Why is My Boiler Losing Pressure?

Written by :
Last updated: October 25, 2023

To work properly and efficiently, your boiler requires constant and stable pressure. So, if you’re experiencing any problems with your boiler – be it a lack of hot water or heating – it’s more than likely that you’re dealing with faulty pressure. The good news is, low boiler pressure is pretty simple to diagnose and, in some cases, you can fix it yourself. To help you do just that, here’s our comprehensive guide to solving the most common boiler pressure problems.

What Is Boiler Pressure?

Before we jump to the most common problems that lead to a boiler losing pressure and their possible fixes, let’s first talk about boiler pressure itself and why constant and stable pressure is important for your boiler to work efficiently.

So, first things first: what is boiler pressure anyway? Since a boiler moderates both your central heating and hot water by heating cold water and moving it around your pipes and radiators, boiler pressure simply refers to the pressure of hot water that’s running through your central heating system. Now, since most modern heating systems are sealed – meaning there are no air vents to control or manage water as it contracts and expands while it cools and heats – the system is pressurised and, as such, it must maintain a constant and stable pressure to move the water around the pipes in your home efficiently.

In essence, to function properly, your boiler requires a particular balance of pressure (air) and water. If this balance somehow gets out of whack and your boiler starts experiencing either too low or too high pressure in the system, it won’t be able to function properly, leading to an inadequate flow of water in the pipes. The result? In the case of low boiler pressure, you’ll have little to no hot water or heating; in the case of too high pressure, the system can get over-pressurised resulting in water coming out from the pressure relief valves.

What Should My Boiler’s Pressure Gauge Read?

On most modern combi boilers, you’ll find a pressure gauge right on the front of the boiler, showing what the current pressure is. Typically, a pressure gauge will have a needle with low and high pressure shown by green and red areas on the dial. Bear in mind that while some pressure gauges come with a single needle, others come with two, in which case a red needle acts as a marker for where the boiler pressure should be, and a black one shows where it currently is. Certain models may not have any needle at all but an LCD screen instead, which will show an error message if the water pressure is either too low or too high (alternatively, the screen may flash).

Whichever pressure gauge your boiler may have, one thing is sure: the ideal pressure is between 1 and 2 bars, although on some models it may go as high as 2.5 when the heating is on. Of course, you should check your boiler’s manual to find out exactly what the ideal pressure is for your make and model. Using this range, it’s easy to conclude that if the pressure gauge goes below 1 bar, you have low boiler pressure. If the pressure gauge reads above 2.5 bar, you have high boiler pressure.

Obviously, neither situation is good, but in the case of low pressure, you’ll have no hot water or heating, while in the case of high pressure, your system won’t work as efficiently. It’s good to know though that high boiler pressure is not dangerous in most cases. This is because all systems have pressure relief valves that will be triggered if the boiler pressure gets too high. In this case, you’ll notice water coming out from the pressure relief valve.


Causes of a Pressure Loss in Your Boiler

If your boiler keeps losing pressure, it’s important to investigate and identify the cause as soon as you can. The sooner you’re able to do this, the better, as this way you’ll protect the boiler from any further damages, as well as prevent any damages to your home.

While the pressure gauge will be able to show you whether the boiler has low or high pressure, it won’t be able to tell you what’s causing the problem. For this reason, you’ll have to manually investigate possible issues yourself. There are two main culprits when it comes to the boiler losing pressure: a leak in the system (pipework) or a leak in the boiler.

In this section, we’ll deal with finding and fixing a leak in the system. If you already know you don’t have any leaks in your pipework, you can continue onto the next section; if you’re starting from the scratch, read on!

A leak in the system can be quite tricky to spot, and therefore fix. This is because of the complex nature of the heating system: most pipework is concealed within the walls or behind panels, making it difficult to perform a proper visual inspection. To make matters worse, most leaks are tiny and difficult to notice. Fortunately, the majority of leaks in the heating system occur at fixtures or joints which are practically always in visible locations. To be more specific, they can typically be found where the pipes are connected to the radiators. So, how can you check if your system has any water leaks?

