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Why is my heat short cycling?

Is your heating system turning off and on? Here’s how to easily diagnose the problem.

Is your heating system turning off and on? Here’s how to easily diagnose the problem.

If your home heating system is turning on and off again, and on and off again… well, first off, that’s annoying! 

Secondly, it’s called short-cycling, and it’s a major sign of other problems—and definitely not the behavior of a correctly functioning HVAC heating system.

So if you’ve been wondering, Hey, am I hearing things? Why does my furnace come on every 5 minutes?—rest assured. 

What you’re hearing is an actual problem. And you’re in the right place to figure out how to fix it.

In this article, you’ll cover:

Let’s dive in.

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How to tell if your furnace is short-cycling (we’ll cover boilers, too!)

The obvious first step is to understand what, exactly, short-cycling is.

Short-cycling occurs when your heater turns on, attempts to heat the room for a little while… and then immediately shuts off again.

A few minutes later, it’ll turn back on and do the same thing all over again. 

And again and again… and again.

On the surface, this may not seem like a big deal, right? After all, your home is probably getting heated—just in short bursts, rather than one long cycle.

But actually, short-cycling is a huge problem for two reasons:

  1. A short-cycling heating system is hugely inefficient—wasting both energy and money. (Learn more about energy waste in your house here.)
  2. It’s not how your heating system was designed to work, so it puts unnecessary wear and tear on your furnace or boiler (leading to breakdown, repair bills, added stress, general grumpiness, and fighting amongst household members).

Short-cycling puts unnecessary wear and tear on your heating system, which could lead to a breakdown.

So how many times should a furnace cycle in an hour? 

This depends on several factors, including the model of your furnace, but most furnaces are designed to turn on 2–3 times per hour, although some models cycle up to 8 times per hour (1).

If that seems like a lot of variance, you’re right. Luckily, there’s a pretty simple way to tell if your furnace is really short-cycling: Pay attention to the amount of time that your furnace stays on. 

If it’s working normally, a furnace should run for a decent amount of time (several minutes at least!) before it shuts off—while a short-cycler may only turn on for a minute or two.

Boilers, on the other hand, use very different technology than furnaces, and they typically have shorter cycles than furnaces.

Trying to stay cozy while you figure out your heater issues? Read How to stay warm in a cold house for 21 tips.

How long should a boiler cycle?

If you’re wondering if your boiler is short-cycling, a good rule of thumb is that one total cycle (that is, the time it takes the boiler to run plus the rest period afterward) should take at least 10 minutes in total (2).

But again, this will vary depending on the model and make of your boiler. Check the manufacturer’s directions for your model to be sure.

How to know if your heater is short-cycling: Pay really close attention

Finally, another way to tell that your heating system is short-cycling is simply instinct and experience.

If you’ve been living in your house a while and you notice your boiler or furnace keeps shutting off much more rapidly than it used to—well, you’re probably on to something.

And in that case, it’s time to figure out what’s going on. To do that, we’ll break down some common causes of short-cycling—and for clarity’s sake, we’ll separate those reasons by technology.

First, we’ll start with furnaces. 

(Don’t have a furnace? Tap here to jump directly to the boiler section.)

How to fix a short-cycling furnace

Here are the most common reasons for short-cycling furnaces. We’ll cover the fixes for each below.

  1. Your thermostat isn’t working correctly (or it’s placed incorrectly)
  2. Your furnace isn’t the right size
  3. Your furnace has a clogged or missing filter
  4. Something is blocking the airflow to your furnace
  5. The blower motor is broken
  6. The ignition isn’t working properly (gas furnace only!)
  7. Your flame sensor could be corroded (gas furnace only!)
  8. Your home is losing heat via air leaks

Your thermostat isn’t working correctly (or it’s placed incorrectly)

Can a bad thermostat cause your furnace to short cycle? Absolutely. In fact, if your furnace is short-cycling, your thermostat is one of the first things you should check. 

If it’s not working correctly (or if it’s not placed in the right spot), it could be feeding wrong information to your central system—and that could easily cause your furnace to turn on and off again (and on and off again).

The solution: 

Check for easy solutions first by making sure the thermostat has working batteries. If they’re weak, well… you know what to do!

If changing the batteries doesn’t work, move on to recalibrating your thermostat. (You can find instructions for how to do that in your thermostat’s user manual.)

