Worcester boiler pressure

Worcester Boiler Pressure Help Guide: Too Low & High Fixes

Last updated on October 5th, 2023

When your Worcester boiler pressure drops too low the boiler will stop working, which means you will have no heating or hot water.

Repressurising a boiler is easy but very important to keep your central heating system running.

I have been repairing boilers and heating systems for over a decade, and have probably come across every boiler pressure problem.

Here I’ll share my thoughts on how to fix pressure problems including how to repressurise a Worcester boiler.

Worcester Boiler Pressure

The boiler pressure should be between 1 and 1.5 bar when the central heating is off and the radiators are cold.

The boiler pressure rises when the heating is on, so you will not get the proper reading until it has cooled down.

  • If your boiler pressure drops below 1 bar you should top the pressure up
  • If it is above 1.5 (when cold) you should release some pressure
What pressure should be on Worcester boiler gauge
Correct Worcester boiler pressure

Worcester Boiler Pressure Low

Your Worcester boiler will usually stop working when the boiler pressure drops too low, typically around 0.5 bar, but they will sometimes keep working on 0.

Low boiler pressure can be fixed easily by topping the pressure up, if you have to keep topping the boiler pressure up, you have a problem that needs to be fixed.

Worcester Boiler Keeps Losing Pressure

If your Worcester boiler is losing pressure you will have a leak somewhere on your heating system or the pressure relief valve is letting the water out.

To find out which it is, you can tie a carrier bag to the end of the copper blow-off (overflow) pipe outside.

Boiler Leaking Water From Overflow Pipe
Boiler blow-off pipe

If your Worcester boiler has lost pressure again, you should check the bag to see if it’s wet and if it is, that means the pressure relief valve is letting the water out.

This is most likely an expansion vessel problem, if so, you will need to recharge the expansion vessel, or you’ll have to replace it if the diaphragm inside is split.

If the bag is dry after your Worcester boiler loses pressure you must have a leak on the central heating system which will need to be found and fixed.

Leaks can be anywhere on the full central heating system, boiler, radiators, or pipework under floors.

Worcester Boiler Leaking Water

A Worcester boiler leaking water is quite common as they have a few places they regularly leak from.

  • Hot water flow turbine adaptor
  • Left-hand flow block
  • Right-hand block
Worcester boiler flow turbine adaptor leaking
Worcester flow turbine adaptor leaking

These Worcester boiler leaks tend to be on the hot water side of the boiler so shouldn’t affect the boiler pressure.

Worcester boiler losing pressure but no leak

A leak is the most common cause of a boiler losing pressure, but it could also be caused by:

  • An expansion vessel problem
  • A pressure relief valve problem
  • A filling loop problem
  • Bleeding the radiators

How to Repressurise a Worcester Boiler

To repressurise a Worcester boiler, you must add more water (pressure) to your heating system from the cold water supply via a filling loop.

Worcester boiler filling loop key
Worcester boiler filling loop key

Repressurise Worcester Boiler With a Key

Unfortunately, some idiot at Worcester Bosch thought it would be a good idea to use a key to repressurise their boilers.

Thankfully modern Worcester boilers don’t use a key to increase the boiler pressure anymore.

How to repressurise a Worcester boiler with a key:

  1. Insert the white plastic key
  2. Turn the white square plastic knob anti-clockwise (like a tap) to open the filling loop and let some water in
  3. Keep turning the white plastic knob until you can hear the water
  4. When the boiler pressure has reached 1 to 1.5 bar close the valve
  5. Reset the boiler (if needed)
  6. Turn the heating on
Repressurise Worcester boiler with a key

Repressurise Worcester Boiler Without Key

To increase the pressure on a Worcester boiler without a key you will need to:

How to repressurise Worcester boiler without key:

  1. Open one valve fully (only if there are two valves)
  2. Open the other valve slowly until you can hear the water
  3. When the boiler pressure has reached 1 to 1.5 bar close both valves
  4. Reset the boiler (if needed)
  5. Turn the heating on

Do not open both filling loop valves fully straight away as the water pressure in the house might be too much for the boiler and cause the boiler pressure to shoot up too much.

How Often Should You Need to Repressurise a Worcester Boiler?

If you need to repressurise your Worcester boiler often you have a problem that you need to get fixed.

A combi boiler is a sealed pressurised system so you shouldn’t need to repressurise it at all.

If the boiler pressure drops, it means the water is getting out which should not be happening.

Low pressure on Worcester boiler gauge
Worcester boiler low pressure

If you need to repressurise your Worcester boiler once or twice a year I wouldn’t worry about it as it’s such a small leak that it’s barely noticeable.

Worcester Boiler Pressure Too High

Having your Worcester boiler pressure too high (above 1.5 when the system is cold), means you will need to let some pressure (water) out of the system.

