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Boiler seemingly overpressuring

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NomoneyNohoney
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Boiler seemingly overpressuring

#24074

Postby NomoneyNohoney » January 18th, 2017, 10:44 am

We have a Worcester Greenstar boiler, and we also have a small leak from the gland, on one of the radiators.
Over time the pressure drops, so I recharge the system and all is well.
Today, the pressure must have dropped too far, as the boiler didn't work for either hot water or radiators. Looking at the circular pressure gauge, the pressure was virtually zero, so I recharged the system to the lowest point of the "green - ok" indicator on the dial (about the ten o'clock position.)
Hot water and radiators now working.

After about half an hour, glanced at the pressure dial and saw that it had gone, from the 10 o'clock position to about the two o'clock position, which presumably means the pressure had built up in the system, much greater than when I'd set it.
I wonder why the (indicated) pressure has shot up so much. I've turned it off at the moment, hoping for inspiration as to why its gone up so high, and now its switched off, the pressure dial has dropped to the 12 o'clock position - in the green, but higher than I like it, and I suspect it will rise again, once the system switches on.
I assume I need to bleed off some of the water, but would be pleased for any guidance and thoughts. Could it be something as mundane as a radiator now needs bleeding?

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Re: Boiler seemingly overpressuring

#24119

Postby Fingers » January 18th, 2017, 1:11 pm

System is behaving in a manner that shows lack of room for expansion.
First thing to check is the pressure vessel, it probably needs its air space restoring. A car foot pump and some thought should do it.

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Re: Boiler seemingly overpressuring

#24132

Postby Dorn1 » January 18th, 2017, 1:44 pm

Re-pressurising the expansion tank will only work if its lost air (implies a faulty air valve), the other failure mode is that the rubber diaphragm inside the expansion tank splits. If you press the air valve and water comes out - that's what 's happened.
We've just replaced our WB, as it had its 2nd cracked main heat exchanger in 10 years, higher pressures will push water out of that very quickly. From what you say any loss of the scale you suggest would soak a carpet, so I would be suspicious of the Heat Exchange in the boiler. The leaked water tends to travel out through the condensate drain, although if it builds up it can create a noise like an aircraft taking off.

HTH
C

NomoneyNohoney
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Re: Boiler seemingly overpressuring

#24139

Postby NomoneyNohoney » January 18th, 2017, 2:03 pm

Thanks for your help.
Since posting I found the following link: http://www.ggasheat.co.uk/common-boiler-problems.html and that seems to point at the expansion vessel, whether needing repressurising or a new diaphragm needs to be seen. I've got a guy coming to look at it tomorrow so hope it's just needing a recharge.
Thanks for your thoughts.

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Re: Boiler seemingly overpressuring

#24170

Postby Itsallaguess » January 18th, 2017, 3:59 pm

Don't be too concerned with a little water in the expansion vessel if you ping the inflation valve and some comes out.

Expansion vessels will get some condensation inside, and if they've deflated completely then this water may be all that's left in, so when you ping the valve it might look like a blown bladder, but might not be if it's not full of water. It'll be worthwhile understanding this difference if your man tomorrow heads straight for the 'new vessel' theory if he perhaps just sees a small amount of water...

My system displayed the same problems as yours NmNh, and showed a small amount of water when pinging the expansion vessel valve. I reduced the system pressure to zero, bled the small amount of water from the expansion vessel, then inflated the vessel to the required pressure with a bike pump (it's important to have a low system pressure at this point, as you'll not be able to fill it with air easily if you're pumping the bladder up against any system pressure, then I increased the system pressure to the normal level (with system cold...), and it's been fine ever since, and that was a few years ago now.

During investigation of the above problem on my own system, I decided that if my boiler expansion vessel ever goes completely in the future, I'm going to leave it in-situ and just install a new, generic expansion vessel elsewhere on my system pipework, in an accessible place. I'm lucky enough to have a really good place for such an addition to my central heating system, although I appreciate this may not always be the case.

