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Boilers make our lives convenient. They provide us with heating and hot water when we need it, especially in the cold months. However, like many human inventions, boilers may require attention from time to time. One of the most common problems with modern combi boiler systems is low boiler pressure. You usually see low pressure in your boiler after it has been idle for a long time, like in the summer months.

Low Boiler Pressure Explained

Low boiler pressure is certainly an annoyance. But what is boiler pressure in the first place? And why is it so important to maintain boiler pressure? How do you know you have low boiler pressure? These questions are important to answer if you want to understand what low boiler pressure is and what causes it.

What is Boiler Pressure?

Your boiler provides hot water for circulation in the radiators and pipes in your system. This hot water circulates constantly through your system, making sure every radiator receives its supply. The radiators, in turn, heat your rooms using the hot water they get from your boiler.

Most British homes use gas combi boilers for heating and hot water. Modern boilers are a sealed system, unlike older open systems that require an expansion tank. This means the amount of water in a sealed modern system remains constant. You can manually top up the amount of water in your system using your combi boiler’s filling loop. Boiler pressure is the amount of water circulating in the system. The unit of measurement for boiler pressure is in bars.

How to Detect Low Boiler Pressure?

Most modern combi boilers come with a built-in pressure gauge on the boiler. Usually, this pressure gauge is visible on the control panel of your combi boiler. Most pressure gauges are marked with green and red zones. The green zone indicates the minimum safe boiler pressure for your boiler to operate efficiently. The red zone indicates the maximum safe pressure for your boiler to function. A correctly functioning combi boiler should have a pressure gauge reading of 1 or 1.5 bar.

If you have low boiler pressure, and your gauge is not faulty, the needle should drop below the minimum safe pressure. When this happens, you need to take action to restore pressure in your system. Another way to tell if your boiler pressure is low is to check if your radiators heat up properly. Low boiler pressure may cause your radiators to heat up inefficiently, which can indicate a problem with your system.

Why is Boiler Pressure so Important?

Maintaining a constant boiler pressure in your system is essential for it to work efficiently. Your boiler and heating system require a steady and stable pressure to continue to heat your home and supply hot water. If your boiler pressure drops below the minimum safe pressure, your boiler and central heating will not function efficiently. It will instead spend more energy trying to heat your house with low pressure.

Low boiler pressure will not necessarily damage your boiler or central heating system. However, it will affect their performance. Lower pressure means your system has to work harder to heat your home. The harder it works, the more energy it consumes, and the higher your costs go. Obviously, inefficient heating will also take longer for your rooms to warm up. If efficiency and heat are a concern for you, you may want to pay attention to your boiler pressure. Low boiler pressure can indicate if there is a drop in the amount of water circulating in your system. The lower the pressure, the more energy your boiler spends in heating water and circulating it.

What Causes Low Boiler Pressure?

Low boiler pressure is a common problem with most modern boiler systems. It most often happens after your boiler has been idle for a long time, such as during the summer. Low boiler pressure means the amount of water circulating in your system has dropped. There can be two main causes behind low boiler pressure, which follow below.

A Leak in the System: A water leak in the system can be the cause behind low boiler pressure. A water leak may not always be easy to find. This is because much of the pipework of your system is not accessible. However, what you can do is check for signs of leakages everywhere your system is accessible. Pay special attention to radiators and pipes. A simple visual examination should be enough to find signs of leaks in your system. Water or other signs of moisture on your pipework can be indicators of leakages. Stains and damp patches on your walls or ceiling can also point towards leaks in the system. Joints in your pipework are good places to check for signs of leaks, as well as the pipes that connect to your radiators. However, be careful not to confuse condensation in colder rooms with a potential leak. You will need a professional to repair any leaks in your system, do not risk a DIY job.

In some cases, your boiler itself could have a leak. This can be the case if you cannot find signs of a leak elsewhere in your system. If you suspect your boiler is leaking, you may need to contact a Gas Safe registered engineer. The engineer will inspect the boiler and check for any leaks inside it that could be causing low boiler pressure.

Bleeding the Radiators: Many people bleed radiators in the summers to improve the system’s heating. In some cases, people also bleed radiators when they are not heating properly because of air pockets in the radiator. If you have recently bled your radiators, this could be a big reason why you have low boiler pressure. Bleeding radiators release air trapped in the system, which often causes the radiators to work properly again. However, the escaped air also reduces water pressure. If you have bled your radiators recently, you may have to take a few steps to ensure your boiler pressure returns to normal.

These two are the most common causes behind low water pressure in your boiler. If you have not bled your radiators, and you have not found any signs of leakages, you should contact a professional. However, if you have found a leak, or have bled your radiators recently, do not worry. There are several easy steps that can help you re-pressurise your system. But before you do follow them, be sure to read the user guide thoroughly to make sure your boiler allows re-pressurising.