Start by inspecting all radiator valves or joints to see if there are any obvious leaks. Now, in most cases, there will be no visible leaks, but just to be sure, visually inspect everything in detail. Bear in mind, however, that not all wet pipework is leaking – wetness on pipes and even joints can be caused by condensation due to temperature differences. Still, there’s a way to check for water leaks if you suspect that one of the fixtures is the problem. Firstly, you should thoroughly dry off the radiator valve with a paper towel or tissue, then, place a paper right underneath it. Wait a little while and if no drip develops, this is not your broken fixture; if you do notice a regular drip, you’ve found the leakage.

Before we get into the possible fixes, it’s important to add that there can be other indicators of water leaks in the system, including:

  • green buildup around a copper pipe
  • flaking of paint on the pipe(s)
  • surface corrosion on the pipework
  • damp patches around the pipe and/or radiator

If you find a leak in the system, you should get it examined and either repaired or replaced by a professional such as an experienced plumber or a heating engineer, ideally Gas Safe engineer. It is crucial not to try to do this yourself if you have no experience in this matter because you can cause further damage to your central heating system.

Once the leak has been fixed by a professional, your boiler should be able to maintain proper pressure.

Can a Boiler Lose Pressure Without a Leak?

If there are no leaks in the system but your boiler keeps losing pressure, the fault could be with the boiler itself. Likewise, if you’ve found a leak in the pipework and a professional has fixed it but your boiler keeps losing pressure anyway, the problem is not (only) with the pipes, but with the boiler itself as well.

Most modern combi boilers use a system called filling loop, which is a connection to the water mains that you can use to repressurise and fill your heating system with water. Note, however, that while many boilers allow the user to regulate the amount of water circulating in the boiler and system, not all boilers do. It is essential to consult your boiler’s manual first to ensure that you can actually repressurise it yourself. If you find that your boiler has no instructions whatsoever on this topic, or if you have any doubts, hire a professional and do not attempt to repressurise the boiler yourself.

If, however, your boiler’s manual states that it’s safe for the end user to repressurise it, continue to the next step!

How to Top-Up (Repressurise) Your Boiler

If your boiler is experiencing pressure loss (and the heating pipework is not the problem) and your user manual states it’s safe to re-pressurise it yourself, in most cases, you’ll be able to fix it at your home in just a few minutes. Aside from looking at your user manual, it’s also a good idea to check your boiler’s brand website to see if they offer any specific tips for safe and quick boiler top-up at home.

While every boiler is different, most modern boilers have filling loops which you can use to re-pressurise the system. It’s good to know that some boilers come with external filling loops while others have internal filling keys; either way, you can use this system to repressurise your boiler in a few simple steps.

But before we get down to the nitty-gritty of repressurising a boiler, let’s quickly talk about what re-pressurising really is. Topping-up your boiler (or repressurising it) simply means allowing more water to enter the heating system from the mains cold water supply. This will increase the pressure in the boiler (which, as mentioned, you should only do if the gauge on your boiler reads at less than 1 bar), allowing it to properly heat your home and provide hot water efficiently. Topping up your boiler will also help maintain its life cycle. To do this, you’ll have to locate the filling loop on your boiler first.

  • Step 1: turn off your boiler and let it cool completely.
  • Step 2: locate the filling loop – a braided, flexible silver cable – and check that both ends are securely attached. Also check if there is any water around the valves because a leak here could be the reason for losing pressure.
  • Step 3: Open the valves to allow water from the mains to enter the system. At the same time, keep an eye on the pressure gauge.
  • Step 4: Wait for the pressure gauge to reach 1.5 bar (or until you’re in the green zone) and then close both valves.
  • Step 5: Remove the filling loop if it isn’t built-in the system and turn the boiler back on.
  • Step 6: Observe and re-check the pressure every couple of hours.

