Old furnace cutting off and on during operation

I inspected a very old, evidently c.1980 American Standard gas, forced air furnace that functioned, and did not blow any measurable CO after having been run a long time, but kept cutting off and back on while it was running.

Is there likely to be some sort of safety mechanism to cut the flame periodically when the inside temperature reaches a certain point to protect the heat exchanger and perhaps improve efficiency or is there a problem with the ignition control?

I recommend a servicing checkup when reporting units that are past their typical life expectancy anyway.

There is a such a mechanism - A high-limit switch.

Can’t tell you if that was the issue with this furnace.

It could just be the limit switch setting in normal operation. It seemed to be cutting off and on very frequently, though. Thanks.

Its short cycling, just recommend evaluation by HVAC tech.

The heating unit was short cycling (kept going on and off but the thermostat was not satisfied). Also due to the system design, there was limited visibility and restricted access to the heat chamber (we were unable to fully view or examine the entire heat exchanger and burn chamber … verifying the integrity of these components would require intrusive testing by a heating specialist). We recommend having a competent and licensed HVAC contractor service the unit; check it for proper operation; and verify the full integrity of the heat exchanger. Cleaning and servicing sometimes reveals hidden defects if present. Repair or Correct any defects.

Before calling for a technician check the filter, blocked air flow through the furnace can cause short cycling.

However for this furnace its a good call, someone has replaced the burner compartment cover with sheet metal, which may be preventing combustion air from reaching the burners, there is a return air opening right beside the furnace which should be 10 feet away if it is in the same room as the furnace, and the furnace age has exceeded its estimated lifespan.

Few if any furnaces have heat exchangers that are more than just a little bit visible.

Therefore as a national HI trainer (and past HVAC contractor) to help protect your client, we train inspectors to … ANYTIME they have a problem with a furnace, an older furnace, jerry rigged components, an obvious lack of maintenance, etc to add to their comments describing the obvious issue something like one of the following …