My Lennox Furnace Ignites & Stops

Dirty Filters

• Dirty filters are one of the most common causes of short cycling. As filters become increasingly clogged, hot air has a hard time getting through. This air backs up and overheats your furnace, causing it to shut down shortly after starting up. In most households, furnace filters should be cleaned or replaced every three months or so to keep your furnace running at peak efficiency.

Blocked Registers

• Blocked registers can also potentially cause short cycling. Some homeowners close some of the registers in their house to save money or direct hot air to certain rooms. However, if more than 60 percent of the registers are closed, the hot air has no place to go and your furnace can overheat and shut down. This can eventually lead to serious damage to your furnace, so make sure the majority of your registers are open. In some cases, registers may also be blocked by furniture, which can add to the problem.

Clogged Ducts

• If your ducts have not been cleaned in a long time, they may be clogged with dirt, dust and the countless other particles that are carried through them with the air. The dirtier ducts get, the less room there is for hot air to pass through. This can overheat your furnace and cause it to repeatedly turn on and off. If your ductwork is accessible, you can check it yourself to see if it needs a cleaning. If not, you may need to contact a professional.

Considerations

• Most Lennox furnaces run on gas, but oil-burning models are also available and may be prone to a slightly different set of problems. Each furnace is different. For information on your particular model, consult the directory of use and care manuals on the Lennox website (see Resources).