Worcester boiler pressure too high
Boiler pressure too high

You can do this via a drain-off valve. A drain-off valve can be on the boiler, a radiator valve or anywhere on the central heating pipework.

Or, you can bleed a radiator from the bleed valve into a bucket. 

You will need to open the bleed valve with a bleed key and let the water out and into a bucket until the boiler pressure has dropped back down.

bleeding water out of a radiator
Bleeding water out of a radiator

How to Release Pressure on a Worcester Boiler

Knowing how to release pressure on a Worcester boiler might save you from some boiler problems. If you repressurise a boiler too much and cause the pressure to go too high, you will need to release the boiler pressure.

There are a few ways to release the pressure on a Worcester boiler.

Here are my go-to ways when out on boiler repair jobs:

  • Open a drain-off valve on the Worcester boiler or central heating system with a hose attached and let the pressure drop.
  • Bleed a radiator from a bleed valve and let the water squirt out into a bucket. This can take a while for the boiler pressure to drop.
  • Crack a nut on the Worcester boiler heating pipes or a radiator valve and catch the water in a Plumb Tub or similar.
  • Open the pressure relief valve on the Worcester boiler by pushing the red plastic lever.
Worcester boiler pressure relief valve
Worcester boiler pressure relief valve (red)

Worcester Boiler Filling Loop

There should be one of these types of boiler filling loops under your Worcester boiler:

  • A keyhole for a plastic key with a white square knob to turn: This knob will need turning like a tap to open and close.
Worcester boiler filling loop with key
Worcester boiler filling loop with key

  • Easy-Fill filling link with blue handle: These are on the latest Worcester boilers (thankfully). This will simply need to be pulled to top up and then released to stop filling.
Worcester boiler pressure filling link
Worcester boiler filling link

  • An external filling loop:

A filling loop will be built in on the bottom of your boiler on most modern and small combi boilers, but there could also be an external filling loop on the pipework.

Usually, this is below the boiler but there could be an external flexible filling loop anywhere on the heating system, sometimes hidden away in cupboards or behind boxing.

Filling loop open
External filling loop

Expansion Vessel Undersized

Worcester boilers are sometimes undersized when fitted and are not large enough to cope with a large house’s central heating system.

This means the expansion vessel might not be big enough and can cause problems with the pressure.

Getting the best combi boiler for a large house is very important, but adding an external expansion vessel that is large enough for the central heating system is also an option rather than upgrading to a bigger boiler.

External expansion vessel fitted
External expansion vessel fitted

Worcester Boiler Problems

Here are some other common Worcester boiler problems you can learn about:


It’s good to know for sure what the problem is when your boiler is not working but the first thing you need to check is the pressure gauge.

If the pressure gauge is reading zero (or close to zero) your boiler needs to be repressurised to get it working.

Repressurising a Worcester boiler is an easy job. It can save you from having to pay a plumber to do such an easy task.

If you have to keep repressurising your Worcester boiler, you should get that problem found and fixed as soon as possible.

Feel free to leave feedback or ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you found it helpful.

Worcester Boiler Pressure FAQs

Can you repressurise a Worcester boiler by yourself?

Yes. You can repressurise a boiler by yourself, and you should definitely learn how to repressurise a combi boiler if you don’t already know.

Some boilers can be tricky to repressurise but most are very simple.

What causes a Worcester boiler to lose pressure?

There are many causes for a Worcester boiler to lose pressure. The most common cause of a Worcester boiler losing pressure is a leak, this could be on the boiler or anywhere on the central heating system.

Another cause could be the expansion vessel has lost its air and needs to be recharged or replaced.

The pressure relief valve could also be letting by. Or, the filling loop on the Worcester boiler could be slightly open or damaged.

Do I need to turn my boiler off to repressurise?

No, you do need to turn your boiler off to repressurise.

You should ensure the heating is turned off before repressurising the boiler as it will sometimes fire up as soon as it reaches the minimum working pressure.

This can cause the pressure gauge to move around so you don’t get an accurate reading but it isn’t a big deal.

How do I increase the pressure on my Worcester boiler?

To increase the pressure on your Worcester boiler you must add water to the heating system using a filling loop.

The filling loop should be under the boiler on a combi or could be elsewhere on the system if it’s not a combi.

You must open both valves on the filling loop (or one if it only has one) to increase the pressure then close both valves when it reaches 1 bar on the pressure gauge.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

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24 thoughts on “Worcester Boiler Pressure Help Guide: Too Low & High Fixes”

  1. Hi, I have a Worcester greenstar 4000 the pressure is low and pulling the blue lever isn’t doing anything, I managed to increase the pressure 0.3 bar in 45 mins!! Is there something else I could do??

