Short cycling is a condition where your furnace will run for a short period and shut off before fulfilling your thermostat's setpoint. Most systems should run as long as necessary to meet your thermostat setpoint, so short cycling typically indicates an underlying issue. However, you may occasionally experience a situation where your furnace barely runs and instead rapidly turns on and off.
Rapid cycling usually points to a problem with your furnace's ignition sequence. Many furnaces will attempt to start several times and then engage a safety shut-off after multiple failures. The furnace may attempt to start again after a cooldown period. If you're noticing these issues with your furnace, here are three tips to get your heat working again.
1. Check Your Filter
You can think of the airflow through your vents as serving two surprisingly different purposes. On the one hand, adequate airflow is necessary to bring warm air to your rooms and heat your home. On the other hand, the cool air that returns to your furnace helps to keep your heat exchanger cool. Without enough airflow, your heat exchanger can overheat and even crack.
Furnaces use limit switches to detect high heat conditions and shut down to prevent heat exchanger damage. Your furnace may rapidly overheat and shut down if you have severely restricted airflow due to a dirty filter. Check your filter and shut your furnace off if it's severely clogged or dirty. Once you install a new filter, run the system and see if the problem resolves.
2. Listen for Ignition
If you stand near your furnace, you should be able to hear the burners igniting. Your furnace may also have a sight glass that will allow you to observe the burners with the cabinet closed. In either case, have a helper turn your thermostat up slightly as you stand near your furnace and see if it ignites. You should hear the draft inducer motor turn on first, followed by the sound of ignition shortly after.
A furnace that ignites and then shuts down likely has a clogged or faulty burner. Furnaces use a flame sensor on the final burner to confirm complete ignition and will shut down if even a single burner doesn't light. In these cases, you should stop using the furnace and have a technician determine if your burner needs cleaning or if there's another problem with your burner assembly.
3. Look for Error Codes
Most furnaces now include a control board that can display error codes, although the method can vary. Some furnaces may show an alphanumeric code on a small LED display, while others use flashing lights. These codes likely won't mean anything to you on their own, but you can cross-reference them with an error code table in your owner's manual.
In most cases, you shouldn't attempt to repair your system based on this error code. Instead, contact a professional and let them know what you found so they can use it as a starting point. Like check engine light codes on cars, furnace error codes are not a fool-proof diagnostic method but one tool that a professional will use as part of a larger diagnostic process.
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