Question about gas furnace

short cycling and

heat anticipator setting 

John Maurice; South Paris, Maine wrote:
on Thursday, January 4, 2001 at 09:20:50
> Brand: Miller
> type: hot air furnace
> fuel: kerosene
> location: living quarters
> Problem: temperature
> My furnace is running through it's cycle and heating the house as it should, however, the burner turns off and on about 3 times during it's normal cycle. We had the furnace checked recently, and I think the serviceman may have accidentally bumped the temp controls. We can't pay to have him come out here again. I see three controls labeled off-on-off, with
(I assume) a temperature setting label. The first off is set to 100, the on is set to 160, and the off is set to 200. How should I change these to make the burner run once during the cycle? Thank you very much for your time and this service!
> -John

Date: Thus, 04 Jan 2001 17:38:38 -0500

I am not sure why it would go off on high limit unless you have a clogged filter. 200 degrees is plenty high for the high limit. If it does cycle on high limit it should be no cause for concern.
Sometimes the gas companies change the btu content of the gas too. Check your thermostat under the cover is a little electric heater called the anticipator. its job is to cause the thermostat to turn off the burner before the temperature can over shoot. You will find them on forced air systems that use gas, oil or electric. Some hot water (hydronic) systems don't have them and most heat pump systems do not use them except for the backup heat function. Usually it is set to the current draw on the gas valve or oil primary. If it has been moved to a lower number that would cause the burner to short cycle. The way to know is to remove the cover on the thermostat and watch the mercury bulb to see if it is falling to the off position. If it is not check to see if the limit (shown below) is opening.

Image of sperry Digisnap ammeter with one turn measuring the current draw of a heat pump contactor.
    The correct way to set the anticipator is to measure the current draw of the gas valve/oil burner plus the heat assist device (show below) if it has one. If you have a clamp on ammeter wrap 10 (ten) turns of wire around the pick up and read the current and divide by 10. Ex: 6 amps =600MA or .6 amps. Set the scale on your thermostat to this number but don't be afraid to change it if you have too short or long a cycle.
WARNING: If  you short circuit the gas valve terminals or oil burner W terminal you will burn out the heat anticipator. You will have to replace the thermostat! This is how a service person can sell you a new thermostat too.
Scott 240-460-3820
Image of Fan Limit control, This one has an assit heater as evident by the coonections in the center
This fan/limit has an "assist heater"  the wires in the center. Which adds load to the heat anticipator and must be compensated for or short cycling of the valve or burner will result. If you do not see the wires in the center then you have a conventional control. The two wires you see in the center are connected across the gas valve in this system.

Related pages:
    Other heat anticipator page.
    Thermostat page.
    How fan and blower controls work.
    Gas valve system page.
    Oil burner page.
    Control wire colors and terminal designations.   

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