    1. You could check the copper pipe outside to see if water is coming out. If not then it sounds like there might be a problem with the filling loop or really low water pressure in the property.

  2. Hi Steve
    Thank you for this site. It’s very helpful. I have an old Worcester Bosch that has 2 valves for repressurising the boiler. The one in the right is horizontal when at rest and the one on the left is vertical. I’ve been repressurising it myself for years, once a month, but this month the left one will not turn fully so I’m not able to fill the boiler fully. What should I do please?

  3. Steve Cuthbertson

    I’m a fellow geordie. My Worcester Bosch has slowly required repressurising and recently needed doing twice a day after reaching zero. I have called out the engineers who fitted it twice and both times they couldn’t find the problem. However after both visits the pressure held for a week and then gradually started dropping again. Any ideas on what could be happening?

    1. It could be anything mentioned in this post. You should try isolating the boiler overnight to work out whether it’s a leak on the heating system or a problem with the boiler.

      1. Steve Cuthbertson

        Sorry to ask again. The pressure hasn’t dropped since they were here on Friday but it was dropping to zero every day whether it was on or off before they came. I was wondering if some valve was staying open after the heating has been on. Is that a possibility?

        1. I doubt it’s a valve problem. One possibility is they might have recharged the expansion vessel but the vessel is faulty and slowly losing its pressure.

  4. My green star heatslave ii oil boiler is at full pressure, I’ve been able to de pressure it but it keeps going up. I’ve bled radiators but it keeps going up, I’ve switched it off and the radiators keep filling with water. Please help

    1. Hi Shona,
      It sounds like the filling loop for repressurising the boiler is open or not closed fully. It could also be damaged and constantly letting water into the system.

  5. Hi Steve.
    I had a new Worcester Bosch 4000 boiler installed on February 20th and it’s already showing low pressure. It took me by surprise as the British Gas engineer never mentioned a word about things like that happening and certainly didn’t show me what to do if it happens.
    Good old YouTube. Is this something that happens on a regular basis?

    1. Hi Sara,
      It’s quite normal for the boiler pressure to drop after a new boiler replacement as the system gets drained and refilled, it could just be trapped air working it’s way out.

      If the pressure keeps dropping then you’ll have a problem that needs to be found and fixed but repressurising the boiler once is fine.

  6. I have a Worcester ( British Gas) 532i which drops pressure on DHW demand. This still provides hot water and pressure recovers when demand stops. This happens when changing from CH to DHW. Is this normal. ?

    1. Hi Rowland,
      Yeah that can happen, it’s nothing to worry about as long as the boiler pressure is good when there’s no hot water demand.

  7. Hi Steve, I have a Worcester Bosh 29L, 9 years old but serviced every year. When cold the pressure drops to 0.5 bar, when on it rises to 2.5 bar. I can’t see any leaks, I’ve bled all rads and the filling loop is closed. The plumber at last service said it might be the expansion tank losing pressure, does that sound reasonable?

    1. Hi Tony,
      Yes, definitely sounds like an expansion vessel problem causing the boiler pressure to keep dropping. You’ll need to recharge it or possibly replace it if the diaphragm inside the vessel has split. You should be able to see water coming out of the copper pipe outside when the pressure drops as the pressure relief valve lets the water out when the pressure reaches 3 bar.

      You could turn the heating (radiator) temperature down at the boiler (not the thermostat) to try to keep the pressure down until it’s fixed as this can sometimes help.

  8. Hi Steven
    I have an older Worcester Bosch combi boiler with a big plastic key to repressurise the boiler.
    I have noticed the boiler pressure dropped below one. One day I turned the heating on, radiators weren’t warm as all. I thought I had to bleed the radiators, then found out the water pressure dropped close to zero. I followed your instruction to repressurise the boiler, nothing happened, and water started dripping from the key, black needle still on red zero.
    What can you suggest, there is not water but not the heating and there isn’t a external filling hose.
    Thank you, lynn

    1. Hi Lynn,
      It sounds like you haven’t inserted and turned the key properly. You must ensure the key is turned and lined up correctly before repressurising the boiler.

      Or, you might have a faulty filling loop which will need to be replaced, or you can fit an external filling loop on the pipes.

  9. Hi Steven, I have a WB Greenstar 25si compact boiler with I’m pretty sure an external filling loop. I need to repressurise the boiler as the level is on zero (hot water is fine but I’ve yet to put or try to put the heating on, since March-April this year. There is a single flexible loop that goes upwards and connects to two valves under the boiler but no obvious cap to unscrew to attach one end of it. I can send a picture! Can you help? I’m used to the key method and have no idea what I’m meant to do with this system. Thank you very much! Debbie.

    1. Hi Debbie, yeah sure. I’ll turn on image uploads so if you upload a photo to these comments I’ll have a look and let you know.

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