Glow-worm boiler pressure

How to Repressurise Glow-Worm Boiler Pressure

Last updated on February 19th, 2024

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

Boiler pressure should be between 1 and 1.5 bar on the pressure gauge when the radiators are cold.

I have been repairing and servicing boilers for over a decade and have fixed my fair share of boiler pressure problems.

Glow-Worm Boiler Pressure

If your boiler pressure is under 1 bar then you should increase the boiler pressure.

If your pressure is over 1.5 bar when the system is cold, then you should reduce the pressure.

Boiler Pressure When Heating is On

The pressure gauge on your Glow-worm boiler when the heating is on can rise to over 2 bar.

This is perfectly normal as the heat expands the water inside the central heating system when the heating is turned on.

Boiler Pressure Too Low

Low boiler pressure is very common and can cause your boiler to stop working. If your pressure becomes too low then you need to repressurise it.

How to Repressurise a Glow-Worm Boiler

To repressurise a Glow-worm boiler, you must open the two valves at either end of the boiler filling loop to let some water in.

  1. Open one valve fully
  2. Open the other valve slowly until you can hear the water
  3. When the 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 Baxi boiler pressure to shoot up too much.

Filling Loop on a Glow-Worm Boiler
Filling Loop on a Glow-Worm Boiler

Boiler Keeps Losing Pressure

Your Glow-worm boiler pressure loss is most likely for one of these reasons:

  • A Leak on the central heating system
  • The expansion vessel has lost its air
  • Pressure relief valve passing water
  • Filling loop open

Boiler Pressure is Too High

Your boiler pressure is too high most likely for one of these reasons:

  • Somebody repressurised the boiler too much: Reduce pressure in the boiler by letting water out
  • The expansion vessel has lost air or the diaphragm inside is split: Recharge or replace the expansion vessel
  • The filling loop is not closed fully or damaged: Close the filling loop fully or replace it
  • Pinhole leak on plate heat exchanger on a combi boiler: Replace the plate heat exchanger

How to Reduce the Pressure

To reduce boiler pressure you must let some air or water out of the heating system.

Here are the 5 best ways to reduce the pressure:

  • Bleed a radiator
  • Open a drain off valve
  • Open the pressure relief valve
  • Crack a nut
  • Magnetic filter

Bleed a Radiator

The easiest way to do this is to bleed the radiators using a radiator bleed key.

You should bleed all the air out of all the radiators. When there is no air left, you can bleed the water out of one radiator until the pressure drops to around 1 to 1.5 bar.

Put a towel or two down on the floor under the radiator and catch the water in a bucket.

Open a Drain Off Valve

Most central heating systems have at least one drain-off valve.

You can find them on radiator valves, radiator pipes, and on the bottom of boilers. They are for draining water out of the central heating system.

You can connect a hose to a drain-off valve using a hose clamp and run the pipe to an outside drain or down the toilet or sink, then open the square nut using an adjustable spanner until the water starts coming out.

Keep checking the pressure gauge on the boiler until it has dropped to around 1 to 1.5 bar.

Open The Pressure Relief Valve

Another way to reduce boiler pressure without bleeding radiators is to open the pressure relief valve.

A pressure relief valve is on every pressurised central heating system.

They are usually inside the boiler but could potentially be in the hot water cylinder cupboard.

You simply turn it anti-clockwise (or pull the lever on some boilers), and the pressure will drop very quickly.

boiler pressure relief valve pulled
Boiler pressure relief valve handle being pulled

This is the easiest way to reduce boiler pressure without bleeding radiators and the most common way I do it when I’m on a job.

The only problem with opening the pressure relief valve is that once they’ve opened they might not close again properly.

This will tend to happen on dirty systems but 9 times out of 10 it’s fine. If it does happen, you need to change the pressure relief valve.

Crack a Nut

Cracking a nut is another way to lower the pressure without bleeding radiators when your boiler pressure is too high. This is one of the messiest ways to do it but it is possible.

You can put towels and a bucket under a nut on the boiler, radiator valves, or anywhere on the central heating pipes.

Then crack the nut until water starts coming out and keep an eye on the boiler pressure gauge.

Magnetic Filter

It is also possible to reduce boiler pressure via the central heating magnetic filter.

You should place a towel and bucket underneath the magnetic filter then open the nut on the bottom (if it has one) or you can open the bleed valve with a bleed key.

Let the water come out into the bucket and close it when the pressure gauge has dropped to around 1 to 1.5 bar when the system is cold.

Glow-worm Boiler Guides


If your Glow-worm boiler pressure is too low or too high then fixing this is important.

Repressurising a 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 it, you should get that problem found and fixed as soon as possible.

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

Please share this post if you find it helpful.


  • 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.

    View all posts
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7 thoughts on “How to Repressurise Glow-Worm Boiler Pressure”

  1. Mary Morton

    I have tried to increase the pressure to 1.5 but unfortunately it increased to 1.8. I’ve tried to turn the yellow knob which I think is to release some of the water but its too stiff. Is there a problem with leaving this at 1.8. I don’t know how to do anything else. Thank you.
    Mary Morton

    1. Steven Reid

      Some filling loops only have one valve to open to repressurise the boiler. Just make sure the other handle hasn’t snapped off or been removed

  2. William

    I have recently fitted new parts on leaking valve on radiator, pressurised system back up and bled radiators, OK. But since then pressure keeps on dropping down to zero overnight takes about 6 hrs?

    1. Steven Reid

      It might be a leak under the floor downstairs that you can’t see. You should check the copper blow off pipe outside for signs of water coming out and also check to see how much the boiler pressure increases when the heating is on.

  3. William

    Fixed leak on radiator valve, repressurized system all ok, but keeps dropping off to zero overnight. Cannot find any leaks in apartment?

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