Why Is My Furnace Short Cycling?

If your furnace is short cycling, it probably needs repairs or maintenance. However, sometimes your unit is working perfectly and short cycles due to a sizing issue. Either way, short cycling is a costly problem to have since it causes more wear and tear on your furnace and higher utility bills.

The original surge as your furnace turns on uses a lot of power. A short cycling furnace uses that extra power repeatedly, driving up the cost of heat. Let's look at some of the issues that can cause short cycling and how to fix them.

Most malfunctions that cause a shorter heating cycle are easy for a professional to fix and relatively inexpensive repairs. If you've noticed your furnace turning on and off after just a few minutes, call an HVAC professional to find out what's causing the problem.

The thermostat is the control device for your furnace. It tells the unit when to come on and when to shut off. If your thermostat isn't working properly or has incompatible settings (it's set to cool when you want heat, for example), you might notice your unit turning on and off several times per hour. Check your thermostat settings first to make sure there are no issues. If everything is set correctly, it could mean your thermostat sensor is bad, or you need a new thermostat.

Either way, both are inexpensive fixes and putting in a new thermostat offers you the option to upgrade to a smart unit for better energy efficiency and long-term cost savings.

Air filters trap pollutants and give you better indoor air quality, but when they fill up, they can also impact furnace performance. A dirty or clogged air filter creates more drag on the air passing through and can even totally block the air from moving into your ducts. When that happens, it traps heated air in your furnace, triggering overheating and automatic shutdown. If your home is cooling off even though your furnace is turning on and off regularly, it could mean you require new air filters. This is a simple DIY repair; you should change your filters every 30 days or so, depending on the size.

If your furnace is too large for your home, you might be kept comfortably warm but spend too much on your monthly utility bills. An overly large furnace won't need to run as long to heat your home. This sometimes happens when a home has large drafts or insufficient insulation, so the owners opt for a more powerful heater. If those air leaks are then fixed with new windows or doors or better attic insulation, the result is an overpowered heater.

The only way to fix this problem is with a new unit. Talk to your HVAC pro about the options and expenses involved with replacing the unit and the potential savings in the long term.

The flame sensor ensures the gas shuts off when there's no active flame to burn it for fuel. If your flame sensor isn't working properly and doesn't sense the flame, it will shut off your unit before it can heat your home. Several shutoffs in a row may lock the unit until you get help from a professional. Replacing a bad flame sensor is a fairly economical repair, typically costing under $300. Sometimes, we can even clean the sensor, making it a cheaper fix.

If your blower never stops, it's a sign that your limit switch has gone bad, but before that happens, you might notice consistent short cycling. The limit switch causes a short cycling furnace because it senses the blower motor is overheating and shuts down the cycle. If it's not reading properly, it will cause frequent shutdowns even when everything else works fine.

The thermocouple is another gas shutoff safety feature. It prevents unburned gas from sitting in the combustion chamber. If your thermocouple goes bad, it might think the pilot light is out, even when functioning properly. The result is a system shutdown, which causes a short cycle. Rust and faulty wiring or exposure to extremely high temperatures can all cause issues with your thermocouple.