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How To Stop a Boiler From Short Cycling (DIY Guide)

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Boilers use water to generate heat, but if your boiler keeps turning itself on and off before your building warms up, it may have a short cycling problem. System clogs, faulty thermostats, or improper installation can cause short cycling. To fix it, you’ll need to identify the root cause. 

Here’s a DIY guide on how you can stop a boiler from short cycling: 

  1. Clean or replace boiler air vents.
  2. Clear out blocked pressuretrol lines.
  3. Fix your boiler pressure levels.
  4. Replace faulty steam traps.
  5. Replace an oversized boiler.
  6. Move the location of your thermostat.
  7. Replace a broken thermostat.

So, let’s discuss ways that you can identify what’s causing your boiler to short cycle. I’ll give you all the details and help you fix your boiler so that you can stay warm and keep your boiler running. 

How To Tell if a Boiler Is Short Cycling

Boilers create a high-pressure, hot environment where water can boil. As the water in your boiler heats up, it can generate enough steam to push the water up through metal pipes, heating an entire building or room. 

When your boiler shuts itself off, then turns itself back on every one to five minutes, it “short cycles.” Short cycling indicates that your boiler is malfunctioning. 

When your boiler is working, it should run for at least 15 minutes before it turns itself off. If you notice that it’s shutting down after running for less than 15 minutes, you have a short cycling issue. 

Boilers with a short cycling issue turn off, then on, then off again in a short period. Your boiler won’t create enough pressure in the water or steam reservoir to warm up your building when this happens. 

Is a Short Cycling Boiler Dangerous?

The most noticeable symptom of a short cycling boiler is a cold home. However, leaving your boiler unrepaired could result in some dangerous conditions that’ll put you and your family at risk.

Short cycling boilers are dangerous. Left unrepaired, your boiler won’t last long. It may also overheat, or some components may break under stress, potentially causing a fire, an explosion, or a gas leak in your home. 

That means that during short cycling, your boiler will waste energy trying to heat your home, although you’ll notice that it won’t provide much warmth at all. 

Short cycling can also put excess wear and tear on your boiler, which could lead to more issues in the future if you leave it untreated. 

Since short cycling reduces efficiency, increased monthly HVAC costs are a common sign of something wrong with your boiler or thermostat. 

Why Does My Boiler Keep Going On and Off?

There are several reasons why your boiler might be short cycling, and, luckily, most of these issues are easy to understand and repair. 

Your boiler will keep going on and off if the air vents aren’t working, the pressuretrol line is blocked, the pressure is too high, the steam traps aren’t functioning, your boiler is too big, or the thermostat is in the wrong place. 

So, let’s get down to the details so that we can diagnose your boiler’s short cycling issue together. 

Your Boiler’s Air Vents Aren’t Working

Air vents allow air to escape from your boiler’s system, keeping the air from blocking steam’s movement through your piping. 

When a boiler is cold, it’s filled with air. Yet, when the boiler heats up, the steam pushes the air out of your pipes through the air vents. 

Air trapped in your boiler can cause short cycling since trapped air will change the pressure of your entire boiler system. Leaving the air inside with no escape could quickly result in a blockage that won’t let steam through. 

When this happens, your pressuretrol won’t get an accurate reading, and the pressure could build up to dangerous levels. The boiler could also overheat, causing excess stress and hazardous conditions. 

Likewise, air vents that release steam will keep your boiler running almost constantly. So, if your air vents seem clogged or leaky, that’s probably why your boiler is short cycling. 

The Pressuretrol Line Is Blocked

A boiler’s pressuretrol is a mechanism that measures how much pressure your boiler is creating with air and steam. 

This device keeps your steam boiler from building up so much pressure that it explodes. It also ensures that the boiler is creating enough force to push hot steam into your pipes. 

Sometimes, debris, fungus, or mineral deposits from the water can build up in the line that connects the pressuretrol to your boiler. This line is called the pigtail line since it looks a bit like a curled-up pigtail. 

Since they’re curved and winding, pigtail lines often get clogged with gunk accumulating in your boiler water. 

When the pigtail line is blocked, the pressuretrol mechanism cannot get a good reading of how much pressure is in your boiler. When this happens, your boiler will short cycle. 

