Why is My Boiler Turning On and Off Constantly?

Oscar Garner

By Oscar Garner

Last Updated on July, 2024

Is your boiler turning on and off like a hyperactive light switch with a mind of its own?

We’ve all been there, and I understand this on-and-off cycle can be frustrating. I am here to help you with this. 

In this article, I’ll explain the common reasons behind your boiler’s frequent on-and-off behaviour and explore some solutions to bring peace back to your home’s heating system. 

If you’re ready, let’s begin.

Quick Summary

  • Several factors can cause a boiler to turn on and off frequently, including the preheat function, a broken heat exchanger, low or high water pressure, a faulty thermostat, a frozen condensate pipe, a faulty water pump, issues with the fuel supply, a faulty temperature control valve, and boiler short cycling.
  • It is important to properly maintain and service your boiler to prevent potential issues and ensure it functions effectively.
  • If you are unsure of the cause of your boiler’s frequent on-and-off behaviour, it is recommended to contact a gas-safe certified engineer for assistance.

Reasons for Your Boiler to Turn on and off Frequently and How to Fix Them? 

an engineer opening a water boiler to examine

Why is your boiler turning on and off frequently? There’s no single reason for this question. Many factors can cause your boiler to fire up and shut down suddenly. 

Let’s discuss the causes and ways to resolve each one in detail. 

1. Preheat Function

Some boilers, such as the combi boiler, turn on and off occasionally to preheat water. Preheat function is designed to provide you with hot water on demand. The boiler heats the plate heat exchanger periodically, so hot water is readily available when required. 

This function is completely normal and doesn’t consume extra energy. You don’t have to worry if your boiler is turned on for preheating. It’s not an error in the boiler. You can turn off the preheat function if you prefer. Go through the manual to find out how it’s done. 

2. Broken Heat Exchanger

If you’ve been using your boiler for many years, around 7-10 years, it’s more likely that the heat exchanger is damaged. A damaged heat exchanger can cause your boiler to turn on and off frequently.

A heat exchanger is one of the most expensive parts of the boiler. Hence, it’s better to install a new boiler than replace a heat exchanger if the boiler is too old, out of warranty period, and the heat exchanger is almost equal to the price of a new boiler.

If you decide to install a new boiler, make sure to choose a quality brand.

3. Too Low or Too High Water Pressure

water flowing faster in a water boiler due to high pressure

The boiler pressure must be at optimum level for it to circulate hot water effectively. Ideally, boiler pressure must be around 1-2 bars. If it becomes too high or too low, your boiler will shut off, displaying an error code. 

There’s a pressure gauge in front of the boiler. You can use it to check the water pressure. Usually, low pressure is caused by leakages in the system. It can be a pipe joint, radiator, or boiler leak. 

You can repair minor leaks in the pipe joint if you’re skilled and comfortable with DIY pipe repairs. Below are the steps you must follow:

  • Switch off the boiler.
  • Isolate the power to the central heating system at the mains. 
  • Dry the area of the pipe joint from where the water is dripping. This helps you confirm the source of the leak. 
  • Tighten the fitting a tad. 
  • If the leaking stops, repressurise your boiler. Check your instruction manual to know the correct method for repressurising. 
  • Finally, turn on the power at the mains and switch on your boiler. 

If you’re unsure where the leak is or are not skilled in pipe repairs, contact a professional. If the water pressure rises above 2.5 bar, it’s considered too high. Too high pressure can switch off your boiler. 

Too much water in the boiler and damaged parts can cause high boiler pressure. Another reason your boiler turns off frequently is trapped air inside the system. When too much air is trapped, water cannot flow through the central heating system, causing it to shut off. 

Hence, the radiator will have cold spots or not be as warm as it should be. Bleed the radiators to remove the trapped air and effectively get hot water flowing again.

4. Faulty Thermostat

Thermostats monitor the external temperature. It turns on automatically when it drops below a certain level and turns off when the temperature rises above a certain level. Check the location of the thermostat too. It’ll turn on frequently if it’s placed in a particularly cool spot.

If the thermostat is broken, faulty, or miscalibrated, it won’t record the temperature accurately. Hence, your boiler will turn on and off unexpectedly. Contact a heating engineer to confirm the cause. 

If your thermostat is old or broken, then you must replace it. Consider buying a smart thermostat as it gives more accurate temperature control and helps to save your heating bills. 