Or—and this fix is more complicated—it’s possible that your thermostat isn’t placed optimally to keep your home or that it might be time for a new thermostat altogether.

Estimated cost to fix thermostat:

Replacing batteries: $0-$15

Recalibrating your thermostat: Free, if you’re the adventurous sort

Replacing your thermostat: $50-$250+, depending on who installs it and what kind of thermostat you get. We recommend getting a smart thermostat if it’s time for an upgrade. Read how smart thermostats save energy here.

On average, the Nest Learning Smart Thermostat saved 10% to 12% on heating costs and 15% on cooling costs.

Nest Thermostat Study

Your furnace isn’t the right size

This is where the age-old Goldilocks principle comes into play—a furnace can’t be too big, or else it will heat your home too quickly. And it can’t be too small, or it will have to work extra hard to heat your home.

Your home heating system has to be just the right size. 

And if it isn’t, that could easily lead to short-cycling. Unfortunately, this is a common problem: Many furnaces are incorrectly sized when they’re installed (which leads to all kinds of heating problems down the road).

The solution: 

If your furnace is the wrong size, it needs to be replaced with a correctly-sized model.

Estimated cost if your furnace isn’t the right size: 

$3,000-$6,500+, depending on the size of your home and which unit you choose. 

(Keep in mind that, if your house qualifies, you can replace your furnace with a super-efficient heat pump for NO upfront cost.)

Your furnace has a clogged (or missing!) filter

A clogged furnace filter can restrict airflow—and when airflow is restricted, that can cause your furnace to overheat and shut down prematurely (i.e., short-cycling).

If this is the problem, you might start to notice that the house doesn’t warm fully before the furnace shuts off.

The solution: 

This one is easy: Just replace your filter with a new one! We recommend that you do this at least every 90 days anyway (though make sure you check your user manual to see what the manufacturer says).

The basic steps of filter replacement are pretty easy, too. Simply locate your furnace’s filter compartment, remove the old filter, and insert the new one—with the arrow pointing toward the furnace.

Estimated cost to fix a clogged furnace filter: 

$10–$50 per filter, depending on the size and type

Something is blocking the airflow in your furnace

When it comes to airflow, the filter isn’t always the problem. If there’s something else blocking the airflow in or around your furnace (say, a bird’s nest near an exhaust vent), that could cause it to overheat and short-cycle.

The solution: 

If you’ve already replaced the filter, the best way to solve this problem is to call a professional HVAC technician. They’ll be able to take a look at your furnace and figure out what’s causing the airflow issue.

Estimated cost to fix furnace airflow: 

$75-$200+ depending on the problem and how long it takes to fix it.

The blower motor is broken

The blower motor is responsible for circulating air throughout your home—and if it’s broken, that can cause your furnace to overheat and shut down prematurely.

(You’ll be able to tell it’s the blower motor because your furnace will be running, but no air will be coming out of the vents.)

The solution: 

If the blower motor is the problem, you’ll need to replace it. Frankly, this is a job best done by professionals, though you might be able to find the DIY instructions in your furnace’s user manual. (Our advice: Don’t go there. You want this done right.)

Estimated cost to fix a furnace blower motor: 

$450-$2000+, depending on the furnace model and who does the work (3).

The ignition isn’t working properly (gas furnace only!)

Have a gas furnace? If so, have an HVAC tech check the ignition system.

If the ignition—which is responsible for igniting the gas in your furnace—isn’t working properly, it could absolutely cause your furnace to turn on and off again.

The solution: 

This is one for the HVAC techs! If you suspect something is up with the ignition, give a professional a call so they can clean or replace it.

Estimated cost to fix gas furnace ignition: 

$150+, depending on the cost for the service call and any additional parts or service needed.

Your flame sensor could be corroded (gas furnace only!)

The flame sensor sensor is responsible for telling your furnace when to ignite the burner.

If the sensor is corroded, it may not be able to do its job properly, which can cause the furnace to turn on and off repeatedly.

The solution: 

The best way to diagnose (and then solve!) this problem is to clean the flame sensor with a wire brush and see if the problem goes away. If that doesn’t work, you’ll need to replace the sensor.

Estimated cost to fix a gas furnace flame sensor: 

$0-$250—depending on whether you’re cleaning or replacing (as well as who is doing the work!)

illustration of a furnace and heat pump demonstrating the best furnace replacement is a heat pump

Tired of dealing with a malfunctioning furnace?

You could get a high-performance heat pump at no upfront cost with Sealed.

