How to Replace the Fan Limit Switch in a Furnace

Does the fan on your furnace keep running but you're not getting any heat? You may have a problem with your limit switch.

There are many common furnace problems that can make the fan keep running. This is because when the furnace defaults, the fan can be used to help reduce risks caused by the problem. In this case, it would help to cool down the furnace if it has overheated. The problem with that here is that if the limit switch is bad, it is sending a false message to the furnace and causing it to believe it is overheating when really it's not.

Often times when a furnace malfunctions, it will run the fan for a minute and then try to restart itself in case it was just a one time glitch in the system. It will try to start usually 3 times before going into a "lockout". After 3 times, the furnace figures there's a problem and it shouldn't try to run again. The "lockout" will last a while then it will try, and try, and try again before locking back out.

The following will tell you what a limit switch is for and how to replace it. This information will also offer some photos of a couple common style limit switches and tips on finding the right replacement.

A furnace's limit switch or fan/limit switch has 3 main functions.

1. Tell the fan when to come on. When the furnace is warm enough upon start up to begin heating the home, (about 20 degrees above room temperature) it will tell the fan to come on. (temperature setting #1) This helps to avoid cold air drafts.
2. Tell the fan when to shut off. Once the thermostat is satisfied, the gas shuts off but the fan will continue to run until the furnace's temperature is cooled to a certain temperature. (temp. setting #2) This helps to make use of residual heat that is left in the system after the gas shuts off but not too long to again avoid cold drafts.
3. Tell the furnace to shut down because of overheating. Let's say the flames are burning but the furnace fan breaks down...there would be nothing to push the heat out of the unit and thus creating extremely high and potentially dangerous temperatures in the furnace. At the "high limit" setting (temp. setting #3) the switch would "trip" and cut power to the gas valve thus eliminating the danger of fire or explosion.

These three functions are based on 3 different temperature settings. Today, these are factory set and unchangeable without changing the limit switch which is a bad idea. This is because the temperatures are based on the furnace's capabilities and ratings. Playing with these temperatures can cause your furnace to run less efficiently, possibly again damage the limit switch, or worse, let your furnace get much hotter than it was designed to be. You can see how this would be bad, right? Older models had a much different style of limit switch which had adjustable temperature settings but they are not used in modern style furnaces.

Furnaces these days are very intelligent. Most modern furnaces have a "spyglass" that you can look through and see a blinking light. This blinking light is saying, "Hey, this is probably what's wrong with me" by blinking you a sort of Morse Code. Dash, dot, dot may mean the number 12 and if you locate the code chart on your unit it might say, "Limit switch is open" or "Ignition failure". Now it doesn't tell you what happened but it does point you in the direction of the problem. This is quite handy when troubleshooting your furnace.

As always, when looking for parts for anything really, it's important to have as much information as you can about the furnace. Model number, serial number, voltage/amperage ratings, etc... Most all of this information will be located on the rating plate of the unit or part. You may not need it all but if you do, you have it and won't be making a second trip to the hardware store. You're also more likely to get respectable attention from your local supplier who may very well be used to dealing with contractors. Often, wholesale suppliers get frustrated with homeowners because they don't realize what information is needed and then get upset with the wholesaler for asking so many questions. This isn't like buying pencils. The proper information is needed to protect you and your HVAC equipment.

With limit switches, the temperature range will be an important piece of information as well as with the older ones, the length of the rod that protrudes from the back. The best thing to do with this part is to bring the old one with you to the dealer. I say dealer because you won't likely find these in your big box home improvement stores. Now if you're shopping online, be sure you read all the specifications carefully and compare them to yours. Don't guess and don't vary or you will be wasting your money, time and probably, more money since you will be buying another part or worse, damage other parts in your furnace by using the wrong limit switch.

Once you've found the correct replacement, replacing the limit switch is quite easy really.

First, be sure you've shut of the power to the furnace either with the furnace service switch or at the breaker. These are the best ways to insure the power to the unit is off.

Now that the power is off, you simply remove the old limit switch and wire the new one back on the exact same way. With the new switches, this is even easier because there are likely only 2 wires coming off of the switch and 2 screws holding it in place. These 2 wires will also likely be held on by spade terminals which are very easy to work with.

The older limit switches are a bit trickier because there will probably be more than 2 screws holding it in, 3 wires to reconnect and there may also be a tab on the switch that you will have to read your installation instructions to determine whether it is to be snapped off or not. Be sure to look at your old one to see if it was an option on it as well and you could copy what you see. Even at that, be sure to read your instructions for your new part and be sure that the settings on the dial are the same as the old.

NOTE: The older switches are a bit more complex so be sure you're comfortable in what you're doing. This may be one that you'll want to turn over to a professional if you're not sure about what it is you're doing.

Once the part is back in place, turn your power back on and set the thermostat so that it calls for heat so you can see that your furnace is working again.

I hope this hub has helped or will help you when the time is needed and that you will have that furnace back up and running in no time. Remember, don't guess or assume anything when it comes to your furnace. When in doubt, call a professional to take a look at it and know that you'll feel better knowing about the services you're being given.