Why Is My Furnace Overheating?

Is your furnace turning off and on all the time? Or do you smell a strange odor near your furnace?

If so, those are signs that your furnace is overheating.

As you may have already guessed, an overheating furnace is highly problematic. Besides potential safety risks, overheating also damages essential parts in the furnace, leading to expensive repairs.

So, what causes a furnace to overheat in the first place?

The following issues are the most common culprits of an overheating furnace:

  • A dirty air filter
  • Closed or blocked vents
  • A dirty or faulty blower
  • Electrical malfunctions

We’ll go into more detail about each of these issues below.

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A dirty air filter

The most common cause of an overheating furnace is also the easiest to fix: a dirty air filter.

We’ll explain how a dirty air filter leads to overheating.

For the furnace to properly function, it needs to take in a constant supply of air. Cool air coming into the system helps keep the heat exchanger (the part that heats air) at a safe temperature.

The heat exchanger, located in the heart of the furnace

A clean air filter allows air to enter the furnace unrestricted. However, air will struggle to reach the heat exchanger with a clogged air filter. The reduced airflow entering the system causes the heat exchanger to get incredibly hot, triggering an internal safety switch to shut down the furnace.

Besides an overheated heat exchanger, a dirty air filter also causes a host of other issues, including:

  • Increased energy bills
  • Wear and tear on furnace components
  • Overheated blower motor

To avoid these issues, we recommend that you check your air filter every month during the winter. If the air filter is dirty, you should replace it with a new one.

Clean (left) vs dirty (right) air filter

Closed or blocked return vent

The return vent is where air enters the furnace. Similar to a dirty air filter (which is often located directly behind the return vent), a closed or blocked return vent will restrict airflow into your system. As we described above, restricted airflow can lead to an overheated furnace and other serious issues.

A return vent, with a dirty air filter behind it

You can typically find return vents at the lowest point of a wall. Sometimes people accidentally block the vent with furniture or unknowingly close it.

Look for the return register (some homes may have more than one) and make sure it is unobstructed. You can quickly tell if your return vent is correctly working by turning on your furnace and holding a piece of paper next to the vent. If the paper drifts towards the vent, then the vent is open.

A dirty or faulty blower

The blower pulls in cool air from your home and draws it over the heat exchanger. After the air is heated, the blower pushes the warm air back into your home.

That said, if the blower is dirty or malfunctioning, the furnace will struggle to bring in sufficient air, which causes the system to overheat. You may have an issue with the blower if you notice loud sounds or strange smells when your furnace is running.

If you suspect the blower is dirty or faulty, you’ll need to contact a furnace technician to inspect the furnace. They can quickly clean the blower and fix any problems that could be causing it to malfunction.

Electrical malfunctions

A furnace has several electrical components that can malfunction and cause the furnace to overheat.

The most common electrical issues that cause overheating include:

  • A malfunctioning blower motor
  • Loose or faulty wiring connections
  • Broken temperature-monitoring sensors

If you’ve checked your air filter and return vent, but the overheating problem persists, there may be an electrical issue that a professional will need to fix. Only a licensed electrician should fix electrical issues for safety reasons, so you should contact a certified furnace professional to diagnose and fix the problem.

Want a Portland professional to repair your furnace?

Call Four Seasons today at 503-538-1950. We provide convenient scheduling times and fast, same-day service for every repair job.

Visit our furnace repair page to learn more about what to expect when you work with us.

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