And we can help optimize your home to keep all that paid-for heat inside, which also makes your new heating system a lot last longer.

Learn how to stop heat escape and heat your house efficiently with powerful energy upgrades at no upfront cost.

Heat pumps are up to 3 times more efficient than traditional heating systems.

Energy.gov

Also, there’s one more reason your furnace could be short cycling, and it’s a problem in the majority of American homes: air leaks.

(Tap here to jump ahead and read how air leaks can contribute to short-cycling.)

How to fix a short-cycling boiler—a troubleshooting guide

Now, let’s go over some reasons why your boiler might be malfunctioning—because boilers are very different technology than furnaces.

Don’t have a boiler? Tap here to skip this section.

Here’s why your boiler might be short-cycling:

  1. The steam isn’t being separated from the water
  2. There’s excess dirt or a clog
  3. Steam traps aren’t working
  4. Clogged (or a compromised) pressuretrol
  5. Dirty condensate trap
  6. The boiler pressure is just too high
  7. Your home is losing heat via air leaks 

The steam isn’t being separated from the water

If the steam in your boiler isn’t being properly separated from the water, the steam condenses—and ultimately causes the boiler to shut down prematurely.

This is often a problem with the piping around your boiler, and it’s pretty difficult to diagnose on your own (unless you’re secretly an HVAC expert). (5)

The solution: 

In this case, you’ll need to call a professional HVAC technician. They’ll be able to take a look at your boiler and figure out what’s causing the problem.

Estimated cost to fix a steam issue in boiler: 

$75-$200+, or the amount for a service call in your area plus any additional parts, labor, or repairs needed.

Excess dirt or a clog

If the boiler is short cycling, it may be due to some kind of dirt buildup. One common version of this problem is a clogged air intake, which can prevent the boiler from getting enough air to ignite the fuel.

The solution:

If you’ve got time and your hands and love YouTube HVAC tutorials, you may be able to find and clear the clog yourself (usually with a vacuum cleaner or a pipe cleaner).

In general, though, if you suspect some kind of clog, we’d recommend calling in a pro.

Estimated cost to fix a boiler clog: 

$0-$300+, depending on the type of clog and whether you hire a professional to do the work.

Steam traps aren’t working

Steam traps can malfunction in a number of ways, causing the boiler to short cycle. One way is for the trap to fail to release condensate, which can cause the boiler to overheat.

Another way is for the trap to release too much condensate, which can cause the boiler to lose its firing rate and shut down.

The best way to tell whether the steam traps are the problem? Check the water level in the boiler: If it’s too low, that’s a sign that the traps aren’t releasing enough condensate. 

If the water level is too high, that’s a sign that the traps are releasing too much condensate.

The solution: 

You’ll need to have a professional HVAC technician take a look at your boiler for this one.

Estimated cost to fix steam traps in boiler: 

$75-$200+, or the amount for a service call in your area plus any additional parts, labor, or repairs needed.

Clogged (or a compromised) pressuretrol

A pressuretrol is a device that measures the pressure of the water in a boiler and signals the burner to turn off when the correct pressure is reached. The word, by the way, is a combination of “pressure” and “control.” 

(It is not, as commonly assumed, an extremely stressed bridge troll.)

Anyway! If the pressuretrol is clogged, the burner will turn off too soon, causing the boiler to short cycle.

You can sometimes tell whether it’s the pressuretrol that’s the problem by checking the water pressure at the boiler.

If the water pressure is low, it’s possible that the pressuretrol is clogged and needs to be cleaned or replaced.

A pressuretrol is a device that measures the pressure of the water in a boiler and signals the burner to turn off when the correct pressure is reached.

The solution: 

Sometimes it’s possible to clean the pressuretrol mechanism (especially the piece of “pigtail” piping that leads up to it).

But if you’ve cleaned everything and you’re still having pressure issues, it’s time to replace the pressuretrol.

Estimated cost to fix a clogged boiler pressuretrol: 

$75-$200+, or the amount for a service call in your area plus any additional parts, labor, or repairs needed.

Dirty condensate trap

A condensate trap is a device that prevents water from back-flowing from the boiler.

If the trap is dirty, it will not function properly and the water will flow back into the house, causing the boiler to short cycle.

The solution: 

To determine whether the condensate trap is causing the problem, remove it and give it a good cleaning. (Be sure the boiler itself is shut down completely first!).