Steps to Correct Low Boiler Pressure

So you know the drop in water pressure is not because of a leak in the system. Alternatively, if there was a leak, the professional fixed it. Now you can start taking steps to re-pressurise your boiler and central heating system. You need to bring your boiler pressure back up to 1 bar. However, the recommended boiler pressure can vary from boiler to boiler, so check your boiler’s user guide beforehand.

You can re-pressurise most gas combi boilers by following the steps below. However, if you feel it is too complicated for you, you can always contact a professional to do it for you. Re-pressurising your boiler and central heating system is not too difficult. This step-by-step guide should work for most modern boiler models:

  • First, you need to switch off your boiler and let it cool. This is very important, as you do not want to risk coming into contact with hot boiler pipes or water. Hot water can cause scalding, so be sure to put safety first and allow your boiler time to cool off.
  • Second, you need to ensure that the filling loop is firmly attached at both ends. A filling loop is a pipe that connects your combi boiler to its mains water supply. Depending on the model, your boiler could have either an external or an internal filling loop.
  • Third, once you have made sure the filling loop is secure, open the valves at both ends. This is will allow water to start flowing into your system. You should be able to hear it filling up your boiler.
  • Fourth, watch the pressure gauge. As the water begins to flow into the system, your boiler pressure will begin to rise. Allow the gauge needle to rise between 1 to 1.5 bar depending on the manufacturer’s recommendation.
  • Once the temperature reaches the green zone, turn off both valves on the filling loop. This will shut off the water flow into the system. You need to close the valves one after the other, not both at the same time.
  • After you have closed the valves on the filling loop, switch your boiler back on. If your boiler controls include a manual reset switch, press it. Keep an eye on the pressure gauge to make sure the water pressure is stable.
  • If the filling loop is external, undo both ends and remove it. This will cause some spillage, but do not worry, this is entirely normal. You can easily wipe away the spilled water. Be sure to keep the filling loop where you can access it again if needed.

When to Call an Engineer

After following these steps, your boiler pressure should be back to normal. However, if you have already tried re-pressurising your system according to the boiler’s manual, consider contacting an engineer. If you are still having problems with your boiler pressure, an engineer should be able to find and fix the problem.

Always be sure to use a Gas Safe registered plumber or engineer for any repairs to your home. Many people often get confused between whether to call a plumber or an engineer in case of low boiler pressure. Leaking pipes may lead you to conclude you need a plumber. But when the plumber arrives, he or she tells you that you need an engineer instead. Let’s take a look at the difference between a plumber and an engineer.

Engineer: A heating engineer usually specializes in working on heating problems. They tend to work only for domestic or commercial properties instead of the wider industry. Heating engineers usually have a lot of expertise in gas and heating work. They can help install and maintain heating pipework. They can also take care of malfunctioning heating appliances like your boiler. Whenever you require an engineer, you should check to make sure he or she is on the Gas Safe register.

Plumber: Plumbers usually work on kitchens, bathrooms and dry and wet pipes. They also work on boiler and central heating system maintenance. This can cause some confusion when your boiler has low pressure due to a leak in the system. However, there is no formal qualification to become a plumber. Anyone can refer to themselves as one. While some plumbers may be Gas Safe registered, you cannot expect it from all of them.

So, who should you call in case of low boiler pressure? The answer is quite simple. Repairing or servicing a boiler involves gas work. This means only a Gas Safe registered engineer has the legal authority to work on your boiler. The United Kingdom thoroughly regulates the gas industry to maintain high service and safety standards. Only an engineer on the Gas Safe register is allowed to work on boilers and other gas appliances.


To summarise, boiler pressure indicates the amount of water circulating in your sealed central heating system. A drop-in boiler pressure indicates a drop in the amount of water circulating in the system. Low water pressure normally has two common causes. The first may be a leak in the system. Your central heating system consists of pipes and radiators that hot water flows through. A leak can cause water to escape the system and the boiler pressure to drop. The second reason may be because you bled your radiators recently. When you bleed radiators, air trapped in the system escapes, lowering water pressure.

If you have a leak in your system, you should not attempt to fix it yourself. To fix your boiler pressure you first need the leak fixed. The abled radiator is not a problem. If there is no leak, or a professional fixed the leak, you can start re-pressurising your boiler and central heating system. Bear in mind that modern combi boilers and heating systems are sealed. This means the water circulating in it remains constant. To correct low boiler pressure, you need to allow more water into the system.

To re-pressurise your system, follow the steps listed above. These should be enough to fix the low boiler pressure. However, if the problem persists, reach out to a Gas Safe engineer to assess and fix the problem.

What Causes Low Boiler Pressure How to Fix That

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