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Troubleshooting and Repairing your Furnace

Updated Aug 5, 2016
By Gary Sprague

See also: Manufacturer's Defect or Installer Error: Who's to Blame When Your Furnace Dies Young?

Whether your furnace is conventional or high-efficiency, proper maintenance can give you years of problem-free comfortable heating. However, as with all complex machines, things occasionally go wrong. If you're handy, and you feel comfortable working on your furnace, you can solve many minor problems yourself.

However, if you feel over your head, you probably are. In this situation, it’s a good idea to contact a heating contractor to troubleshoot and solve your problem. Having a professional repair your furnace will keep you from making the problem worse and save you money in the long run. Many heating contractors today charge a flat fee for different service calls. For instance, a furnace cleaning is a certain price, a thermocouple replacement is a set price, etc. This eliminates overcharging and allows the homeowner to know the total cost of the job upfront.

You should also remember to have a qualified service technician perform an annual inspection and maintenance on your furnace.

The most common furnace problems are related to:

There are certain problems, particularly if you have a gas furnace and you smell gas, which you should not attempt to repair yourself. A leaking gas line can lead to injury or death. If you suspect a problem with your gas line, do not attempt to fix it. Instead, leave the house immediately and turn off the gas at the outside supply valve. Then call your utility company from a remote location.

Blower Problems

There are several possible causes of blower issues. The first thing to check if your furnace seems to be blowing cool air, or not enough air, is the filter. A dirty filter can block airflow, resulting in less air through the ducts while forcing the furnace to overheat and shut itself off. A dirty filter can be the cause of several furnace issues Cleaning or repairing a filter is usually quite simple and can be done by the homeowner.

A wall-mounted thermostat or the limit switch located on the furnace below the plenum can cause the blower to run continuously. The plenum is the box that distributes heated air to the ducts. If the air here gets too hot, the limit switch is set to shut off the furnace. Check to see if the fan switch on the thermostat is on. If it is on, turn it off or to auto. Also, if you have a digital thermostat, check the batteries. Bad thermostat batteries can cause the furnace to run continuously or not at all. If it is already set to off or auto and the batteries are fine, call a technician to adjust the limit switch.

Heat Exchanger

The heat exchanger is a sealed compartment inside the furnace where air is blown across a hot metal surface and then distributed throughout the house. When a heat exchanger fails or cracks, dangerous gasses such as carbon monoxide can leak into the house. Heat exchangers, like everything else, will wear out with time. A broken heat exchanger cannot be repaired; instead, it must be replaced by a heating professional. If the heat exchanger is not still covered under the warranty, replacing the entire furnace is often a preferable and more cost-effective option.

Circuit Board Errors

The circuit board, or control board, is basically the brain of a furnace. It controls ignition and blower functions as well as system diagnostics. As part of the diagnostics control, the circuit board has diagnostic indicators, usually flashing codes, to let the homeowner or service tech know what is wrong with the furnace. There is usually a chart on the inside of the front or access panel of the furnace which explains the flashing codes. Many problems can be fixed with a reset button or by changing a switch, but sometimes the problem is the board itself. A homeowner with experience with electronics can replace a circuit board, but it is usually a good idea to have it professionally done.

Noise Issues

Your furnace may make a variety of noises that don’t seem normal. Some are nothing to worry about, but others can indicate a problem. A squealing sound can be worn motor bearings, while a loud thumping noise is usually caused by a blower wheel that is out of balance. This should be repaired before it causes further damage to the furnace. If the furnace is making a humming noise but not running, this indicates a faulty fan motor or capacitor. Rattling from the ductwork is often caused by a loose hanger, which can be fixed by tightening any loose screws or adding duct tape to any loose pieces of ductwork. A loud bang when the unit turns on or off can be from the duct work popping in or out from pressure changes, or it could be a potentially dangerous ignition malfunction. The best thing to do if you are concerned about a sound from your furnace and can’t figure out the cause is to contact a heating professional. Chances are it’s a simple fix, and if not, you’ll be glad you called.

Ignition Problems

A furnace that will not ignite can lead to a very cold home, and nobody wants that. There are several possible causes for a furnace that won’t light. One reason is a lack of fuel. Be sure to make sure you have enough gas or oil. Sounds simple, but it’s very easy to overlook. Check that the power switch is on. If it is, check the master switch and circuit breaker or fuse. There's a possibility that the electrical system may have overloaded. If there is no circuit breaker trip or a blown fuse, the problem may be with a faulty thermostat or old batteries. If the pilot light does not light or remain lit, check the thermocouple to see if it is positioned properly in the pilot flame.

Most furnaces will provide dependable service for decades, if you provide them with annual maintenance, and you solve problems as they arise.

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