Some smart thermostats can learn your behaviour such as when you’re home, when you’re not, which rooms you use, etc, and they adjust the heating accordingly.

5. Frozen Condensate Pipe

a picture of a Frozen Condensate Pipe in a water boiler

Boilers generate acidic water while operating. A condensate pipe is a small pipe that carries the wastewater from the boiler into the drain. Usually, these pipes are installed externally. Hence, they can freeze during colder months. 

Frozen water blocks the condensate pipe, and the wastewater is backed up into the boiler. Luckily, boilers are designed to detect frozen condensate pipes. 

Therefore, your boiler will turn off and display fault lights or an error code, depending on your boiler model. You will also hear a gurgling noise while operating it. 

A frozen condensate pipe is the easiest to fix. Locate the condensate pipe and thaw it with hot water. Ideally, the temperature of the warm water must be 60-70 Celsius. Don’t use boiling water as it might damage the pipes. 

Your pipe might not thaw at once; you will have to pour hot water several times, and it can take hours if it’s been frozen for a long time. You can thaw a condensate pipe by yourself. However, if you’re unsure how to do it, call a heating engineer.

6. Faulty Water Pump

A faulty water pump is another potential cause for your boiler to turn off suddenly. When the water pump is broken, water cannot circulate the central heating system properly. 

This could lead to overheating of the boiler. Hence, it shuts off completely to prevent overheating. This is a simple issue, but you can’t fix it by yourself. Contact a gas-safe registered engineer to replace the broken water pump. 

7. Issues With the Fuel Supply

Your boiler needs a consistent supply of fuel to work properly. Interruption to the fuel supply can be a cause for your boiler to turn on and off frequently, but it’s something we usually ignore. 

Hence, check out the other gas appliances that work with the same fuel as your boiler. Are they working? If they’re also not working, contact your energy supplier, figure out the issue, and resolve it promptly. 

If the fuel supply is fine, check the prepayment metre and ensure you have enough credits. This could sound silly, but at times we tend to forget these. If you cannot figure out the problem with the fuel supply, call a gas-safe engineer to identify and resolve the issue. 

8. Faulty Temperature Control Valve (TCV)

faulty temperature control valve (TCV) in a water boiler

If the temperature control valve is closed, water cannot flow through the boiler. Hence, it will turn off automatically. To resolve this issue, check beneath your boiler and set the valves to open. 

If they’re already set to open, then the problem is with the temperature control valves inside your boiler. They regulate the water temperature inside the boiler. 

When the internal temperature control valve is broken, the water temperature cannot be regulated. This can lead to overheating of the boiler system. To prevent this, the boiler turns off. 

You must remove the boiler cover to fix the internal temperature control valves. Therefore, you can’t do it by yourself. 

Call a gas-safe registered engineer to do it. Remember, any problem that requires you to remove the boiler cover cannot be fixed DIY. You must call a professional.

9. Boiler Short Cycling

If your boiler is turning on and off every few minutes, it’s more likely that your boiler is short-cycling. 

Why does your boiler short cycle? Let’s discuss this in detail.

Inside a combi boiler, there’s a heat exchanger with a small reservoir of water to provide hot water on demand. 

When the water starts to cool down, the internal thermostat detects it and triggers the heat exchanger to reheat it. The heat exchanger turns off after the water is heated to the ideal temperature. 

When the amount of water to be heated is small, the above process will repeat often. So, your boiler will turn on and off every few minutes. Every time your boiler fires up, it consumes gas. This leads to higher heating bills, fuel wastage, and carbon emissions. 

You must repair a short cycling boiler as soon as possible as it leads to many other boiler problems, such as:

  • Damage to pressure vessel components.
  • Premature damage to burner components. 
  • Changes in water chemistry due to inadequate heating time, which leads to the corrosion of boiler components. 
  • Cracked heat exchanger.

The cost of replacing a heat exchanger varies according to the boiler model, but it’ll cost you an enormous repair bill. If the boiler damage is irreparable you’ll have to get a new boiler installed. 

Oversized boiler, water flow issues, issues with burner tuning, and incorrect boiler setpoint are the causes of boiler short cycling. Let’s look at each of the above causes in detail.

Oversized Boiler

a picture of an oversized water boiler

An oversized boiler is the most common cause of boiler short cycling. A short-cycling boiler will turn on and off frequently. 