The Boiler’s Pressure Is Too High

Although it seems like most boilers would need a high-pressure environment to force steam throughout your pipes, it doesn’t take much for your boiler to work. 

A boiler’s pressure should be around 1.3 bar (about 13 psi) while it’s operating, and it should never go any higher than 3 bar (30 psi). If your boiler reaches too high of a pressure, it’ll automatically shut down, resulting in short cycling. 

If your boiler’s pressure is too high, you might have air trapped inside your piping system. Otherwise, you might have a faulty pressure relief valve or  air vent. 

Too much air trapped in your boiler can interfere with the steam pressure, causing a short cycling issue. 

That’s because air condenses and pressurizes with the steam your boiler creates. Yet, when air gets stuck in the system, the moisture can’t pass through your pipes completely. The air acts like a clog, blocking the water from fully cycling back to your boiler. 

The Steam Traps Aren’t Functioning

Steam traps filter condensed steam (AKA water) from the steam that rises from your boiler. 

As the steam moves through your pipes, some of it will turn into condensate. Steam traps pull the condensate from the steam in your pipes while forcing the steam to stay, keeping your home warm for as long as possible. 

However, if the steam traps are no longer trapping the steam in your pipes, they’ll release all the heat. This process will trick your boiler into short cycling to warm your building. 

Another issue that could cause short cycling is if your steam traps aren’t draining the condensate they collect. 

When this happens, your pipes can become waterlogged, causing corrosion or damage. It could also interfere with the amount of steam that can make its way through your piping system. 

Your Boiler Is Too Big

When it comes to boilers, bigger isn’t always better. 

When your boiler is too large for the room or building you’re heating, you’ll likely experience short cycling. 

That’s because an oversized furnace will heat your home quickly and efficiently, then it’ll turn off again once your space is warm. 

However, if your boiler is too big, its short cycling can damage the burners, gas valves, and other essential components. Boilers need at least 15 minutes between cycles to dry out condensed liquid or fuel on the burners and valves. 

If they don’t have enough time to rest between cycles, the burners can rust or wear out, breaking your oversized boiler completely. 

Your Thermostat Is in a Bad Place

Boilers start to heat your home when the temperature falls. Most of the time, a thermostat measures the temperature. It’ll turn the boiler on when the temperature is five or more degrees below the desired heat level. 

However, if your thermostat is in a humid, drafty, or cold spot in your home, it may misread the overall temperature. When that happens, it’ll turn the boiler on and off frequently. 

Your Thermostat Is Broken

If your thermostat breaks, your boiler won’t know when to turn on or off.

A broken thermostat can cause short cycling since the boiler will try to heat a cold home. However, when it can’t connect to your thermostat, it’ll turn itself off to prevent overheating. 

To check if your thermostat is broken, look at the temperature reading or thermometer. If the temperature isn’t accurate, it’s likely broken. 

In addition, thermostats with water damage or loose wires won’t work. So, you should check the inside of the device to ensure that it’s dry and properly connected to the wires in your wall. 

How To Stop Your Boiler From Short Cycling

Now that you know the possible reasons why your boiler keeps going on and off, let’s talk about the different ways you can stop your boiler from short cycling.

1. Clean or Replace Boiler Air Vents

If you suspect that your air vents are the issue, you can clean or replace them to remove blockages. 

Usually, a thorough scrub with detergent or vinegar will do the trick for unclogging gunked-up air vents. However, if yours seem old, corroded, broken, or if they’re leaking, you may need to replace them altogether. 

When replacing an air vent, be sure to get the same model as your old one. That way, you can be sure that it’ll be compatible with your boiler.

You’ll also need a wrench, towels, and Teflon sealing tape to get the job done. 

