Help! Mysterious Pressure Drops in My New Glow-Worm Combi Boiler

hobbits · 24 Jan 2025

Hi everyone,

I'm hoping someone here can shed some light on a frustrating issue I'm having with my brand new Glow-Worm Compact LPG combi boiler. I had it installed in my small bungalow about two months ago, and for the first month, it worked like a charm. However, for the past month or so, I've been experiencing random pressure drops.

The error message on the boiler display indicates that the pressure is too low. When I check, it's often at zero. I've been manually refilling the system to bring the pressure back up, but the problem keeps recurring at seemingly random intervals throughout the day and night, even when the heating and hot water are not in use.

I had my original installer come back to inspect the system. He suspects a potential leak in the central heating loop. However, there are absolutely no visible signs of leaks anywhere in the house – no damp patches, dripping pipes, or anything like that. This makes me wonder if the leak theory is correct.

If it's not a leak, what else could be causing these random pressure drops? Could it be a faulty expansion vessel, pressure relief valve, or something else within the boiler itself?

At what point should I contact Glow-Worm directly? Is this something covered under warranty?

Any help or advice would be greatly appreciated. I'm at a loss as to what to do next!

Thanks in advance!

jj4091 · 24 Jan 2025

You should be able to isolate the flow and return pipes to the boiler and if you do that then top up the boiler and leave off overnight and there has been any drop in pressure it is the boiler at fault. If it is ok then drops when you open the flow and return valves again you have a leak in your radiators or pipe work.

Harry Bloomfield · 24 Jan 2025

hobbits said:
Could it be a faulty expansion vessel, pressure relief valve, or something else within the boiler itself?

It will be a relatively tiny amount of water loss, to cause the drop, and so difficult to spot.

It could be the expansion vessel, if it has lost air pressure, or leaked air pressure. You can easily check the PRV, by putting a sandwich bag, over the exit pipe to see if any water is collected. The system turned off, topped up, then flow and return valves under boiler turned off - if the pressure still drops, then it has to be a leak in the boiler.

hobbits · 25 Jan 2025

Thanks for the suggestion!

I have three valves under my boiler: two red and one blue. I believe these are the isolation valves for the flow and return. Just to confirm, to isolate the boiler, should I be turning all three of these valves (both red and the blue) to the "off" position?

jj4091 · 26 Jan 2025

Post a picture please.

hobbits · 26 Jan 2025

jj4091 said:
Post a picture please.

Note that on the boiler the two pipes to the right have an arrow pointing up and the two pipes to the left have an arrow pointing downwards. Example below...

dilalio · 26 Jan 2025

Blue is cold mains in.
Yellow is gas.
The two red ones are your flow and return pipes and are the ones you need to turn off once system pressure is up to normal.

You say bungalow. Do you have heating loop from boiler going up into loft and then dropping down to each rad?

This is common for bungalows and there should be AAVs on the pipework at highest point in the loft - these may still be purging air from the system and causing your pressure Loss. See if you can locate any, above the boiler, in the loft.

hobbits · 27 Jan 2025

Hi everyone,

Thanks for the advice. I ran a test last night to try and isolate the problem, as follows;

Pressurised System: I ensured the boiler pressure was at approximately 1 bar (which seems to be the maximum it will hold).

Isolated Boiler: I turned off both of the red handled valves, which, based on previous advice, are likely the flow/return valves for the central heating and domestic hot water. I left the gas valve (yellow handle) and the fresh water valves (blue handles) open.

This morning, I checked the boiler pressure, and it had dropped to zero.

From what has been mentioned here, since the boiler was isolated from the central heating circuit by closing the flow valves, the pressure drop to zero strongly suggests the issue lies within the boiler itself and not with a leak in the external heating circuit.

The plumber who installed the boiler and recently inspected it was insistent that the pressure loss was due to a leak in the central heating system. However, my test results seem to contradict this.

I plan to repeat the test tonight to double-check the results, but it seems fairly conclusive at this point.

I have some follow up questions which I hope the community can help with.

-Does this test definitively rule out a leak in the central heating circuit?

-Since the boiler is less than a year old, should I contact Glow Worm directly? What are my rights regarding a warranty claim? Will they send an engineer to inspect it?

-How should I approach the original plumber given his previous diagnosis? Should I involve them at this stage?

Any advice or insights would be greatly appreciated!

Johntheo5 · 27 Jan 2025

hobbits said:
Hi everyone,

Thanks for the advice. I ran a test last night to try and isolate the problem, as follows;

Pressurised System: I ensured the boiler pressure was at approximately 1 bar (which seems to be the maximum it will hold).

Isolated Boiler: I turned off both of the red handled valves, which, based on previous advice, are likely the flow/return valves for the central heating and domestic hot water. I left the gas valve (yellow handle) and the fresh water valves (blue handles) open.

This morning, I checked the boiler pressure, and it had dropped to zero.

From what has been mentioned here, since the boiler was isolated from the central heating circuit by closing the flow valves, the pressure drop to zero strongly suggests the issue lies within the boiler itself and not with a leak in the external heating circuit.

The plumber who installed the boiler and recently inspected it was insistent that the pressure loss was due to a leak in the central heating system. However, my test results seem to contradict this.

I plan to repeat the test tonight to double-check the results, but it seems fairly conclusive at this point.

I have some follow up questions which I hope the community can help with.

-Does this test definitively rule out a leak in the central heating circuit?

-Since the boiler is less than a year old, should I contact Glow Worm directly? What are my rights regarding a warranty claim? Will they send an engineer to inspect it?

-How should I approach the original plumber given his previous diagnosis? Should I involve them at this stage?

Any advice or insights would be greatly appreciated!

Have you carried out the test as suggested in post#3, very important.

"You can easily check the PRV, by putting a sandwich bag, over the exit pipe to see if any water is collected. The system turned off, topped up, then flow and return valves under boiler turned off - if the pressure still drops,"

This valve should not lift until at least 2.5bar, normal setting is 3.0bar.

dilalio · 27 Jan 2025

Yep. If you have isolated a pressurised boiler from the CH system and it loses pressure with no signs of leaks from within the boiler (ie underneath) then the first thing to check is the discharge pipe that connects to the PRV and (in most cases) pokes through the wall to outside to allow water from the CH to be expelled safely.

If the PRV has failed, this pipe will emit water and will be running, dripping or wet inside.

This does not rule out a split/pinholed Hex or a leak somewhere on the CH system.

Speak to your installer, explain the tests you've done and ask if he wants to come and double check prior to calling manufacturer.

Harry Bloomfield · 27 Jan 2025

hobbits said:
Isolated Boiler: I turned off both of the red handled valves, which, based on previous advice, are likely the flow/return valves for the central heating and domestic hot water.

Easy to double-check.... With system cold, and those valves open run the boiler for heating. the flow pipe will become quickly warm, followed a while later, by the return pipe. Mark the pipes with an F and a R, for future reference.

hobbits said:
-Does this test definitively rule out a leak in the central heating circuit?

If the valves have been closed over-night, and the pressure has been lost - if you now open a valve, whilst watching the gauge, and the gauge rises, then it confirms a boiler internal leak.

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Help! Mysterious Pressure Drops in My New Glow-Worm Combi Boiler

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