When you sense that your furnace's heating cycles are coming off and on often without your space reaching the set temperature on the thermostat (and there are no other issues), your furnace may be short-cycling. This is also a condition that prevents your home from heating up to a comfortable temperature. Some furnace short-cycle issues can easily be fixed, while others will require the assistance of a qualified HVAC technician.
What Is Furnace Short-Cycling?
A furnace short-cycles when its heating cycles turn on and off but do not allow the room to reach the set temperature. Short cycles will also happen more frequently than cycles of normal duration.
What Furnace Short-Cycling Is
When a home's furnace short-cycles, its heating cycles run often but stop short of reaching the thermostat's set temperature.
Short also refers to the length of the heating cycles: The heating cycles run shorter and more frequently than they should ordinarily be. The furnace's resting cycles might be short, as well.
An Example of Short-Cycling
Your home's thermostat is set to 72 degrees. When the home's temperature dips below a pre-set minimum level of 65 degrees, the furnace turns on in an attempt to bring the house up to 72 degrees again. Yet the furnace's heating cycle stops five degrees short of 72 degrees (67 degrees). After a while, this on-off cycle repeats.
Signs of Furnace Short-Cycling
A cold house can be attributed to any number of causes: poor wall insulation, inadequate furnace, faulty or outdated windows, or inadequate attic insulation. A furnace that short cycles might be one cause since the desired temperature is never achieved.
Short Heating Cycles
The length of furnace heating cycles depends on the temperature needs. A house that has been shut down for months in the cold season may require the furnace to run for hours. A well-insulated, heated home may have heating cycles as short as five minutes. If those cycles are less than five minutes, the furnace is likely short-cycling.
Frequent Heating Cycles
Frequent heating cycles are a function of heating cycles that run for shorter times. These short cycles may occur five to eight times an hour, depending on the outside temperature and how well-insulated your home is.
Room and Thermostat Difference
The most positive identifier of a short-cycling furnace is when the two temperature readings on the thermostat never match: room temperature and thermostat set temperature. If the temperature reading of the room steadily increases during the heating cycle but stops short of the set temperature—and does so continuously—then the furnace is short cycling.
The U.S. Department of Energy recommends that you set your thermostat at 68°F in the winter and 78°F in the summer.
How to Fix Furnace Short-Cycling
If your furnace has no other issues and the thermostat is functionally properly, the following tips can help fix the short-cycling issue.
Dirty Air Filter
The furnace's fan needs to be able to move a sufficient volume of air over the heat exchanger. A clogged or dirty air filter can limit the volume of air. This activates the fan limiter.
This is an easy, do-it-yourself fix. Remove the air filter and replace it with a fresh filter. Make sure that the arrows on the frame of the filter run in the same direction as the system's airflow.
The thermostat may be improperly set, the batteries may be weak or dead, or the thermostat itself may be defective.
To fix this, start by changing out the thermostat's batteries with fresh batteries. Check the lower end of the thermostat's temperature settings. If set too high, the furnace will frequently cycle on. Fix this by lowering the temperature differential.
The furnace flue is designed to expel hazardous gasses. If the flue is clogged with any kind of object, whether a bird's nest, dead vermin, or leaves, this will trigger the furnace to shut down. If not, the furnace will keep running and hazardous gasses will build up inside the house.
Fix this by turning off the furnace and disassembling the flue. Check every section of the flue with a flashlight. Clear out any obstructions. Reassemble the flue and start the furnace up again.
Dirty Flame Sensor
If a furnace's gas valve emits gas but no flame is present, gas can build up in the home—an extremely dangerous condition.
The furnace's flame sensor is an L-shaped metal rod that tells the furnace that a flame is present while the gas valve is open.
If the flame sensor doesn't detect a flame, it shuts down the furnace. Flame sensors can become dirty over time. When dirty, flame sensors cannot properly detect flames.
Fixing a dirty flame sensor is a simple project. After shutting down the furnace, the sensor is removed with a hex driver or socket wrench. Then, lightly rub down the flame sensor with fine steel wool or fine-grit sandpaper.
In some cases, it's best to replace the flame sensor with a new one. Sensors cost about $10 to $20.
Defective Fan Limit Switch
The fan limit switch lets the furnace blower know when to turn on or off. It's the gatekeeper that controls the movement of the hot air. Since the fan limit switch is a safety switch, it will shut down the furnace if it senses any overheating.
Fan limit switches are sealed devices that should not be repaired. Replacing the switch is the fix for this problem. Fan limit switches cost $100 to $200, labor not included.
When to Call a Professional
Most furnace short-cycle problems should be repaired by a qualified HVAC technician. Homeowners can replace air filters and reset thermostats or change their batteries, but more complex problems will require an expert.