Combi boiler high pressure

fantastapotamus

A few weeks ago, I noticed that my combi boiler pressure was getting higher and higher. It was resting on 2 and if the heating was left on getting up to 4. Everything was working fine, but I contacted my landlord who eventually sent a guy out. I was more concerned about a wet patch near the boiler, but that turned out to be damp. Anyway, the plumber said the pressure was getting a bit too high so he said he'd change the starting pressure.

After he'd left I think the pressure was too low because none of the heating would now work. I had an old boiler which used to lose pressure and remembered there was a valve which let water in to increase the pressure, so I turned a small black lever to increase the pressure (not remembering at the time that the old boiler was NOT a combi). That seemed to do the trick, and everything was fine. The pressure was resting on 1 and not getting past 2.5.

However, the resting pressure gradually started rising before stopping eventually at 2.5. I made sure the black lever was definitely set back to the off position and wasn't too sure about what had caused it (obviously it was that and I shouldn't have touched it). A friend told me that to reduce the pressure I could bleed one of the radiators, so I did that last night. It got the pressure down to about 1.5, but after the heating had been on it went back to resting on 2.5.

If I remove more water from the radiators, will that help or have I royally messed up? It took about a week for this other guy to come out (and that was at the mention of a wet patch on my wall) so I'd rather try and fix it myself if possible. I didn't remove a lot of water from the system, about a cupful or maybe more.

ariba10

Water expands as it heats up.

Have a look to see if the pressure drops when the boiler is cold.

roddydogs

Are you sure that a filling "loop" is not bypassing? Should be 1 bar when cold.

fantastapotamus

Had it on at 6.45am this morning for 15 minutes, went from 2.5 to 4. It's back down to 2.5 now, so it is returning to the original level (although that is clearly too high).

Will the radiator trick work again? I'm not sure why it went from 1.5 after I'd removed the water back up to 2.5.

fantastapotamus

Also I'm not too sure if it is bypassing the filling loop. That's the braided loop that's hanging down? The technician who came out didn't see anything wrong with it, he just thought the pressure was too high.

matelodave

Sounds like the pressure vessel has failed and is now full of water and either needs emptying and re-pressurising or replacing.


The pressure vessel is a tank, either inside the boiler or attached to the outside pipework which is designed to absorb the increase in pressure when the water is hot and to maintain pressure in the system when it's cold. It has a rubber bag or membrane inside it and is pumped up with air to about 2-2.5bar.


Expanding water squeezes the membrane and allows space for the water. If the tank gets fully filled with water either because of over pressure or the membrane splits then there's no space for expanding water and the system pressure will increase, sometimes to the point where the safety valve operates. Get the vessel checked (try tapping it gently - it sounds hollow when it's operating correctly, a dull thud when it's full of water.

You might also find that the filling loop valve is letting by and slowly pressurising the system but if it gets too high then there's a possibility of boiler damage - hopefully you'd see water coming out of the safety valve before that happens though

roddydogs

fantastapotamus wrote: »

Also I'm not too sure if it is bypassing the filling loop. That's the braided loop that's hanging down? The technician who came out didn't see anything wrong with it, he just thought the pressure was too high.

No, it looks just like Washing Machine hose (cause thats what it is) Not really supposed to be left in situ, but a lot are for convenience. Look underneath the boiler or at the back of cupboards.

fantastapotamus

Thanks for the replies. It's now sitting at 2.5 and it shoots up to 4 (full pressure) when the CH has been on for more than 10 minutes. It starts dripping at that pressure too, so we're going to leave it off until someone can come (God knows when). I tried draining some off a radiator again to reduce the pressure but it doesn't make a difference now.

The wire hanging at the bottom looks like this http://i.ebayimg.com/t/Combi-Boiler-Filling-Loop-straight-/06/!Bj42!5!!2k~$(KGrHqMH-CsEs-Gy7hmoBLVQORHVJg~~_35.JPG

so isn't that the filling loop? I turned that black valve to increase the pressure slightly the other day, is there a way to definitely know I've closed that? I'm pretty certain I did and I can't hear any noise coming from it, but turning it back 'on' doesn't do anything now. I'm guessing counterclockwise is the off position?

ariba10

The handle across the pipe is Off.

In line with the pipe is On.

macman

Yes, that is the filling loop.

fantastapotamus

ariba10 wrote: »

The handle across the pipe is Off.

In line with the pipe is On.

That makes sense - does it matter which way across the pipe it goes? Does the boiler have some sort of cut off so you can't put too much water in the system? I'm just wondering why that isn't working now - obviously I don't want to increase the pressure further, but that's how I did it previously.