My Gas Furnace Cycles On and Off Frequently

When your furnace turns ON and OFF frequently we call this “short cycling”—and short cycling is not good for your furnace or your wallet (more on that later).

So what makes a furnace short cycle?

Well, the reasons behind furnace short cycling can be lumped into 3 general categories:

1. Overheating issues
2. Thermostat problems
3. An oversized furnace

We’ll explore each of these problems and what you can do to fix them. But first, let’s look at why short cycling is so harmful in the first place.

Short cycling causes 3 major issues:

1. Furnace damage—Turning on and off frequently puts stress on parts in your furnace, causing them to wear out faster. And furnace repairs can get expensive, so this will end up costing you a lot of money down the road.
2. Expensive energy bills—Larger motors, like your furnace fan motor, use more energy starting up and shutting down than they do when they’re running. So a furnace that’s constantly turning on and off works much harder to heat your home and uses more energy, which raises your monthly utility bill.
3. Discomfort—Because your furnace is turning on and off all the time, it won’t run as long, which means you’ll have uneven temperatures throughout your home.

​Now that you know how harmful (and expensive) short cycling is, let’s take a closer look at what’s causing the problem and what you can do to fix it...

When your gas furnace’s heat exchanger—the part that heats your home’s air—overheats, your furnace will shut down to protect it from further damage. After the heat exchanger cools off, it’ll turn back on and continue in an endless cycle of overheating and then shutting off.

And if your furnace overheats over and over again, you run the risk of ruining your heat exchanger (spoiler alert: replacing a heat exchanger can cost up to $2000).

The main causes of overheating are:

1] A dirty filter—A clogged air filter blocks air from entering your furnace, which means the heat exchanger will retain heat and eventually overheat.

If your filter looks like the one on the right, it’s dirty and you should change it.

Here’s what to do: Change your air filter if it’s dirty.

2] Closed or blocked air vents—When you close or obstruct your vents, pressure increases in your air ducts, which makes the blower circulate less air. Low airflow means the heat exchanger can’t transfer enough heat, and when that heat builds up, it could cause your furnace to short cycle.

Keep your supply vents (top) and return vents (bottom) open and unobstructed

Here’s what to do: Make sure you keep ALL your vents open—even in unused rooms.

3] Blocked exhaust vents—A gas furnace exhausts certain gases during normal use. These gases escape outdoors through an exhaust vent called a flue pipe. If the flue pipe gets blocked by debris or leaves, those hot gases will return to the furnace and cause it to overheat.

If you have a blocked exhaust vent, you’ll also be at risk for carbon monoxide, a dangerous, odorless poisonous gas that can cause serious health problems and even death.

Exhaust air is carried out of your home through the flue pipe (red arrow). Source: www.basc.pnnl.gov

Here’s what to do:Have a professional check your flue pipe and clear it if it’s blocked.

Your thermostat controls your entire heating/cooling system

Your thermostat is essentially the brain of your heating system. It controls when your furnace turns on, and to what temperature it heats your home’s air.

If your thermostat is malfunctioning or broken, it may cause your furnace to short cycle.

Here’s what to do:

Step 1: Check the thermostat screen. If the screen is blank or seems glitchy, try replacing the batteries. If this doesn’t solve the problem, contact a professional to inspect and repair your thermostat.

Step 2: Check the thermostat location. Is it close to a heat source? If your thermostat is near heat it skews the temperature reading on the thermostat, which could make your furnace turn on and off irregularly.

If you notice your thermostat is close to a heat source, contact a professional to remove it and reinstall it in a better location.

Step 3: Have a professional inspect your thermostat for wiring issues. Sometimes the wiring gets old and causes the thermostat to malfunction.

Did you get a new furnace recently? If so, it may be too big for your home’s heating needs, which could cause short cycling.

You see, furnaces are sized according to how much heat it can put out in an hour. So, if your furnace is “too big” it will quickly heat your home to the desired temperature then shut off abruptly. Once your home starts to get cold, it’ll repeat the short cycling process, which can seriously damage your furnace.

If the HVAC tech who installed your furnace didn’t perform a Manual J Heat Load, there’s a good chance your furnace isn’t sized correctly for your home. A Manual J Heat Load calculation basically measures how much heat your home really needs and takes into account many factors like your home’s square footage, insulation and ductwork.

Here’s what to do: Have a professional perform a Manual J Heat Load calculation to determine if your furnace is the correct size.

Unfortunately, if your furnace is too big, you may need to replace it with one that’s sized correctly for your home. So, while the cost to replace a furnace might seem daunting, it will likely save you thousands in the long run.

Contact MSP Heating & Air to schedule an appointment with one of our trusted furnace technicians. We’ll fix whatever is causing your furnace to short cycle so you can have normal heat again.