When a furnace is operating normally, it will go through 2 to 4 heat cycles per hour, each lasting 5 to 15 minutes depending on how cold it is outside and whether you’ve just turned up the thermostat or it’s been set where it is for more than a few hours.
The blower kicks on 45 to 90 seconds after the burner fires, and it runs for maybe a minute after the burners quit in order to expel hot air ,kstill in the combustion chamber.
But the blower should not shut off before the thermostat is “satisfied,” meaning the temperature you set it to is reached.
Why your Blower Turns On and Off Repeatedly
There’s either a problem with your blower or there’s an issue with the furnace called short cycling.
Blower Problems – Let’s Rule These Out
If the blower shuts down before the thermostat is satisfied, the issue is with the blower.
Possible causes include these:
Dirty air filter(s): If the filter is so dirty that the blower is overheating trying to pull air through it, the blower might stop. When it cools off, it starts again. This is an easy fix.
Wrong fan speed: Same issue as above, but a different cause. Did someone just work on the furnace, or is this a new furnace to you? It could be that the blower is running too fast and overheats. If your filter is clean, and the blower is shutting off before the thermostat heat requirement is hit, this is a possibility. Especially for new or newly worked-on furnaces. Call the person who installed it or worked on it last and explain the issue – and that it is probably their responsibility to fix it at no cost to you.
Furnace Short Cycling – The More Likely Cause
If your furnace blower is constantly turning off and on, it is most likely short cycling for an internal reason other than the blower. When a furnace is short-cycling, it’s turning on and then quickly turning off, and running through many short heat cycles per hour. The shorter the cycle, the more harmful it is to your furnace.
Short Cycling is Harmful to Your Furnace
When your furnace short cycles it may be overheating inside and shutting down. When it cools down enough, it restarts. Constant overheating and turning on and off creates excessive wear and tear on the electrical and lubrication systems in your furnace and will shorten its lifespan.
Causes of Short Cycling
There are a number of reasons that a furnace will short cycle, some are simple, inexpensive fixes and some are serious, but all need immediate attention to keep your furnace operating safely and in peak condition. Here are common causes of short cycling and what can be done to solve them.
Is it short cycling during an AC cycle? If so, here is the article for that problem.
Now, if it happens during heating, here are the most common causes and what to do to solve the problem.
Issues with the Thermostat
It’s possible that our thermostat is malfunctioning and may need to be tested to see if this is the case. If you are thinking of an upgrade to a smart or WiFi thermostat, this is a good time to make the switch and see if it also fixes the short-cycling issue.
When a thermostat is placed in direct sunlight or near a heat generating appliance or light source, the thermostat will reach temperature too soon and signal the furnace to shut off. Thermostats must be in a central location in your home and away from heat sources to operate properly. The solution is to move your thermostat to an appropriate location or move the space heater or lamp away from it.
An Oversized Furnace
This is a failure of the furnace installer, and the installer should be notified and asked to fix the issue. The adage used to be “bigger is better,” but that’s been proven wrong – decisively so.
A furnace that is too large for your home might not seem like a problem at all but it can cause areas closest to your thermostat to heat up quickly, turning off the furnace and areas furthest away, to remain cold. It can also cause your home to quickly become too warm. An oversized furnace will use more fuel than necessary and raise your energy costs. The only solution is to replace the furnace. You can use our furnace sizing calculator to check if your furnace is oversized.
Clogged Air Filter
One of the most common causes of short cycling is restricted air flow. You’ve probably already checked your filter and changed it if it was dirty. But if not, now is the time to do it. When the filter is filthy, fresh air cannot get into the furnace, causing it to overheat and shut down. Pulling air through a dirty filter puts stress on the equipment too, causing reduced efficiency and early mechanical failure.
It’s also possible that you have the wrong air filter in your furnace – one that is too dense or has a MERV rating that is unacceptably high. If you can’t otherwise determine why the blower turns on and off repeatedly, check out the information on the Right Air Filter for your HVAC System.
Blocked Exhaust Vent Outside
The exhaust vent can become blocked by animal nests, beehives, debris from trees, and snow and ice. Remove the blockage yourself or call an HVAC professional. If an animal nest is causing the blockage, it’s best to call an animal control specialist.
Blocked Heat Register or Closed Dampers
This is an easy fix.
If the registers are blocked, or too many of the dampers are closed, the warmed air cannot exit the furnace, the furnace will overheat and shut off. In this case you will notice the furnace shuts down in 3 to 10 minutes after starting and the air coming out of the open registers may be hotter than normal. If too many of the registers are blocked or closed, the furnace can’t dissipate the heat. Unblock the registers and make sure at least 70% of the dampers are open.
Gaps in Ductwork
Do you have ductwork in a cool area like the basement, garage or attic? If the home is warm, but cold air gets into the ductwork from one of those locations, it can trigger the thermostat to start the furnace, but it will quickly shut down since the house is already pretty warm. Inspect ductwork for leaks and other damage.
And be sure to seal and insulate those ducts for efficiency and indoor comfort.
Other Overheating Causes
There are a number of other issues, besides restricted airflow, that can cause a furnace to overheat and short cycle. These issues include dirty internal components like the coil or blower motor or fan, high gas pressure, a weak inducer motor, a bad control board, and a bad heat exchanger.
This might be the right time to have an HVAC technician give your furnace a good cleaning and cover the full checklist of items that can cause short cycling and other mechanical issues. Most HVAC companies charge $100 to $250 to properly clean, check and tune a gas furnace.
Faulty or Dirty Flame Sensor
A flame sensor is a safety device in your furnace that determines if the gas in your furnace is actually burning. If the flame sensor doesn’t recognize a flame, it will shut the furnace down, and the blower will shut off too. But since the thermostat isn’t satisfied, the furnace might soon start again, and then shut down, repeat, repeat, repeat. The flame sensor is inside of the furnace and will eventually become covered with soot keeping it from recognizing the flame.
You may be able to determine if this is the cause by removing the furnace cover and watching the furnace start. The inducer motor will run about 30 to 60 seconds before the burners fire. As the burners fire, they make a “whooshing” noise. If you don’t see the flames, or the furnace shuts down just after it flames, the flame sensor may be the problem.
Cleaning or replacing the flame sensor is a simple fix and if you have HVAC experience, you should be able to remove it and clean it yourself, otherwise, call an HVAC professional. Or you can troubleshoot the flame sensor with this information.
Problems with the High Limit Switch
A high limit switch, also called a fan limit switch, is another safety feature in your furnace. It shuts down your furnace when it overheats, preventing fires and damage to the heat exchanger. One of the most common reasons for short cycling due to overheating is restricted airflow, and if you’ve addressed those issues as detailed above, you may have a faulty high limit switch that keeps tripping.
A high limit switch should be inspected by an HVAC professional to determine if the switch is the problem or if there are more serious reasons the furnace is overheating.
A Damaged Ignitor
As the furnace turns on, the ignitor heats up creating the flame as the gas is released. If the hot surface ignitor (on newer units) or the pilot light (on older units) is not working, the furnace will shut down preventing unburned gas from accumulating inside the furnace.
Hot surface ignitors generally last about 7 years and, as they get older, can warp, crack, and become unable to heat high enough or long enough to maintain the flames. You can watch your furnace turn on and if you don’t see flames, or the flames quickly go out, the problem is probably the ignitor. Replacing the ignitor requires an HVAC professional.