Problem: Gas on furnace is shutting off after 3 minutes of burn time
Solution: You would need to test some controls to see what the problem is. Why is the gas shutting off after 3 minutes of burn time.
I would start with testing the thermostat to make sure it is getting a constant 24 to 28 volts between the W and C (com) wires when calling for heat. If the thermostat isn’t getting 24 volts after the 3 minutes then you might try adjusting the anticipator if equipped. The new digital thermostats do not have an anticipator setting, but some of the new digital thermostats have instructions on how to adjust the thermostat to make a longer run cycle. I would suggest reading the instructions that came with the thermostat. If the thermostat is not the problem then I would suggest testing all the safety controls, ie limit, pressure switch, rollout switches to make sure they are all staying closed. Test the controls to make sure they are staying closed when the furnace shuts off after the 3 minutes. This sounds like the furnace might be overheating (going off on limit) and shutting down. This overheating could be caused by a dirty filter, dirty blower wheel, slow blower motor (could be a weak capacitor) gas pressure set to high on the gas valve, improper setting on the thermostat anticipator (if equipped), over-sized furnace (too big for your home or duct work) or a stopped up evaporator coil. Below we have a YouTube videos that show how to test a limit switch made by grayfurnaceman. Thank you grayfurnaceman! We also have another YouTube video that shows how to clean a blower wheel by DrZarkloff. Thanks so much for making these informative videos! I hope you can get the problem figured out easily.