Note: In the case of an external filling loop (not the built-in type), you’ll have to attach it yourself during Step 2. Then, as mentioned, you should remove it in Step 5, before turning the boiler back on.

If you accidentally over-pressurise the system, don’t worry – there’s a solution to this problem too. The best thing to do in the case of over-pressurising the boiler is bleed the radiators as this will reduce the pressure to the optimal level. While pretty simple, this task is time-consuming so it’s best to keep a close eye on the pressure gauge while you’re topping up your boiler.

As for how often should you repressurise your combi boiler, there isn’t a specific timeframe you need to stick to. Simply repressurise it when you see the boiler pressure dropping below 1 bar. Note, however, that you shouldn’t do this too often; if you find that your boiler has to be topped up more frequently than you think it should be, it’s time to call a heating engineer or a plumber as you boiler could be leaking.

Pressure gauge

Boiler Keeps Losing Pressure, Even After Being Repressurised?

If fixing a pipe leak and/or topping up the boiler hasn’t helped and you keep losing pressure, it’s likely that the problem is one of the components in the boiler itself. This is a serious issue which will require the help of a qualified engineer. The sooner you get a professional to inspect your boiler and fix the issue, the better, as you’ll prevent further damage to the boiler and its components from occurring. Make sure you hire a person who is on the Gas Safe register!

While it’s best to let a professional do its job, it’s good to know what the possible causes for the boiler leak could be. These include:

  • Damaged seals: over time, seals can get damaged and even broken, causing a boiler leak. Thankfully, this is a pretty inexpensive and quick fix.
  • Faulty expansion vessel: the expansion vessel’s job is to act as a buffer in the case the pressure gets too high. Unfortunately, this component can sometimes fail, forcing the pressure elsewhere, thusly causing leaks.
  • Loose water pump: the longer you own a boiler, the higher the chances that a water pump will move around and become loose, causing leaks. Luckily, this is not a particularly expensive fix.
  • Heat exchanger: a faulty heat exchanger is a worst-case scenario as the solution is to buy a new component which costs a few hundred pounds. However, if your boiler is still within warranty, you can call the manufacturer to inspect the problem and replace the heat exchanger. If your boiler is no longer under warranty, you need to weigh up whether it’s worth spending that much money on the replacement.

Bear in mind that if your boiler is over 10 years old, it might be better to invest in a new one instead of spending money on fixing frequent boiler issues. This is especially true if the problem is a heat exchanger. Also, if your boiler is getting on in years and is around or older than a decade, it may be difficult to find replacement parts as sometimes manufacturers stop producing parts for older models.

Is Low Boiler Pressure Dangerous?

While low boiler pressure can cause your central heating to stop working, it’s no something that you should panic about. In fact, faulty pressure – whether low or high – is typically not dangerous as it’s highly unlikely to cause any significant damage to the boiler or your home. In most cases, low boiler pressure simply means that water cannot circulate around your radiators properly be delivered to your shower and taps.

This being said, low system pressure can have a big impact on the efficiency of your central heating system, making it difficult to keep your entire house consistently warm. In turn, this can cost you more on your energy bills. So, if you want to stay comfortable in your home and save money on heating bills, it’s essential to fix the problem as soon as you notice it. As always, it’s recommended that you consult a professional such as a Gas Safe engineer or at least an experienced plumber if you’re not sure what the problem or the solution is. A qualified professional will be able to tell you precisely what the issue is as well as what it will take to fix it. Prices will vary depending on the problem at hand, as well as the company you have hired. But generally speaking, the average boiler repair cost is between £300-£500. Again, don’t hesitate to consult a professional as soon as you notice your boiler losing pressure!


  1. What should I do if my boiler loses pressure? – Viessmann
  2. What To Do If My Boiler Keeps Losing Pressure – Baxi