With the correct installation of the right isolation valves and appropriate pressure gauge, as well as a well-placed bleed-off valve and outlet, I'll be in a much better position to maintain the vessel pressure, and also be in a better position if the vessel itself needs replacing again. The boiler-replacement vessels are quite expensive and quite often difficult to remove and replace, but the generic ones are much cheaper.

Cheers,

Itsallaguess

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Re: Boiler seemingly overpressuring

#24263

Postby NomoneyNohoney » January 18th, 2017, 9:21 pm

That was fulsome: thanks! Something I read online was, as a temporary measure, take out about half a radiator's worth of water from the system, then the radiator acts as a temporary expansion vessel. Interesting concept!

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Re: Boiler seemingly overpressuring

#24318

Postby Itsallaguess » January 19th, 2017, 5:08 am

I made a slight mistake in my post.

I completely drained the expansion vessel, including the remnants of condensed water, whilst the system was still under pressure, before removing pressure fully from the system, allowing me to re-inflate the expansion vessel.

Trying to do it the other way around would have meant not being able to deflate the expansion vessel, as there would have been no system-pressure behind it to allow that to happen.

Apologies for any confusion caused, and good luck with your diagnosis today.

Cheers,

Itsallaguess

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Re: Boiler seemingly overpressuring

#24557

Postby quelquod » January 19th, 2017, 9:30 pm

That's a pretty comprehensive writeup Itsallinaguess - well done.

If you do exactly as you say, is there not a risk that although the expansion vessel is now repressurised the volume of the 'air side' of the vessel will be too small since you've pressurised it against an incompressible system. Should you not repressurise it against either a drained system or at least one with (say) a bleed valve open to allow the diaphragm to 'centralise'? Or will it just spring back when you release the system pressure? When the vessels are delivered I believe they are pressurised against atmosphere.

(I did the latter with mine).

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Re: Boiler seemingly overpressuring

#24726

Postby Itsallaguess » January 20th, 2017, 1:27 pm

quelquod wrote:
If you do exactly as you say, is there not a risk that although the expansion vessel is now repressurised the volume of the 'air side' of the vessel will be too small since you've pressurised it against an incompressible system. Should you not repressurise it against either a drained system or at least one with (say) a bleed valve open to allow the diaphragm to 'centralise'? Or will it just spring back when you release the system pressure? When the vessels are delivered I believe they are pressurised against atmosphere.

(I did the latter with mine).


Yes, when I said I'd released the pressure on my system before re-inflating the pressure-vessel, I actually had it 'less than full' at that point, but not fully drained. There was a drain-off from a local radiator valve into a bucket, so that when I inflated the pressure-vessel, any water forced out would go into the bucket. As it turned out the system must have been drained 'enough' to cope with the expansion of the pressure-vessel, as no more water came out of the radiator valve.

But yes, it needs to be made clear that when re-inflating a boiler pressure-vessel, there needs to be room for that air-expansion to occur without any water pressure at all on the other side of it. Get the pressure-vessel up to the rated pressure, cap the system again so that it's all nice and sealed, and then re-pressurise the system itself using the normal methods.

Cheers,

Itsallaguess

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Re: Boiler seemingly overpressuring

#25047

Postby NomoneyNohoney » January 21st, 2017, 7:08 pm

Don't know quite why but thought I'd close out this post.

So, the pressure shooting up to over 3 bar indicated the pressure vessel was awry.
Drained down the system, and used a bike pump to pump up the pressure, to about 14 psi - which is approx 1 bar.
Refilled system, and then found that hot water and heating had both failed now.

Boiler repair man came and diagnosed it in an instant. Inside the boiler is a plastic bottle, which apparently stores any bits of gravel etc that get into the system, plus small amounts of water. (Sorry, not exactly sure if that's correct, but that's my understanding.) Boiler guy reckoned the bottle was full, and water had backed up the pipe and the boiler had shut down accordingly. Anyway, disconnected bottle, cleaned it out and refitted, and then system came back to life.

Probably a combination of two things, I reckon. Seems OK now, and pressure isn't going up too high, so trust it has now been sorted.


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