Why Does My Gas Furnace Keep Turning On and Off?

If your furnace is turning on and off more than two or three times each hour, there could be cause for concern. This might happen occasionally if temperatures outside are extremely frigid, and your house is a bit drafty in general. But if it continues regardless of conditions outdoors, you should contact your HVAC contractor right away to let them know your HVAC system needs inspecting.

When your furnace turns on and shuts off frequently this is called short cycling, and if you ignore it for too long the results could be catastrophic. Short cycling puts excessive strain on a furnace and will cause a mechanical failure at some point if nothing is done to stop it.

7 Possible Causes of Short Cycling

There are actually quite a few reasons why your furnace might be short cycling. Some of them can be addressed with some simple troubleshooting, but others will require the assistance of a trained HVAC technician.

Generally speaking, your best course of action is to try what you can do first, and if that doesn’t work summon the experts.

The possible causes of short cycling include:

1. Dirty Air Filters

This is the most common reason why a furnace short cycles. A clogged and dirty filter will restrict air flow through your HVAC system, causing the heat exchanger to overheat and triggering frequent shutdowns.

Fortunately, changing an air filter is a simple troubleshooting task. Filters should be changed at the start of every heating and cooling season, and possibly more often if you have significant indoor air pollution problems.

The filter is normally found adjacent to the furnace, in the return air duct or blower compartment. You can purchase a new air filter at your local hardware store or HVAC supply outlet. Fiberglass models are the cheapest, but pleated air filters will last longer and do a much better job of capturing contaminants and preserving the health of your furnace.

2. Dirty Blower Wheel

The buildup of dirt, grime and debris can prevent your blower wheel from working properly. If it builds up enough short cycling could happen frequently, and eventually mechanical failure requiring expensive repairs could be the result.

The best way to avoid this development is to have an HVAC technician inspect and clean your furnace before the start of each heating season. Annual maintenance will keep your furnace in tip-top working order.

3. Closed or Blocked Intake Vents

Intake vents supply your furnace with a steady supply of air to fuel efficient combustion. If they are inadvertently closed, blocked by furniture or other objects, or clogged with dust, they won’t feed enough air into the system. This can cause the heat exchanger to overheat and lead to short cycling.

If your furnace is short cycling, checking the intake vents is one of the first things you should do. If they are obstructed, remove whatever is obstructing them and see if that does the trick.

4. Malfunctioning Thermostat

A thermostat might trigger short cycling if it is exposed to direct sunlight or has been placed too close to a heat source, such as a register or heat-generating appliance. This could be the issue if your thermostat is new and recently installed.

One thing to remember with thermostats is that their batteries must be changed regularly. If you haven’t done that in quite some time, that could be the source of the problem.

If thermostat location is not a problem and you know the batteries are fresh, it could mean you have a wiring problem. Needless to say, this is a job that your HVAC technician should be allowed to handle.

5. Dirty or Corroded Flame Sensor

The flame sensor informs the furnace when the gas valve is open, and will shut off the valve if no flame is present. But a corroded or dirty sensor may not always detect the flame even if it is there, causing the valve to shut off and the furnace to stop running.

A flame sensor that is no longer working correctly can cause short cycling. The only way to know for sure if this is the problem is to have it checked by a certified HVAC technician.

6. Clogged Flue Pipe

The flue pipe is the exhaust vent that emerges from the top of your house. Because it is exposed to the open air, it can easily get plugged with leaves, branches, sticks, or other types of falling debris. Sometimes birds or squirrels might build their nests inside.

If the plugging is significant it can lead to short cycling. A visual inspection should tell you if you’re flue pipe is obstructed.

7. An Oversized Furnace

When a brand new furnace is short cycling, there’s a good chance it wasn’t correctly sized by your HVAC contractor. When a furnace is too large and has too much capacity for a home, it will heat up rooms too quickly and shut off sooner than expected.

When a furnace operates in this way, it won’t function efficiently and your heating and repair bills are likely to be disappointingly high as a result.

At ABE Heating and Cooling, our technicians know how to properly size a new furnace, using a time-tested method called the Manual J Heat Load Calculation. When you do business with us, you’re ensured of getting HVAC equipment that maximizes energy-efficiency and keeps your utility bills manageable.

Short Cycling a Problem? Call the Problem-Solvers at ABE

If your attempts to solve your short cycling problem have failed, its time to call in ABE Heating and Cooling. We’re the highly-skilled heating and cooling experts that can find the answers and offer workable, affordable solutions. We’re located in Brighton, Colorado and serve the entire Denver Metro Area, and when you’re having difficulties with your furnace you can call us any time day or night and we’ll be there in a flash.