Category - Boilers
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Ryan Gill, Engineer

31 Jan : Updated 21 Mar ● 8 min read

Why does my boiler pressure keep dropping?

It's normal for homeowners and tenants to experience their boiler pressure dropping at some stage during the time in their home.

Although determining the problem is often as simple as checking the pressure gauge on your boiler, this doesn’t explain what’s causing the problem.

In this guide, we walk you through the most common causes and reasons why your boiler pressure is dropping so you can get the right tools and resources to fix the problem.

What are the signs that your boiler pressure has dropped? 

While boiler pressure is something that you can check yourself, the reality is that most people don’t do this until they’ve noticed a problem with their central heating or hot water.

But what are the signs that your boiler’s pressure is low? If you're experiencing the following signs, you need to go check your boiler pressure:

  • You run the hot tap, but the water doesn't warm up
  • Your radiators aren’t heating up when your central heating is turned on
  • Your boiler has turned itself off or is showing an error code.

What should your boiler’s pressure gauge read?

Your boiler pressure should be between 1 to 1.5 bars when your boiler is turned off and cool. Once turned on, your boiler pressure should increase only slightly, making it 1 to 2 bars.

If you're unsure about the number of bars, your marker should be in the green zone regardless.

How can you check if your boiler pressure is low?

Once you've identified an issue, how can you confirm that the problem is likely caused by low boiler pressure?

Checking the pressure gauge allows you to see whether your boiler pressure has dropped or risen. Here’s how to easily check your boiler pressure:

  1. Locate the pressure gauge on your boiler. This resembles a clock face with coloured bars and numbers typically from 1.0 to 4.0.
  2. If the needle is in the green zone, this means your boiler is at the correct pressure.
  3. If the needle is in the red zone, this either indicates that your boiler pressure is too low (before the green zone) or too high (beyond the green zone).
  4. Some more modern combi boilers or system boilers do not have a visible, clocklike pressure gauge and will instead display digitally - refer to your user manual if unsure, as this should be something that is easily checked. 

Is it normal for boiler pressure to drop?

It’s not uncommon for your boiler to lose a little bit of pressure slowly and only slightly, as this can easily be resolved by repressuring the system.

But a rapid drop in pressure, or a repeated drop after you’ve repressurised, usually means that there’s a problem somewhere. If this is the case, you should call upon a Gas Safe engineer to carry out any boiler repairs.

Is it dangerous for a boiler to lose pressure?

Whenever you discover that there’s a problem with your boiler, however small it may be, there’s always going to be some element of concern.

You’ll be pleased to know that, in most cases, low boiler pressure isn’t dangerous or something that should cause you to panic, although it will prevent your boiler from heating your home and water.

If your boiler’s pressure drops too low, you’ll usually find that it turns off and displays an error code. This can help identify a problem and allow you to swiftly take the necessary steps to resolve the issue.

What causes a boiler to lose pressure?

There could be numerous reasons why your boiler pressure keeps dropping, but the most common are:

1. Recent radiator bleeding

If you’ve recently bled your radiators, then this is likely to be the reason why your boiler has lost pressure.

This is because when you bleed a radiator, air is released from your central heating system, which lowers the pressure.

After bleeding a radiator, be sure to check the pressure gauge and top up the pressure if needed.

2. A leak in your central heating system

This is the most likely reason for a drop in boiler pressure, but locating a leak in your central heating system can be tricky.

The boiler operates with a network of pipes and radiators, so the leak could be anywhere in your home. If the leak in the system is not found and fixed, however, the boiler will continue losing pressure.

How to find a leak in your central heating system

The good news is that a leak in a central heating system tends to happen at the most vulnerable parts; the joints and the fixtures. Any pipework that connects to radiators is one of the most common points for leaks.

Depending on the type of boiler you have, the network of pipes, fixtures, and radiators creates a closed or sealed circuit. So, if you can see or feel water accumulating near a joint or fixture, this can be evidence of a leak.

