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Troubleshooting a Furnace Limit Switch

If you're having trouble with your furnace and you suspect it's the furnace limit switch, read on. We'll give you some ideas of how to troubleshoot the switch on your own. Narrowing down the likely problems can save you some money when you call in a professional HVAC installation and service company.

A Word of Warning
Gas heaters and furnaces do have automatic shut off switches and safety devices built in to avoid gas leaks. That said, they are not fail safe. If you notice an odor of gas in your house, don't attempt to troubleshoot the furnace. Avoid turning on lights and don't try to shut off the gas supply to the furnace. Leave your home, leave the door open for ventilation and call the gas company or the local fire department. Don't re-enter your home until the gas company or the fire department has informed you it's safe to do so.

What is a Limit Switch?
The limit switch is the device that tells when to turn the hot air blower on and off. The limit switch keeps cold air that is not fully heated from blowing into the house. It also acts as a safety device by turning off the burner if the temperature of the warm air supply gets too hot. In this way it works to keep your heat exchanger from cracking from too high of a temperature.

Troubleshooting Basics
The furnace limit switch is generally found on the furnace just below the plenum. If the plenum reaches too high a temperature, the limit switch shuts it off. It also shuts the blower off once the thermostat indicates that the desired temperature has been reached. If you find that your blower is running constantly however, either the blower has been set to be always on or your limit switch is faulty or needs adjustment. Check your thermostat first to be sure that the blower hasn't been set to the ‘on' position. If the blower is already on ‘auto' instead of ‘on' then chances are your limit switch needs attention.

To adjust the switch, go to your furnace and find the limit switch and remove the cover. Underneath you'll find a dial with one side marked LIMIT and the other side marked FAN. Don't adjust the limit side – only adjust the fan side. There should be two indicators on the fan side – one at a higher temperature setting and one at a lower one. They should be around 25 degrees apart. Try setting the upper one to 115 Fahrenheit and the other to 90.

When All Else Fails
If adjusting the limit switch doesn't fix the issue it's time to call in a local HVAC service professional who can help you further troubleshoot and repair the problem. It could be a number of things from a faulty limit switch to a blower that is on the fritz. Most licensed professionals will be able to quickly identify and address the issue and can save you money over the long run versus doing it yourself. When a service company does the work you also have the peace of mind knowing that you have a service warranty to fall back on in case the repair isn't effective.

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