Wondered if someone could help diagnose my boiler?

[uptown47]

Hi there,

I have a combi boiler (Buderus 500-24C) and have had a few problems with it recently. I wondered, if I explain the problems, if someone on here could maybe point me in the right direction as to what the problem could be.

Firstly, I had a downstairs bathroom renovated. The radiator / sink etc were removed and then replaced when the job was done.

After that I noticed that the pressure was dropping from my boiler. I can't guarantee it wasn't dropping before the work was done. But it was down to 0.5 bar after the work was done. I topped it up to 1.5 bar and within a few days it was down to 0.5 bar again.

I spoke to the plumber to ask if there was a chance there was a leak. He said to turn off all the heating and put the boiler to 1.5 bar and then leave it. I left it over 24 hours and the pressure didin't change. He assured me that this means there isn't a leak.

Next part of the puzzle...

When the boiler was running I noticed the pressure going up to 3.5 bar. I spoke to another plumber about this and he said that (as I'm sure you will know) this points to a problem with the expansion vessel causing the Pressure Relief valve to operate. He came out and re-pressurised the vessel. The next time the heating was on it only rose from 1.5 bar up to 2.0 bar and I thought it was sorted.

The next day however the pressure had dropped again.

I telephoned the plumbing company and a different guy came out and re-pressurised the expansion tank again. His gauge seemed to show that the expansion tank was holding pressure (he had thought that it might be leaking hence losing pressure again so quick).

He duly left a few hours ago and the pressure has dropped again.

So... an hour or so ago, I pressurised the system to 1.5 bar.
I then put the heating on after disconnecting the Pressure Relief pipe
The pressure rose to just over 2.0 bar but nothing seemed to come out the PR pipe
The boiler got to temperature (65 deg) and turned itself off.
I then turned the boiler off and let it cool down.
When the temp got below 50 the pressure started coming down
And went all the way down to below 1.0 bar

I've now just re-pressurised the system to 2.0 bar
And I'm going to leave the heating off to see what happens.

Very sorry for the long rambling post but I wanted to give you all the facts.

I just wondered if there was any diagnostic tests I can do in order to find out what the issue is or do you have any ideas what the problem is and/or ways to check each potential issue?

Many thanks for any help you can give me

Cheers

John

 

[Dan Robinson]

Expansion vessel is shot.

 

[picasso]

As you have checked all the usual suspects the heat exchanger could be leaking down the condensate pipe, not easy to diagnose as it seems to leak only when its hot.

 

[Rampage77]

The valve on the expansion vessel will be leaking, it will hold pressure while he has his pump connected but slowly leak air out when he disconnects it.

 

[uptown47]

A big thanks for all the replies so far Dan, Picasso, Rampage.

I pressurised the boiler to 2.0 bar and then ran a bath (not to test.. just cos I needed a bath

I've just checked and the boiler is still exactly at 2.0 bar.

** Edit ** bear in mind the heating is off at the moment. I've switched it off to see whether it leaks with the heating off.

Does this point to a leak outside of the boiler in the radiators somewhere (i.e. maybe in the downstairs bathroom that was refitted)?

Or can it still be indicative of one of the suggestions already made?

Also, please don't be fooled by any of the buzz words I used in the original post, I know zero about boilers so please bear that in mind if I ask any 'stupid' questions

Cheers

John

 

[flameport]

** Edit ** bear in mind the heating is off at the moment. I've switched it off to see whether it leaks with the heating off.

It's the expansion vessel.
The problem won't occur with the heating off, as virtually all of the water in the system is not being heated when using hot water only.

The expansion vessel is a bag of air, and when the heating is on, the water contained in the radiators and pipes expands as it heats up. The air bag is there to allow the water to expand - air can be compressed, water cannot.

When the expansion vessel is faulty, the air leaks out of it and when the water heats up it cannot expand, so the pressure in the system increases until the relief valve operates, letting water out.
When off, it cools and the pressure in the system drops to less than it was to start with, typically too low for the boiler to operate.

The solution is to have the expansion vessel replaced.

 

[uptown47]

Hi Flameport,

Thanks for the reply and the explaination.

What I don't understand though is that after the guy left this afternoon and I pressurised the system to 1.5 bar and then put the heating on, I watched the pressure rise to just over 2.0 bar (I also had the PR pipe disconnected to see if anything came out of it) and then the heating got to temperature and switched off. I then turned the heating off and the pressure (eventually) dropped (when the temp dropped below 50 ish) to under 1.0 bar.

Bear in mind during all this time the pressure hadn't gone over the 3.0 bar that I understood was needed to trigger the PR valve?

Based on that info is your idea of the expansion vessel still correct?

Thanks again for your help (and patience) with this

Cheers

John

 

[Fallingdown]

Put som leak detection fluid or fairy liquid and water on the Schroeder valve on the expansion vessel(the part the engineer pumped it up with) turn on heating and look for bubbles.

 

[uptown47]

Hi FallingDown

I'm not sure I can get to it as its situated behind all the other gubbins but I'll have a look and try the fairy liquid idea.

So going back to my post to flameport a couple of posts ago. Based on that info could it still be the expansion vessel then?

Thanks

John

 

[Fallingdown]

That would be my guess

 

[uptown47]

Ok thanks for your help. Will try the fairy liquid trick and see what happens. Thanks again

 

[Agile]

Put som leak detection fluid or fairy liquid and water on the Schroeder valve on the expansion vessel(the part the engineer pumped it up with) turn on heating and look for bubbles.

Are you sure that you don't mean a Schrader valve ?

 

[Dan Robinson]

 

[uptown47]

Hi all,

A MASSIVE thank you for all your help so far. We are getting somewhere now.

I checked the bike tyre valve on the expansion vessel. It's quite a difficult to get to as it's behind the boiler. But when I looked closely at it I could hear air slowly crackling out through it.

See the pics...

So my next questions are....

Is the valve 'fixable'? i.e. will a bit of WD40 and a few on/offs reseat it correctly again?
Is it easy to change the expansion vessel (i.e. do I need a 'gas safe' guy there or is a DIY job - I'm a competent DIY'er and an electrical engineer by trade)?
Do you have any places you would recommend to get a new expansion vessel for a Buderus boiler?

I put the pics up just to show you the setup but also to potentially help someone else who may stumble on this thread in the future.

Cheers

John

 

[Terry lambert]

You just need to change the core, drop the pressure out of the boiler, remove core, insert new core, re charge vessel whilst drain point still open

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