Tell-tale signs of a leak around radiators and pipes are:

  • Discolouration
  • Staining
  • Paint bubbling or flaking
  • Softened or rotting wood
  • Rust marks
  • Swelling
  • Bulging or lifting of the skirting board
  • Other obvious water-related damage

If you suspect there is a leak, you can also place a bowl underneath the joint or fixture and see if water begins to pool.

If you can’t find any evidence of leaks in these areas, any possible leak could be inside a wall, under the floors, or inside the boiler itself.

How to fix a central heating system leak

You shouldn’t attempt to fix a leak in your central heating system yourself. Instead, call a Gas Safe engineer who can resolve the problem far quicker and give you some peace of mind.

If you’ve found a leak, lay down towels and place bowls under the leak to reduce the chance of further damage.

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3. A leak in your boiler

If you can’t spot any leaks from your radiators or pipes, it’s worth knowing that a boiler losing pressure can also be a result of boiler leaks inside the unit itself.

Over years of use, parts inside the boiler can become corroded or loosen, resulting in internal leaking which can lead to low boiler pressure.

A common explanation for boiler leaks is a pressure reducing valve (PRV) not working properly, resulting in a wet patch outside your boiler where the PRV terminates.

Other reasons for a boiler leak include:

  • A damaged expansion vessel
  • The filling loop valve not tightly adjusted
  • A diverter valve not being secure

How to find a boiler leak

The most obvious evidence that the leak is boiler-related is if there is water dripping from the boiler or accumulated around the boiler.

If you see this, your first step is to switch off the boiler to prevent further damage and any potential short-circuiting.

The boiler unit must be opened to find the specific location of a boiler leak but you should not attempt this by yourself.  Call a Gas Safe engineer to arrange for a boiler repair if this is the case.

How to repressurise your boiler

Repressurising your boiler is a simple process that should have your boiler working again in no time.

You won't need to call for assistance, though it is recommended that you consult your boiler’s manual or review the process on the manufacturer's website before beginning.

You can check out our helpful video guide to find out which repressuring method is right for your boiler: 

We’ll take you through the most common way to repressurise your boiler.

Make sure your boiler is turned off and has cooled down to make things safe - when repressurising, you want to base your pressure reading on a cold system as this will allow for more accuracy.

Once you've prepared yourself, you can follow these simple steps:

Step 1 - Locate the filling loop

Your boiler’s filling loop is the hose that connects the boiler to the cold water supply.

The location will differ depending on the brand and model but you’re looking for something that looks like this:


(Source: Worcester Bosch)


If unsure, it's always worth checking your boiler’s manual to find out exactly how to use the filling loop for your boiler, or look on the manufacturer's website.

Step 2 - Increase the boiler pressure

Increase the boiler pressure by opening one or two taps (depending on your boiler) to 45 degrees, letting cold water into the system. Be sure to do this slowly to avoid adding too much pressure.

Continue filling with cold water until the pressure rises to between 1 and 1.5 bars, which is in the green zone on the pressure gauge.

Step 3 - Once in the green zone, close the taps

Once the pressure has reached the ideal level, close both valves.

At this point, you can turn the boiler back on. You may need to press a reset button but this varies from boiler to boiler.

You’ll want to keep an eye on your boiler’s pressure gauge over the next couple of days to make sure the pressure doesn’t keep dropping.

What to do if your boiler keeps dropping pressure after being repressurised

If repressurising your boiler doesn’t stop your boiler from losing pressure, then there may be a more serious issue at play.
Contact a Gas Safe engineer to come and diagnose and fix the problem for you. 

Frequently asked questions

Help keep your boiler pressure balanced with BOXT

Your boiler pressure will need topping up from time to time, that’s completely unavoidable, but problems, most commonly leaks, can cause a more rapid drop and something that needs your attention.

An annual boiler service is a great way to prevent unexpected problems and helps catch minor issues before they amount into something more serious.

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