Furnace Tries To Start But Then Shuts Down? (We Have A Fix)

A furnace that isn’t working problem is more than just an annoyance. Any problem with your furnace can indicate issues that are potentially dangerous to you and your family. A furnace that tries to start and then shuts down requires immediate attention.

A furnace that tries to start and then shuts down suffers from what is known as short-cycling. Several problems can cause a furnace to short-cycle, such as a restricted air-flow. Faulty sensors can also cause short-cycling, as can an improperly sized heater. These are the most common problems causing short-cycling. Other factors can cause short-cycling, but they are rare.

Short-cycling of your furnace is a serious situation you shouldn’t ignore. Some of the more common problems that cause short-cycling of a furnace can be diagnosed and remedied by most homeowners. A few simple steps can help you diagnose and remedy some of the problems associated with furnace short-cycling.

Most new furnace installations include an outside air intake to provide combustion air for your furnace. Most of these installations use an air pipe that extends up through the ceiling and roof. Unheated or cooled air is fed to your furnace to support the combustion process.

If this air intake pipe is clogged or blocked, your furnace can begin to short-cycle. Check the outside air intake in your furnace closet to ensure an unrestricted flow of combustion air to the furnace. If necessary, clean the intake pipe or adjust the roof cap on the pipe.

The space around your furnace must be clear and free of any obstructions. You shouldn’t use your furnace closet to store anything. Not only does this pose a fire hazard, but it can also cause air supply problems to the furnace.

Your furnace also has minimum offsets from walls. Air must circulate around the furnace for proper functioning. If the furnace is too close to walls or other objects, air-flow disruptions occur, and your furnace may not get the air it needs to operate correctly.

The user manual supplied with your furnace should have more information about minimum distances and offsets for correct installations. You may need to consult with a plumber if your furnace needs to be moved or reinstalled correctly.

Even if your furnace is getting plenty of air to combust properly, it may begin to short-cycle if the hot gases created cannot escape. The exhaust flue on your furnace allows the hot combustion gases to escape outside safely.

Checking the exhaust flue may require a plumber. There are safety issues involved when removing or replacing an exhaust flue. If not done properly, poisonous gas can enter your home and pose a health threat to you and your family.

If you suspect that the exhaust flue on your gas-fired heater is blocked or restricted, make an appointment with a plumber to inspect your furnace and repair the flue if necessary.

Believe it or not, a dirty air filter on your furnace can cause short-cycling. If your furnace cannot get enough airflow through the vents, the heating system’s air will get too hot. Sensors in the combustion chamber will trigger, and the furnace will shut down.

Check the air filters in your furnace. Change your air filters regularly, often as much as once a month. The owner’s manual for your furnace has more specific information about the type of furnace filter to use and the frequency of changes.

Your furnace may be getting too hot. Modern gas-fired furnaces have several internal sensors that continually monitor the interior temperature of the furnace. These sensors won’t allow the furnace to operate if any of these internal parts exceed a pre-set temperature.

If one of these sensors is faulty, it may send bad readings to the control board and keep your furnace from operating properly. A bad sensor can result in short-cycling of the furnace.

Unfortunately, finding and testing these sensors is beyond the scope of most homeowners. Call your HVAC technician for a service visit.

Your furnace has a flame sensor as a safety precaution. The flame sensor controls the gas valve on your furnace. If the flame sensor tells the gas valve that the flame is burning, the valve remains open.

A defective flame sensor will allow the burner to light but then sends a signal to the gas valve that the flame is not burning, and the gas valve closes. This situation results in a classic short-cycle in your furnace.

If the flame sensor on your furnace is faulty, have it replaced by a qualified HVAC technician. The flame sensor must be positioned in the burner correctly and calibrated for your furnace.

Many people think that a bigger furnace equals more efficiency and better heating. Nothing could be further from the truth. A heater that is too big for the space it is serving often short-cycles as a result.

The furnace heats the space too quickly. The result is frequent starts and stops of the furnace. Short-cycling of this type leads to a shorter life of the furnace and less efficient operation. It requires more energy to start the motors on your furnace than running them once they are operating.

A qualified HVAC technician can evaluate your home size and heating needs and make recommendations for properly sizing a furnace installation.

Running a furnace that is short cycling is never a good idea. It puts a strain on the furnace. More importantly, a short-cycling furnace is a symptom of bigger problems. Some of these problems are potentially dangerous for you and your family.

We hope that the information in this article helps you diagnose and remedy problems that cause furnace short-cycling. We encourage you to seek the help of qualified professionals for problems in most cases involving gas-fired appliances. Stay safe and stay warm.