Why does my boiler pressure keep dropping? 

Are your hot taps running cold? Have you noticed your radiators aren’t getting warm? It could be a sign your boiler has lost pressure. 

In this blog post, we’ll look at why it happens and how to fix the problem. 

Is it normal for your boiler to keep losing pressure? 

It’s normal for your boiler to lose pressure from time to time. It happens to almost all boilers of varying ages, makes and models, but if it starts to happen frequently, it could be a sign of an underlying problem.

You can check your boiler’s pressure by looking at the gauge on the front. This is usually a clock-like gauge with numbers from one to four. Some combi boilers and system boilers have a digital display instead of a dial. 

Refer to your boiler’s manual for help interpreting the numbers on this display. 

What should your boiler pressure be? 

Regardless of the pressure guage numbers, the boiler dial should be in the green zone. If it’s in the red zone, it means the pressure in your boiler is too high or too low. 

Your boiler’s pressure will naturally be lower when it’s switched off. You can expect it to be between 1 and 1.5 bars in this case. Once turned on, it will increase to around 2 bars. 

If your boiler pressure rises to 2.75, it’s too high. You can read more about high boiler pressure in this blog on fixing boiler pressure issues

What causes low pressure? 

Boiler pressure is dictated by the balance of air and water in your system. If this is off balance, the pressure in your boiler will drop or rise unexpectedly. There are several causes, but the most common are: 

  1. A leak in your central heating system   
  2. A leak in your boiler 
  3. Bleeding your radiators 

Let’s look at each of these in more detail.  

A leak in your central heating system 

A leak in your central heating system can cause low pressure. The leak can be as small as a pinprick, making it difficult to find (not to mention that most of the pipes that make up your system are hidden behind walls and under floorboards). 

If you can’t see any obvious signs of a leak, you may need the help of a professional to find and fix the leak

A leak in your boiler 

A drop in boiler pressure can also be caused by a leak inside your boiler. Old or faulty parts are typically behind this. Some problems can be fixed (assuming they still make parts for your make and model of boiler), but depending on its age, it might be cheaper to replace your boiler.  

If you feel confident looking under your boiler cover, you can locate the leak yourself, but we recommend enlisting the help of a Gas Safe engineer. 

Bleeding your radiators 

Letting the air out of your radiators (known as bleeding your radiators) can lead to a drop in boiler pressure. If you bled your radiators recently, this is the likely culprit of low pressure. In this case, you’ll need to repressurise your system.  

How to top up your boiler pressure 

If this is the first time your boiler pressure has dropped or you recently bled your radiators, it may just need topping up with water again. It’s relatively easy to top up the pressure yourself.   

We’ve included here the most common way to top up a boiler. If this doesn’t apply to your boiler, refer to the manual that came with it.  

1. Find the filling loop

The first step to repressuring your boiler is to find the filling loop that connects your boiler to the cold water supply. It’s typically a silver braided hose with a black tap at either end. 

If you can’t find something that looks like this, your boiler may have an internal filling loop (refer to the manufacturer’s manual for help). 

2. Open the taps

Start by ensuring your boiler is switched off, then slowly open the tap(s) until you start to see the pressure rise.

As water flows back into your system, the dial on your pressure gauge should move back into the green zone.

3. Monitor the pressure

Once you’ve topped up the pressure, close the taps securely (this is important – if you forget and leave them open, you could cause a different problem). Turn your heating back on and monitor the pressure over the following days. 

If the problem continues, it’s time to call the boiler repair experts

FAQs on low boiler pressure 

If you still have questions about your boiler pressure, you might find the answers in our collection of FAQs. 

Do boilers lose pressure in cold weather? 

Boilers can lose pressure in cold weather, but the cold weather itself is not the direct cause of the pressure loss. Pressure loss can occur for various reasons (see above), and some of these problems can become more pronounced in cold weather.

What should boiler pressure be when the heating is off? 

When your central heating is turned off, your boiler pressure should be between 1 and 1.5 bars. 

What happens if gas pressure is too low? 

Low boiler pressure will affect your central heating system. When this happens, you’ll notice your hot water taps running cold and your radiators failing to heat up. The good news is that it’s relatively easy to repressurise your system using the steps in this blog. 

Is it safe to use a boiler with low pressure? 

In most cases, low boiler pressure is not dangerous. It’s still safe to use your central heating system, but it may prevent your radiators and water from reaching the desired temperature. 

Why does my Worcester boiler keep losing pressure? 

A leak in your system is the most likely reason for your Worcester boiler losing pressure. This might be from the boiler or the central heating system, i.e. your pipes and radiators. If you can’t see any obvious signs of a leak, contact a Gas Safe engineer for help locating the problem.  

Can you repressurise a boiler yourself? 

It’s relatively easy to repressurise your boiler. You can top up your boiler in three quick steps following the instructions in this blog. We’ve covered the most common way of repressuring a standard boiler. For additional help, refer to the manufacturer’s manual or call a plumber.  

Post author

Lee Wenham

Date of post

November 10, 2023


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