To replace your air vent: 

  1. Turn the power supply for your boiler off from your circuit breaker. 
  2. Turn the boiler off and let it fully cool. 
  3. Close all the valves on your boiler. 
  4. Unscrew and remove the small, protruding top of your air vent.
  5. Place one or more towels under your air vent to absorb the water that’ll inevitably spill from your air vent and boiler when you remove the vent. 
  6. Use a wrench to unscrew the old air valve from your boiler (warning: water will spill). 
  7. Take your new air valve out of its packaging.
  8. Wrap the screw threads on the bottom of the valve with Teflon sealing tape.
  9. Unscrew the protruding cap of your new valve and wrap the threads with a layer of Teflon sealing tape. 
  10. Replace the cap on your new valve.
  11. Use your wrench to screw in the new air valve tightly. 
  12. Turn your power back on and start up the boiler. 

If you need some help finding your air vent or want some help replacing yours on your own, here’s a tutorial from a fellow DIY-er, CantLetHerDieDIY:

2. Clear Out Blocked Pressuretrol Lines

Clearing out your pigtail line is simple, and it’s very DIY-friendly. 

You can flush out the pigtail line with vinegar and water until it washes clean to fix the issue. For stubborn grime, you may need to soak the line in a detergent such as a dish soap for a few hours, then try to rinse it clean again. You can also scrub it out with a pipe cleaner to ensure that it’s in perfect condition before you reinstall it. 

If a wash doesn’t work, you may need to replace your pigtail line with a new one. 

If you want to learn more about pigtail lines or need help identifying yours, take a look at this brief Youtube video from PowerMechanical: 

3. Fix Your Boiler Pressure Levels

To fix your boiler pressure level, you can:

  1. Check the pressure while the boiler is cool. Turn off your boiler and wait for it to cool. Now, recheck the pressure. Did it go down? If so, the problem was air-related and should clear up unless you have clogged air valves. If the pressure is still above 1 bar, continue to step 2. 
  2. Tighten your filling loop or relief valve. If your relief valve or filling loop (usually used to repressurize boilers) is loose, it’ll interfere with your pressure gauge reading. Ensure that these are screwed in tightly and aren’t leaking steam. 

If your boiler is constantly overpressured, you may need to install an expansion vessel, which is an overflow tank that keeps the water in your boiler from increasing your pressure too much. 

For a helpful video guide to repressurizing your boiler and adjusting the pressure, follow along with this helpful video from boiler specialist Roger at Skill Builder: 

4. Replace Faulty Steam Traps

If your steam traps aren’t working, you’ll probably need to replace them since they’re intricate mechanisms that are prone to breakage. While you can probably do this yourself, it may be best to call in a pro for this since faulty steam traps are usually hard to find. 

To learn more about how steam traps work, take a look at this informational Youtube video from Wareboilers: 

5. Replace an Oversized Boiler

Unfortunately, the only way to fix the issue of your boiler being too big is to replace it with one that’s the appropriate size for your building. 

Still, you’ll save a large sum of money on your utility bill just by replacing your short cycling boiler. So, it’s better to change it to the right sized unit than try to make do with your current one. 

6. Move the Location of Your Thermostat

If your thermostat is in a bad spot, which causes it to misread your home’s overall temperature, moving it is an easy fix for this issue. It’ll also prevent your boiler from overheating and short cycling in the future. 

If you want to move your thermostat yourself, you’ll need to remove the plate and feed the wires through a new hole in your wall. Usually, you can easily change the room your thermostat is in or move it a few feet down the wall by drilling a new hole into your drywall. 

For a quick and easy tutorial, follow along with this Youtube video from Arlo’s DIY Tips: 

7. Replace a Broken Thermostat

Before purchasing a new thermostat, you may want to check the wires that connect it to your power to ensure that none of them have come loose or been chewed up by pests. 

If the wiring seems fine, you’ll need to replace the thermostat or call in a professional to service it. 

For more information about replacing thermostats on your own at home, follow along with this Youtube tutorial from Home Repair Tutor: 

Conclusion

Some of the problems that can cause your boiler to short cycle and their solutions are:

  • For malfunctioning air vents, replace your air vent with a new one.
  • For blocked pressuretrol lines, clean the pigtail line out. 
  • If your boiler’s pressure is too high, repressurize it and tighten the valves.
  • For faulty steam traps, call a professional to install new ones.
  • For an oversized boiler, get one that is appropriate for your building’s size.
  • For a poorly placed thermostat, move it to a new location with fewer drafts. 
  • For a broken thermostat, install a new one.