Central Heating Pressure Rising too high

[danbl]

I recently had a plumber out to sort a leaking radiator valve and during the job he tried to drain the system via the pressure release valve on the boiler but said it wasn't working so ended up bleeding a radiator. The next morning I noticed that the pressure guage was showing 0.5 bar so I presumed that the plumber had forgotten to top up the system completed, so I topped it up to 1bar (when cold). The next morning it was back down to 0.2 bar (cold but the heating had been on inbetween).

I've just had the plumbing company back out again (different plubmer) and they have checked where the plumber repaired the old leak and it was fine, but when I mentioned he'd been messing with the pressure relief valve on the boiler the new guy just said that he shouldn't have done that and they're not qualified to touch it. Obviously I'm not exactly happy with this but my main concern at the moment is getting the system working.

Whilst the plumber was here we topped up the system to 1 bar again and turned the heating on, 20 mins later it's at 3.5 bar. I've just turned off the system as I'm presuming there must be something wrong. Does anyone have any ideas what could be causing this?

 

[denso13]

Filling loop passing, ensure it's fully off after filling or PV needs charging //www.diynot.com/wiki/plumbing:faq:faq7

 

[danbl]

Definately not the filler loop (have just disconnected to prove), thanks for the tip on charging the PV. Will let the system cool and give it a go.

 

[danbl]

For my own curiosity (and to make sure I'm knowledgable enough to argue this with the plumbing company!), if it is the PRV that needs repressurising then where is the excess water going? I.e. how is it going from 1 bar when cold, to 3.5 bar when hot, then to .2 bar when cold?

Thanks

 

[swbjackson]

When the water heats up the pressure rises, this is normally taken up by the expansion vessel but as yours has lost it's charge it rises to above 3 bar and the PRV discharges to bring the pressure back below 3 bar. When the boiler cools again it drops to a lower pressure because some has been discharged.

 

[MANDATE]

Sounds as though your expansion vessel has lost some of its 'air' charge and is partly filled with water.
This leaves less volume of air to compress when the heating is on.
Because of this the water pressure will go well beyond 2 or 2.5bar and when it reaches 3bar the PRV will open and water will be discharged. The PRV will automatically close again but the pressure will have dropped to 0.5bar.
The air pressure in the expansion vessel should be 0.8bar when the vessel is completely empty of water meaning water pressure should be zero and the system open to atmosphere.

 

[xr4x4]

You mean the PRV 'should' open

He mentioned the prv wouldn't open, so maybe it's duff, as well as the expansion vessel.

 

[MANDATE]

Agree PRV 'should' open.
If pressure drops, the water is going somewhere.
Into the expansion vessel? (limited amount)
Out through a unknown leak?
Out through the the PRV when activated?
Also PRV may not close properly and allow pressure to drop to below original
set pressure, even when heating is off

 

[danbl]

Right... I had the company out on 8th April and they repressurised the expansion vessel. All seemed well at first... but shortly after I noticed that the system pressure is still dropping, albeit much more slowly. It goes from 1.2bar to about 0.3 in 3 or 4 days.

Any ideas what this could be? I'm just hoping that as the pressure ended up going to 3.5bar+ before it hasn't caused a leak elsewhere in the system.

Any help is appreciated! Thanks

 

[superman123]

Why didnt the 2nd engineer recharge the expansion vessel & change the prv when he come to check your boiler, dont let them say you need replacement without checking it first prob needs pumping up. not for diyers though.

 

[danbl]

The 2nd engineer did recharge the expansion vessel but didn't change the prv as far as I know - should he have?

After he recharged the expansion vessels he ran the heating for 10 mins or so and pointed out that the ch pressure only rose by about 0.2 bar, which was good. As my previous post, over the next few weeks I was slowly dropping pressure each day (and I was topping up every few days). I watched the system closely on Friday night and realised that the pressure was going right up 2bar+ and then dropping to 0.2 (as it did after the 1st engineers visit).

The only thing I can think of is that the expansion vessel has a leak (hence the initial slow loss of pressure) and has now completely discharged off air and I'm back to the prv dumping water.

Does this sound right, or can anyone think of another explanation? If it is the ev, is it just a case of replacing it?

 

[superman123]

if the pressure is below 3 bar and the prv is still leaking outside then it will require replacing. i would also have the pressure checked again on the expansion vessel, if it has lost all the pressure again then yes there is a leak and that will need replacing.