You can find the condensate trap by tracing the condensate line away from the boiler—the condensate trap looks kind of like an upside-down U.

Estimated cost: 

$0-$75, depending on whether you hire a professional to do the work (which you really should do).

The boiler pressure is just too high

A boiler that’s cycling too frequently (short cycling) is often the result of high pressure.

A boiler’s pressure should never go any higher than 3 bar (about 30 PSI). 

So when the pressure builds to an unsafe level, the boiler will automatically shut down—which means, you guessed it, a shorter cycle.

A boiler’s pressure should never go any higher than 3 bar (about 30 PSI). 

The solution: 

First, check the pressure gauge on the boiler. If it’s showing a higher-than-normal pressure, that’s a good indication that high pressure is the problem.

Another thing to look for is water leaking from the boiler. Evidence of water leakage can be an indication that the pressure is too high and the boiler is starting to overheat.

How to fix the problem? Call a professional. This is a safety issue, and it’s not something you want to try to fix on your own.

Estimated cost to fix too-high boiler pressure:

$75-$500+—depending on the reason the boiler’s pressure is too high.

Air leaks—an issue that affects both furnaces and boilers

Finally, we have to address one of the biggest reasons that furnaces and boilers short cycle: Air leaks.

Here’s how air leaks cause a short-cycling heater: 

Your heater turns on and heats your house to the required temperature. And hot air escapes within minutes through the hundreds of gaps and cracks in the home (often around doors and windows, as well as in the attic and the foundation). The house is cold again, so the heater is forced to turn on again—way too quickly!

This is an incredibly common issue, by the way. Most houses in the U.S. have tons of air leaks (especially large, old houses with under-insulated attics).

For a quick visual of how this happens in a house, check out the video below.

The solution to losing heat through air leaks: 

Hire a professional to seal up all the tiny gaps and cracks in your house by weatherizing your home from top to bottom. That way, warm air can’t escape.

Estimated cost for air sealing alone: 

Between $5,000–$12,000, depending on your home size (or you can get it done for NO upfront cost through Sealed).

Check out our Complete Guide to Air Sealing to learn why air sealing is so important and how it can make your home more comfortable year-round.

It’s estimated that homeowners can reduce energy use up to 40% through air sealing and insulation upgrades.

NAIMA 2022 study

Repairs too costly? Here’s when you should replace your heater—and the best system to replace it with

We’ll cut to the chase: If your heating system is acting up consistently—and it’s 10-ish years old or older—you may be past the point of repairs. It’s time for replacement.

(Especially if you’re constantly asking yourself, “Why is my house so cold?”)

Don’t panic, though! Today’s heating technology is so much better—and much more efficient—than the home heating systems that were being installed a decade ago.

cold climate heat pump outdoor compressor unit on the side of a home

A prime example of that is the cold climate heat pump, which is by far the best heating system for most homes, and uses up to 3x less energy than a traditional heating system.

In fact, installing a heat pump is one of the most energy-efficient home improvements you can make to your home.

Learn more about whole-house heat pumps here.

(If your home heater runs on fossil fuels, check out converting from oil to electric heat pump or converting from gas to electric heat pump to discover how to make the switch.)

Ultimately, if you’re replacing your heating system in 2022-2023, you should at least consider a heat pump. They’re the best option for most homeowners. 

(And believe us—we’re HVAC nerds over here. When there’s a home heating option out there that’s better than a heat pump, you better believe we’ll write 20 articles to let you know all about it.)

At Sealed, we’ve seen homeowners cut energy waste by up to 50% with air sealing, insulation, and heat pump HVAC upgrades.

Get a home heating makeover—for NO upfront cost

More good news about heat pumps: You may be able to replace your aging heating system with a heat pump—for NO upfront cost.

How? 

If you qualify for Sealed, we’ll give your home a complete comfort and energy makeover—we’ll put in all the work (and upfront funds) to make your house feel amazing and keep paid-for heat inside your home. 

Ultimately, your home upgrade plan is completely customized—it all depends on your preferences as well as what your house needs.

You’ll get the improvements you need—and nothing you don’t.

(We’ll also help you understand and apply all the home energy rebates and utility incentives you might be eligible for—and, in the wake of the Inflation Reduction Act, there might be several that apply to your situation…. including heat pump tax credits!)

Learn about the Sealed payment program.

Read to see if your house qualifies? Take our 2-minute quiz now.


November 25, 2022