Boiler loosing pressure? 6 common causes and fixes

For example, your combi boiler may lose pressure because: 

• there is a leak in the pressure relief valve;
• you have an issue with the expansion vessel;
• a leak appeared in the heating pipework;
• you recently bled your radiators.

In this article, we will give you in-depth information about the most common causes behind your boiler losing pressure and detailed steps on how to fix the specific problems.

What is Boiler Pressure and How It Works?

Before we get to the actual problems and solutions part of the article, you’ll need to learn about how your appliance works, and we will start with what exactly boiler pressure is. 

What your boiler does is that it heats cold water and then distributes it in a circuit of pipes and radiators around your property. In order for the appliance to do its job, it needs to have proper pressure to “push” the water. In modern combi boilers, the pressure is controlled by a filling loop that is connected to the cold water supply. It’s simple as that! 

What Should The Normal Boiler Pressure Be and How to Check It?

As a rule of thumb, the normal boiler pressure is between 1.0 and 1.5 bars. If you are wondering how to check the boiler pressure, the process is very simple. First, you need to locate the pressure gauge on your boiler. Usually, it’s on the front part of the appliance. The pressure gauge will have two coloured sections – a red one and a green one. Typically, when the pressure is too low, or beneath 1.0 bar, the needle of the gauge will be below the green section, and if the pressure is too high, over 2.5 bars, the needle will be in the red section. And this is only a part of the boiler maintenance you must know about. 

What Causes a Boiler to Lose Pressure and How to Fix It?

There are many reasons behind a boiler losing pressure. They can range from a faulty pressure relief valve, recently bled radiators, a problem in the expansion vessel, to a huge leak in the actual pipework. You can easily diagnose such issues and some of them you can fix by yourself. 

1. Boiler leak

One of the most common reasons for a boiler to lose pressure is an actual leak in the appliance. Boilers have many weak points – the automatic air vents, the washers, the rings joints, even the diverter valves, and heat exchangers can get damaged. When a big leak occurs, you won’t miss it – your boiler will drip out of the bottom. Smaller, internal leaks aren’t so noticeable because the water frequently ends up dripping on another surface. 

Solution:  

A boiler leak is not a minor issue that you can fix with some duct tape. If you notice a puddle of water underneath the appliance, your best option is to call a registered gas engineer to inspect it. The cost of the repair will highly depend on the type of damage, however, it’s always better to pay for a replacement part, than for a brand-new boiler. And no – you can’t leave it dripping – the issue won’t magically disappear by itself. 

2. A leak in the system

If you suspect that you have a leak in your pipework, you’ll need to do some investigative work. Start tracing the pipes in your property to locate the issue. We recommend looking around the joints and bends as they are the weakest parts of the system. Similarly, to a leaking boiler, you are looking for a small puddle of water.

Solution:

Again, you’ll need a certified professional to resolve the issue. A temporary solution for a pipe leak is to turn off the isolation valves for the heating circuit underneath the boiler, but only until the engineer comes to repair it. Note that this will leave you without heating, so book a technician fast!  

3. Faulty pressure relief valve

As the name suggests, pressure relief valves are designed to let out the excess pressure, and that happens usually when the boiler exceeds 3 bars. When such a valve, no matter internal or external, becomes faulty, it will start leaking water from the central heating system at a lower pressure to a pipe located outside of your home. 

Solution:

In this situation, the only thing you really can do is check if the above-mentioned copper pipe is dripping and to call a professional to change the valve.

4. Damaged expansion vessel

When you have a damaged expansion vessel, your boiler pressure will rise to 3 bars when it heats up. The reason for this is that the expanded water in the system has nowhere to go. What happens next is that the pressure relief valve lets out the excess pressure from the boiler, so the appliance can reach a safe spot. The result is a significant drop in the pressure gauge. 

Solution:

Usually, re-pressurizing your expansion vessel should be a part of your annual boiler service. The procedure prevents issues, such as a damaged vessel, from occurring in the first place. That said, don’t worry – the repair of a damaged expansion vessel is neither expensive nor hard to complete, however, it should be always performed by a licensed professional. 

5. Recently bled radiators

If you have bled your radiators recently, there is a chance that the boiler pressure dropped. The good news is that diagnosing and fixing such an issue is easy. 

Solution:

To fix low boiler pressure due to recently bled radiators, you must re-pressurize the appliance. To do that, locate the filling of the boiler – it comprises two taps and a copper pipe with connection parts. When you need to re pressure the appliance, grab the copper pipe and screw it in between the two taps. Once done with that, open one of the taps and monitor the gauge needle. Then, open the second tap and wait for the pressure to rise – the needle should be between 1.0 and 2.0 bars. Close the taps and you are done!

Typically, when you book professional radiator repairs, the certified expert will bleed the radiators and once he finishes the job will check the pressure of your boiler. If the boiler pressure is too low, he will re-pressurise the boiler, so you don’t have to worry about the health of your heating system.

6. Broken boiler parts

The last reason for your combi boiler to lose pressure is a broken part. If you aren’t gifted with divination abilities, don’t worry – your boiler’s monitor will show that the appliance doesn’t feel very well and needs a check-up. 

Solution:

Anything broken needs to be replaced, right? The same applies to your boiler – if something is damaged inside, you must get a professional to remove it and replace it with a new one. That said, if the unit is on the older side, it could be the perfect time to replace your boiler. For example, a heat exchanger replacement can burn a hole in your wallet.