Why Won’t My Furnace Stay On? A Pittsburgh Tech Answers

Do you hear your furnace turning on and off frequently?

If so you’re probably frustrated—and rightfully so.

The reason your furnace is shutting off is usually due to 1 of these 4 common problems:

1) Overheating issues

2) A dirty flame sensor rod

3) Clogged condensate line

4) Oversized furnace

We’ll explain how each of these 4 problems cause your furnace to shut off, and we’ll share some quick DIY fixes to try before you call a pro.

Ready for your furnace repair?

Problem #1: Overheating issues

If your furnace starts to overheat, a safety device called the limit switch will turn off your furnace to protect it from damage.

What causes a furnace to overheat?

Overheating problems can almost always be traced back to a lack of airflow.

You see, if your furnace isn’t pulling in enough cold air to blow over the heat exchanger (the part that heats your home’s air, pictured below), then the heat exchanger will start to get really hot. Once the heat exchanger reaches a high temperature, the limit switch will shut off your furnace.

Diagram of a furnace

The most likely culprits of low airflow (and therefore overheating) include:

  • A dirty air filter
  • Blocked vents
  • A dirty heat exchanger
  • Malfunctioning blower motor

2 quick DIY fixes

  1. Make sure all vents are open and that nothing is obstructing them.
  2. Check your air filter. If it is dirty, replace it with a clean one.

If your air filter is clean and your vents are open but your furnace is still shutting off, you’ll need a professional to inspect the heat exchanger and blower motor to see if those components are causing your furnace to overheat.

Problem #2: A dirty flame sensor rod

If your flame sensor rod is covered in soot, it could cause your furnace to short cycle (turn on and off frequently).

The flame sensor rod, like its name suggests, detects when there is a flame in your furnace burners. If the sensor rod doesn’t detect a flame, it closes the gas valve as a safety measure.

Normally, this is a good thing. But if the flame rod gets covered in soot, it can’t accurately detect when there’s a flame and it can shut down the furnace when it’s not supposed to.

Since cleaning the flame sensor rod isn’t an easy DIY task, we recommend having a professional clean it for you.

Problem #3: Clogged condensate line

If you have a condensing* furnace, your furnace may be shutting off because the condensate line is clogged.

You see, condensing furnaces produce condensate as part of their normal combustion process. This condensation gathers in a collector box then drains into a line with a P-trap. The drain line carries the condensation outside or into a floor drain.

However, dirt and debris can get into the P-trap and clog the line, which backs up water into the condensate collector box.

If a lot of water backs up into the condensate box, it will likely trip a safety device called the pressure switch, which will shut down your furnace to prevent damage to your system.

A professional will need to unclog the condensate line so that your furnace will stay on.

*Not sure if you have a condensing furnace? Look for two white PVC vent pipes coming out of the side or top of your furnace. If you see a metal vent pipe instead, you have a conventional (non-condensing) furnace, which means this will not be your problem.

Problem #4: Oversized furnace

Has your furnace been struggling to stay on ever since you first had it installed?

If so, your furnace may be too big for your home.

You see, a furnace that’s too big will heat your house really quickly then shut off. Over time, this constant on and off will wear out your furnace components faster and lead to uneven heating throughout your home.

When a tech inspects your furnace to find what’s preventing your furnace from staying on, they’ll check to see if your furnace is the correct size for your home. If it turns out this is your problem, you may need a smaller furnace.

Ready for your furnace repair?

We’ll send one of our trusted techs to diagnose the problem and fix your furnace.

Learn more about what to expect when you hire us by visiting our furnace repair page.