A furnace that is short cycling turns on and then turns off before it has heated your home to the thermostat set point. Sometimes a short cycling furnace won’t make any heat at all before shutting down.
Why does a furnace short cycle?
Reasons for furnace short cycling are:
- A bad flame sensor (flame rod)
- A very dirty furnace filter
- Blocked heat grates
- A blocked exhaust vent
- A thermostat mounted in the wrong location
- A furnace that is too large for your home
Solutions for each of these problems are found below.
Most causes are easy to fix. The last reason can be expensive because it requires replacing the furnace. However, it is best to check these potential other causes before being concerned you’ll have to replace the furnace.
Reasons for Furnace Short Cycling
Let’s explore these one at a time.
1. A Bad Flame Sensor (aka Flame Rod)
The furnace flame will shut off almost immediately after it lights.
Short cycle time: A few seconds after the burner fires. Don’t mistake the draft inducer motor starting for the furnace burner firing. The inducer motor starts 30-60 seconds before the gas fires. Take the cover off the furnace to look for the burner firing. Or listen for the “whoosh” it makes.
The flame sensor’s purpose is to determine that the gas valve has fired and isn’t pumping raw gas into your home where it can explode if it meets an open flame, like on a water heater or gas range. If the sensor doesn’t sense a flame, it will shut off the gas valve.
A flame sensor that is failing might work sometimes – it won’t shut down a burning furnace – and not work other times. Of, it might shut down a furnace flame every time.
Solution: Clean or replace the flame sensor (and we can help!)
This is a common cause of short cycling, so we’ve prepared a guide on How to Tell if the Flame Sensor is Bad. It explains how to diagnose a bad flame sensor. There are links to videos for cleaning it and for replacing it. If you’re handy, either job can be a DIY fix. The jobs take less than an hour.
2. A Very Dirty Furnace Filter
A dirty filter can cause the furnace to overheat.
Short cycle time: 2-5 minutes. The key is that the furnace shuts off before your home is warmed. You might also notice hot air (warmer than usual) coming out of your heat grates.
Your furnace has a part called a high-limit switch. If the furnace gets too hot, the switch shuts down the furnace.
A dirty filter prevents a flow of fresh, cool air into the furnace to be heated and dispersed. Without the fresh air, it will quickly overheat.
Solution: Change your furnace filter. In the future, check it monthly and change it as needed.
3. Closed Heat Grates
This is another cause of overheating inside the furnace. A dirty filter doesn’t let enough air in. This problem doesn’t let enough air out of the system.
Short cycle time: 3-10 minutes. Again, the furnace will stop before your home is warm. The air coming out of open grates might be hotter than normal.
Air flows out of ductwork into living space through the grates, also called vents and registers.
Your home’s heat grates probably have dampers on them, so you can control which rooms get heat.
If too many of these vents are closed, the furnace blower can’t disperse enough heat. The heat will build up in the furnace, and it will overheat and shut down.
Solution: Open some of the closed grates. At least 75% of them should be open. If boxes or furniture are blocking grates, you might have to move the items.
4. Blocked Exhaust Vent
This is another cause of a furnace overheating and short cycling.
Short cycle time: 1-5 minutes.
This issue can be dangerous too, since deadly carbon monoxide is dispersed through the vent. If it is blocked, the gas won’t escape, and it can leak out of the furnace and into your home.
The most common causes of a blocked vent are bird or animal nests. A bee hive can do it too. An abundance of snow or ice will block a vent also. Sometimes screens are put on vents to keep out pests. This is an improper addition, because screens are easily clogged.
The image below shows the exhaust flue exiting the roof. If you have a furnace that is 90% efficient or better, the flue might exit through the wall. Follow the exhaust flue to determine where it is exiting your home, so you will know where to find it on the outside of your home.
Solution: Clear the obstruction from the exhaust flue. If the cause is an animal nest or bees, an animal control or pest specialist can handle the job.
5. A Thermostat Mounted in the Wrong Location
Your thermostat might be getting warmed before the rest of the house.
Short cycle time: Any time short of the home being properly heated.
This happens most often with DIY thermostat installations, but pros can make this mistake too.
If the thermostat is mounted right above a heat grate or on the wall of the utility closet where the furnace is located, it will heat quickly. When it does, it will reach its set point before it should – long before the rest of your home is warmed.
Is there a lamp near the thermostat that isn’t using an LED bulb? The heat from the lamp could be warming the thermostat too, causing it to shut down the furnace prematurely.
Solution: Relocate the thermostat (or the lamp). Choose a central location for the thermostat that isn’t near a heat grate or window that allows the sun to shine directly on the thermostat.
This video discusses where you should and should not mount a thermostat. The helpful information starts at the 0:30 mark.
6. A Furnace that is Too Large for your Home
Many homes have furnaces that are too large because the installer believed that “bigger is better.” That’s not true when it comes to furnace size, indoor comfort and how long your furnace lasts.
Short cycle time: 3-7 minutes in most cases.
When a furnace is the right size for a house, the heating cycles are long. They don’t produce noticeable temperature swings.
When a furnace is too big, it can short cycle – start, heat up air very quickly that reaches the thermostat, and then shut down.
Compare this to driving a car. A properly sized furnace is like driving along steadily at 60 MPH. A furnace that is too large is like driving at 120 MPH for a few minutes and then stopping for a rest, then speeding up to 120 again and repeating the cycle. That’s not comfortable, and it is a major waste of fuel.
An oversized furnace causes more problems than just short cycling. We’ve discussed 6 problems with furnaces that are too large in this FAQ Guide to Oversized Furnaces.
Solution: Replace the furnace with one that is properly sized.
Clues that your furnace is too big include short cycles, temperature swings and heating bills that are higher than expected.
Contact a respected heating company in your area to discuss your concerns. They can do a test called a Manual J Load Test and other tests to determine if your furnace is too large and what can be done about it.
If you’d like help finding an experienced, licensed and insured furnace pro, it is easy with our Free Local Quotes option. There is no cost to you for using the service, and no obligation.
Watch a Technician Discuss These Issues
We’ve covered the causes of 95% of short cycling causes. There are a few more that are quite rare. The common and rare causes are discussed in this video by an experienced furnace technician.