How to diagnose low boiler pressure and how to solve

Signs of low boiler pressure and the solutions available 

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What Is Low Boiler Pressure?

Your boiler pressure is just like your blood pressure. You don’t want it too low and you don’t want it too high, you need it just right! Just like your body, your boiler requires an optimum pressure in order to transport the fluid to where it needs to go inside the system. The ideal pressure (for boilers not humans) is normally 1.5 bar but this can vary depending on the make and model installed at your property. 

If pressure is below 1 bar then it is probably too low for the boiler to function and it will most likely shut itself down. Having low boiler pressure can also affect the efficiency of the boiler system, this is because it has to work harder to get the water to its destination by using more energy to cycle the water and leading to higher energy bills. Luckily diagnosing a low boiler pressure is fairly easy as is fixing the issue.

Tell-Tale Signs

Before determining the cause of low boiler pressure you want to be certain this is the problem you’re experiencing. A couple signs of low boiler pressure are cold radiators despite the heating being switched on or a lack of hot water output coming from your taps. 

If you are encountering these issues the first step is to check the pressure gauge on your boiler. The boiler’s pressure gauge is normally located on the front of the boiler casing, especially on combi boilers; on system boilers it may be located amongst the pipework surrounding the boiler. 

As previously mentioned, the pressure should be between 1.5 bar which is often easily identifiable by a green zone on the gauge, although make sure to check the user manual for the optimal pressure because this can vary depending on the model you have installed. Any pressure lower than 1 bar should be in the red zone, signalling that the boiler pressure is too low. 

Another sign of low boiler pressure would be if your boiler automatically switched itself off and displayed a relevant error code. Modern boiler systems will often do this if they detect a fault within the system and then display a code on the control panel to help diagnose the problem. Simply look up the error code given in your user manual or online to find out if low boiler pressure is the cause of your boiler shutting down.


There are two main causes of low boiler pressure: a leak within the heating system or pressure lost from bleeding radiators. 

A leak within your system or a leak from a valve would explain low boiler pressure as the water escaping the system is integral to maintain the pressure to a certain level. Leaks can be caused by a number of things from corroded pipes to broken valves or faulty parts, if you wish to find out more about leaks please visit this article or our leak detection and remedy service page

Air escaping whilst bleeding radiators is the other explanation behind this dilemma and often goes unnoticed despite being the most common cause. When bleeding radiators you’re letting out trapped air from inside your boiler system, this air will partly make up the pressure of your boiler. Despite the air inside your system not being desirable, if a substantial amount of air is removed it needs to be replaced by water to prevent a significant drop in the pressure level. 

As you can see the three main causes of low boiler pressure revolve around a gas or liquid escaping the system because these are the components that act as the makeup of your boiler pressure. If your system loses a substantial amount of one, it will then need to be replaced with water to increase the pressure to what it once was.

How to fix low boiler pressure

Repressurising your boiler is actually one of the few procedures in boiler maintenance you can perform yourself without a Gas Safety licence. The process of repressurising most boilers is done by an external filling loop, some boilers use an internal filling loop instead (mainly the brand Worcester). To repressurise a boiler via an external filling loop, firstly you will need to locate the external filling loop hose which should be either next to your boiler or underneath. The hose should be a small metal braided hose and roughly 15mm in diameter, one end will be connected to the mains water supply and the other to the heating system. There should be a valve on both ends of the filling loop hose (sometimes there is just one), check that they are both in the closed position e.g. a 90 degree angle to the pipe. Now you just need to turn the valves to open which lets water enter the system, do this whilst keeping an eye on the pressure gauge. Once the pressure gauge reaches just below 1.5 bar turn the valves back to closed and you have successfully repressurised your boiler system.

Some models and brands of boiler have a slightly different process to repressurising boiler systems, this method is done with an internal filling loop. The main brand of boilers which implicate this process is Worcester, some other models by other brands also use this system but it is uncommon. It is easier to show you this method, therefore we recommend watching this tutorial.  We always recommend consulting your user manual before repressurising your boiler as there is such a wide variety of boilers available on the market with constantly evolving technology.

If your still worried, don’t threat! Flair Facilities will perform any necessary pressure adjustments as part of a boiler service or if you still have any questions about re-pressurising your boiler and about low boiler pressure in general, feel free to drop us a call on 0207 9989 005 on our 24/7 customer service helpline.