How does the furnace work?

My furnace works like this: Burners come on, then blower comes on. They stay on until it makes my thermostat setting, say 74.

On some inspections, the furnace does this: the burners come on, then the blower, but after a few minutes the burners go off and the blower stays running. It continues this way until it reaches the set temp. This is on old furnaces.

My question is: Is this a case of “short cycling”, or on old furnaces is this the way they were designed to run?

An indication of short cycling is if the burner comes on, blower comes on, the burner goes off, the blower stays on, burner comes back on again, burner goes back off again, blower keeps running. It may or may not reach set point.

The older version furnace, has a bimetal fan/limit control that heats up and turns on the blower. When the burner shuts off when the thermostat reaches set point, the blower remains on until the heat exchanger where the fans/limit control is installed, cools down. This has a “purge” affect, which is a desirable function and is incorporated even in electronically controlled blower controllers.

Mark, don’t let the ‘cool down’ cycle fool you. Gas heaters will ignite the burners, a limit switch will then turn the fan on once the heat exchanger has come up to temperature ( this keeps from blowing cold air throughout the home), the heater will run until the thermostat set temperature is reached, the burners will turn off, but the blower will continue to run until the heat exchanger has cooled to a lower set value then a limit switch will turn the blower off. Your description could go either way, the clue would be to fully know if the burners turn off once the t-stat is satisfied or if the burner is truly turning off prior to that point. My guess is that the heater is working properly.


There are various scenerio’s for what you are describing.
David gave a good explanation on (2) coun

I think Equipment age, maintenance, visible condition, location, and type (ie: Gas, Electric, Ht.Pump, H.W./Steam, etc.) all play a part in the final analysis by the Pro.

I think you list your observations and recommend further analysis by a Pro.

If it has an older tstat, the anticapater may not be set causing the short cycle.

A couple of thoughts to add.

  1. This may or not be a real problem but some troubleshooting is in order.

  2. The plenunm control may be misadjusted and set too low.

  3. The air flow could be restricted by a clogged filter, clogged A-coil or the registers and any damperes not open alowing the plenumn temp to rise to the limit of the control.

  4. The blower may not be at the cottect speed for the application by malfuction or blwoer speed setting.

  5. An oversized furnace may also short cycle.

Older and lower efficiency systems will run the blower for a while after the system satisfies the T-stat to purge as previously indicated.

Do you turn the T-stat all the way up, so that it should run for quite a while before the T-stat is satisfied, or do you just turn it up enough to fire the heating system?

I would turn the T-stat all the way up, so the system should run for quite a while before reaching the set point (and bake the people inside so they remember the heat was working “at the time of the inspection”) … :wink:


All the way up…

All the above could be the cause

Number one in my book is a dirty air filter

Lack of air through the heat exchanger will cause the hi temp safety to shut down the gas


So it is short cycling ? The older furnaces were not set up to run that way correct?


We just can’t tell from here.:wink:

Please review the following links for tips on this issue.

Hope this helps, good luck and be careful.

Of course you could always refer to a quaified HVAC techincian for evaluation and repair as needed.:wink:

Mark, if you turn the stat up all the way and the flame shuts off before it reaches its designated temp. I will always consider that short cycling and recommend servicing. I ususally check the filter first, try removing it and to see if that corrects the problem or not. Some of the high end filters will even restrict the air flow enough to cause this, even if they are clean.

Thank you Darrell, very concise and to the point.