I have a gas furnace that is 17 years old. It is made by HEIL. It is rated at 175,000 BTU (Input).
The furnace is equipped with FAN/LIMIT CONTROL device. I am just a novice so I am describing what I was told. This device senses temperature in heat exchanger and if the temprature crosses a certain preset limit then the device will turn off all burners but let the fan run for few minutes until the temprature cools down to 90 to 100 degrees. This is a safety device.
In my furnace ever since I know (last 7 years), my burners cycle on and off but the fan keeps running during this burner on/off process. I have timed this. Typically when the room thrmostat calls for heat, the burners turn on first followed by the fan. The burners stay on for about 150 seconds and turn off while blower fan continues to run. After about 95 seconds the burners start again. This process continues until the room thermostat is satisfied.
I was told by a technician yesterday that the funrnace is not supposed to do this. It is doing this because the fan/control safety device is coming in to action. The root cause is my return duct is undersized and so the blower motor is not getting sufficient air so the temperature in heat exchange is frequently reaching the safety threshold. He wants to install couple of return ducts in the basement (furnace is located in the basement in a utility room) and replace the return duct on the side of the furnace with a bigger size.
But another technician, couple of years ago, said that the on/off process is normal and good. When the burners are off I am obviously saving gas while the blower is still working to remove already hot air in the heat exchanger. He described this as a good thing.
Today I did a little experiment. I removed the front panel that covers the blower compartment and put a duct tape on the limit switch. The burner ON/OFF behaviour is totally gone. The burners were on continously until the thermostat temparature setting is reached. I help a toilet paper in front of the blower and I noticed the blower is drawing lot of air from the front. (The return duct is on the side of the furnace.)
So, what is correct here? I appreaciate if you could tell me if this is the way it is supposed to work or if I indeed have a return duct sizing issue.
Thanks in advance for reading this long story...