Lenox G40DF Furnace


My home inspector has reffered me to this site.

I recently purchased a home and have noticed potential furnace problems after the move in. When the temperature is set at for 70 degress, the furnace seems to run for about 8 minutes, then shuts down for 2 minutes, and then restarts for 8 minutes. If I decrease the temperature the amount of time the furnace is off increases and the time the furnace is on decreases for during the cycle. It seems to be running fine, but the shut down and start up does not appear to be normal. I can imagine it being major since the furnace is about 4 years old. It is a Lenox G40DF.

Any information you can provide would be much appreciated, especially during this cold time of year!


Make sure you check your air filter first.

It sounds like the furnace may be short cycling.

There are several possible causes for the short cycling you describe. The unit could be oversized, there could be a problem with the controller or the high limit switch may be sensing high heat and shutting the unit down.
As mentioned, check your filter first. If it is clean, then you should call for service. Also, make sure your returns are not obstructed.


If you have a thermostat with a heat anticipator it may not be set properly or damaged.
This would be the first place to start as the cycling changes when you change the thermostat setting.
There will not be changes in cycle time if it were one of the other many possibilities.
Gas furnaces are quite complex and it is unlikely that you will be able to make the evaluation yourself.

Thank you for all of your responses, it has been very helpful!

I’ve checked the filters and it looks like the previous owner did not change them for years (a single women used to live here)…

After changing the filters, I’ve noticed an improvement in the cycling. Also, I found the heat anticipator located under the thermostat. The thermostat is not digital and is an older model from “White-Rodgers”. I’ve adjusted the heat anticipator to the ‘longer’ setting which has increased the cycles. So far, it seems to be ok. If there are any other problems, I may try a new thermostat.

Albert: if what you stated is true about the filter not being changed you can rest assured that the furnace A-Coil is also matted over and you are severely restricting air flow which can also cause the furnace to cycle. I would caution you about adjusting the heat anticipator as this anticipator has a graduated scale setting that is suppose to be set to match the amp draw of the furnace control gas valve.