Reviewed by George Baral
A standard home furnace generally turns on and off between three to eight times per hour, so its usual heating cycles last from seven to 20 minutes. When it happens every minute or two, your furnace is likely out of whack.
Short-cycling can actually have a silver lining. Although it is a signal that something’s wrong with your furnace, it also demonstrates that the furnace’s safety mechanisms are working and preventing something more serious happening—a fire starting and burning down your home, for example.
Before you investigate the cause of a short-cycling furnace, it’s helpful to understand how your furnace works. Basically, the blower motor—the fan that distributes the air—sends air across a heat exchanger comprised of tubes with burning fuel passing through them, and then onward into the rooms of your home.
Luckily, there are numerous safety features in place that prevent the furnace from doing so in an unsafe manner. A high limit switch turns your furnace off if it becomes too hot, while a flue limit switch shuts off the furnace if it isn’t venting properly through the exhaust flue.
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A dirty air filter is the most common cause of short-cycling. If your air filter is clogged with dirt, it won’t allow proper air flow to the heat exchanger. That can cause the furnace to overheat and forces the high limit switch to ride to the rescue by shutting the furnace off. When it cools, it will restart, and the cycle will continue.
You can easily prevent this type of short-cycling by being conscientious about changing your furnace’s air filter. It should be cleaned or replaced monthly to allow your furnace to work at optimum capacity. Turn off your furnace, unscrew the panel and insert a new filter. Most of the time, this is the easy answer to your short-cycling problem.
If you’ve eliminated a dirty air filter as the cause, the next most likely culprit for short-cycling is your thermostat. Like any piece of mechanical equipment, it’s not perfectly reliable, and may need attention periodically. If your thermostat isn’t working properly, it may be telling the furnace to shut off prematurely.
Shut off your thermostat, open the unit, and replace the batteries. Now, turn the system back on and monitor it as it runs for an hour to see if the problem is fixed.
You may also want to reconsider the location of your thermostat. If it is near a heat register, a space heater or in direct sunlight, it can be unduly affected by these heat sources and prone to shutting down prematurely.
Before you investigate more complex causes for your short-cycling problem, there’s another possible cause with a straightforward solution: poor insulation or air leaks that allow the newly heated air to escape and requires the furnace to fire up all over again.
Check your windows to see if any of them are open or if they aren’t sealed properly. If air is escaping, you may need to do some caulking around the frames to keep that warm air trapped indoors.
More challenging causes of short-cycling could relate to the furnace blower the flue or the flame sensor.
If your blower motor is off, your heat exchangers won’t be warming air and may become too hot, causing the furnace to shut off. If this is the case, air won’t blow through your vents, or if it does, it will be weak. Put your hand in front of a vent to check.
The device that shuts of the furnace if the air becomes too hot could be malfunctioning, and turning the furnace off prematurely. You will need a professional to determine this.
Your flue could be blocked, which means heated gases aren’t escaping properly and the furnace’s flue limit switch is causing the system to shut off before the flue gets too hot. Usually, there is a blockage in the flue that is causing the problem; an animal may have deposited some branches in the opening, for example. Get it checked and cleared out, and your system should be back in business.
Finally, if your furnace’s ignition system isn’t working properly and doesn’t produce a flame within four seconds, the flame sensor will shut the system down. Have a professional check the system if you suspect this to be the problem; a dirty or damaged flame sensor (thermocouple) or damaged ignitor could be the culprit.
If you’ve just moved into your home and find that the furnace is short-cycling regularly, it’s possible that your (HVAC) system is too large for your home.
If the system has greater heating power than is needed, it heats your house rapidly and shuts off. It turns on again soon afterward. Over time, your energy bill will be higher than it should be, since the system will use more power turning on and off frequently than it would if it ran consistently. It also results in more wear and tear on your system and a shorter life span. If you suspect this is the problem, contact an HVAC professional for assistance.
Once you identify the problem and fix it, with or without the help of a professional, your furnace will be able to do its job properly and you’ll be more comfortable.
If your furnace turns on for a period of less than 7-8 minutes, it is likely short-cycling. This span of time isn’t long enough for the furnace to meaningfully heat the home.
It is possible to clean a furnace (at least partially) yourself, but make sure you power it off and shut off the gas before you do. You can open the unit and vacuum as much dust as you can reach. You may also be able to detach the blower and clean the blades with a vacuum and brush. Finally, changing the filter is a simple task you can do on your own (and you should, every few months).
Large air leaks in your ductwork are easy enough to find by listening carefully. Move along your ducts and check for hissing sounds, especially around seams and connections. If the ducts are near insulation, leaks will eventually darken the insulation around them over time. To track down tiny leaks, light a small incense burner and use the smoke trails to seek out small air currents.
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