When your boiler’s power output exceeds your needs, more steam than it can condense will be produced. This will eventually overheat the boiler, causing it to turn off.

In old times, boilers had around 70% efficiency. Hence, having an oversized boiler wasn’t an issue. However, modern boilers have an efficiency of around 90% or more. So, you must get one that suits your home size. 

Most people think that they must select the largest, affordable boiler available, but that’s not the case at all. An oversized boiler can hike up your energy bills, lead to more frequent faults in your boiler, and cost you a huge repair bill and labour costs. 

How to choose the right-sized boiler?

You must consider several factors when choosing the right boiler size for your home. Below are some of them:

  • Number of radiators

Count how many radiators you have around your home. When the radiator number increases, a boiler needs to supply more power. I’ll give some estimates for combi boilers:

  • Up to 10 radiators-24-27 kW
  • 10-15 radiators-28-34 kW
  • 15-20 radiators-32-40 kW
  • Property size and the demand

If you have a large property, then you need a high-power boiler for efficient heating. Additionally, you must consider the number of bathrooms you have, how often they are used, etc. 

  • Fuel type

The efficiency of the boiler varies based on the fuel used. Most homes in the UK use a gas boiler, while some use an oil-fired or liquid petroleum gas system. Some modern boilers can use modern energy sources such as solar power. Take these into account when selecting a boiler. 

  • Heat loss

These days, local heating engineers can calculate your home’s heat loss. This allows you to select the most efficient boiler size. 

To calculate heat loss, the floor area, insulators, window size, and many more factors are considered. It’s better to call a gas-safe engineer to determine the optimum size boiler for your home.

Water Flow Issues 

Water flow is an important factor that decides your boiler’s performance. If the water flow slows down, the temperature rises across the boiler. If it rises above the setpoint, your boiler will shut down. 

Water flow issues can occur due to problems in the wet end of the pump, faulty pump motors, piping issues, and closed valves. To resolve issues related to water flow, you must inspect the hydronic system, including the valve positions, piping arrangement, etc. 

If you’re unsure, call a heating engineer to inspect it and take the necessary actions.

Burner Turndown Issues

Burner turndown is the lowest firing rate for a burner. The optimum turndown for your boiler is mentioned in the user manual, and it must be at the correct level for the boiler to function effectively. Hence, inspect the boiler system thoroughly and tune it for maximum turndown.

Incorrect Boiler Setpoint 

Your boiler has several controls and setpoints, such as the boiler pressure, temperature, etc. You must set them up properly for the effective functioning of your boiler. If it’s incorrectly tuned, it could lead to boiler short cycling. 

Review your boiler setpoints. Compare it with the last setpoints and check if there are any changes. If so, tune them back. You can also refer to the user manual for guidance or contact a professional service. 

Tip: You can use error codes to identify the issue on your boiler. Boilers come with some built-in fault codes. If there’s a fault, your boiler will display it as an error code or a light sequence. 

Refer to the user manual to know the faults related to each fault code/light sequence. If you’re unable to identify the cause of the problem, don’t hesitate to get in touch with our service.

Precautions to Prevent Boiler Issues

an engineer working on a water boiler

Boilers are used often in UK homes, especially during the winter season. When you use something frequently, it’s more likely to get damaged. Hence, you must maintain your boiler to prevent further damage and use it for a long time. 

Maintain your boiler properly by cleaning air vents and inspecting steam traps frequently. This will prevent boiler short cycling. 

You must do an annual boiler service on your boiler and other gas appliances. This will help you to figure out any issues in the heating system and resolve them promptly. 

Additionally, carry out an annual maintenance check at least once a year to identify and resolve existing issues and to prevent potential boiler problems. Consider using a boiler cover to prevent frequent repairs. 

Why Does Your Boiler Keep Turning on and Off?

There are many reasons as to why your boiler keeps turning on and off suddenly. 

High or low water pressure, issues in the heating system, faulty thermostat, frozen condensate pipe, damaged water pump, issues with the fuel supply, faulty temperature control valve, and boiler short cycling are some potential causes. 

Use the error codes displayed on the boiler to figure out the exact cause. You can solve some issues by yourself, but to identify and solve most boiler problems, you need to contact a gas-safe certified engineer.

Useful guides:


Yes, a boiler can catch fire due to overheating, low water levels, issues with the fuel supply, stuck safety valves, and other factors.

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