may have an air restriction et.al. air filters or clogged "A" coil causing the limit shut down. Also could have a bad blower motor or clogged blower wheel. Or, just a bad Fan/limit

It does sound like you have a large furnace, but that depends of course on where your house is. However, an oversized furnace will not go off on higfh limit if the ductwork and air flow matches the furnace. If the ducts are too small, say sized for a 60,000 BTUH furnace, and there are some additional contributing factors such as a dirty filter, then you will see the furnace cycle on high limit.

Are you sure it's going off on high limit? If the burner shuts off and the fan keeps on running and then the burner relights again without the fan turning off, this would indicate a high limit condition. But if the furnace goes through a complete cycle where the burner lights, the fan comes on, the burner goes out then the fan goes out, this might indicate a thermostat problem. The heat anticipator in the thermostat may be set incorrectly or the thermostat may need replacing. If you have an old mercury bulb thermostat, then the anticipator can be adjusted to match the amperage put out by the gas valve. Look on the gas valve. There should be a number like .4 or .5 amps for example marked on the valve. There is a "slider" style arm under the cover of the thermostat. Slide the pointer to match the number from your gas valve. Try this and see if it helps with the furnace cylcing. If the numbers already match up, then a new thermostat may be required. Hope this helps. Post back any more info that might help. See ya.

Thanks for the ideas! Additional Info: I have a digital setback thermostat, and I don't believe that it has heat anticipation that is adjustable. I also do not have central cooling yet, so no coil is restricting air flow. I just had my ducts cleaned by a reputable company which mentioned my short cycling problem, which is why I'm here. My home is a ranch, 1100 sq ft up, 1100 sq ft down located in colorado. The home was built in 1969, so insulation is average at best. It has one return in every main floor bedroom, one larger common return for the living room/kitchen area and no returns in the finished basement. Supply runs are installed in the basement. The blower motor is good, and does not shut off when the burners shut off during the heat cycle.

By the description of the fan control, it sounds like you have an older furnace. This means, unlike the newer furnaces, you don't have a lot of sensors and controls to break down. The problem sounds like the fan control. Try that first and see what happens.