preparing your air conditioning unit for summer use

If your furnace runs for a while and then shuts down, there are many possible causes. In some cases, your furnace may even be suffering from multiple issues at once. Fortunately, you can often get a rough idea of where the problem lies by checking your furnace's error display, typically a single LED that blinks several times to indicate a trouble code.

One common code you might encounter is an "over-limit" code. This code indicates a serious problem that will cause your furnace to shut down, but what does it mean, and what can you do to get your furnace running again?

Understanding Over Limit Conditions 

An "over-limit" error may go by several different names, depending on your furnace's manufacturer. Other typical names for this code are "high limit" or "high temperature" errors. Whatever the name, it means that your furnace detected an unusually high temperature near the heat exchanger. While your furnace's job is to generate heat, it's important that it doesn't get too hot. 

The metal can crack if your heat exchanger exceeds its designed temperature ratings. A cracked heat exchanger will burn inefficiently and may even release toxic gases into your home. A special sensor known as a "limit switch" detects an overheating condition and shuts your furnace down to prevent damage.

Overheating in a furnace is typically the result of poor airflow. The furnace relies on air moving across the heat exchanger to keep it cool. As a result, airflow restrictions can cause the metal to heat up, expand, and crack. A faulty limit switch can also cause false alarms that may force your furnace to shut down repeatedly.

Fixing an Overheating Furnace

If you see one of these error codes on your furnace, don't panic. First, shut the furnace down and don't attempt to run it again. While the limit switch will protect your furnace from immediate damage, repeatedly running an overheating furnace can still stress the heat exchanger. Allowing your furnace to short cycle in this manner may ultimately ruin your heat exchanger.

Next, check your air filter and replace it if it seems dirty. If you aren't sure, it might be a good idea to go ahead and replace it anyway. Air filters are relatively cheap, and dirty filters are a common source of airflow restrictions, so replacing yours can be a cheap diagnostic option. Once you've installed your new filter, restart the furnace and see if it still overheats.

If these steps don't resolve the problem, it's time to call in a pro. Do not attempt to bypass or replace the limit switch yourself since this component is a critical safety feature. An expert will be able to determine if the problem is the switch or some other issue in your system, ensuring a fix that will get your furnace running without causing additional damage to your heat exchanger.

Contact a furnace service to learn more.