Combi Boiler Keeps Filling On It's Own Over Pressure.

New house you should not be using your own maintance contract to sort out issues with the fixtures and fittings. You need to get back to the builders to sort it out for you under the new build insurance scheme which should be valid for up to ten years.

If your taps are the wrong way round you would have cold coming out on the left and hot on the right.

Shower mixer is a very possible option you can test this by isolating the cold inlet undeneath the boiler (flat blade screwdriver needed) relase some pressure from your system via a drain off valve it should no longer rise or if possible shut off both hot and cold isolation valves ad release the pressure.

Its posible but unlikely for the cold to be linked into the heating circuit and i would expect the pressure to be much higher than 2.5 bar.

Try not to use the PRV too much as it an automatic device for pressure release, by using it as a drain off you will damage it and have to have it replaced. Use a drain off point and a hose to release pressure.

 

Blimey! That's a weird one.

I guess one thing to try is to isolate the boiler using the shut-off valves on the flow and return pipes to the rads. Then dump some of the pressure and see if it rises again with these valves shut.

Not quite sure what that will tell us tho'...

 

Have you had any work done recently?

 

This might sound daft but can you hear water running when all the taps are off and toilet(s) are full. Put a screwdriver on the mains stopcock and your ear to the handle any noise at all

 

Blimey even more.

With the boiler isolated from the CH pipes, its internal pressure remains constant? So it cannot be filling from either the mains supply or the PtoP exchanger.

The pressure rises when you open the boiler to the CH system?

You are 100% sure this is a 'combi' boiler, and not even a 'system' type or one supplying a Megaflow hot storage tank?

 

Has the worcester boiler got the internal filling loop fitted? The special key is usually housed on the removable lid under the boiler.

 

Yes Jit he mentioned it was removed in his first post

 

The big mystery is why only up to 2.5bar and no higher, if it was constantly being filled it would be activating the PRV almost continuously as soon as it hit 3 bar. So from that we can almost eliminate the cold main theory. But you say the pressure drops when you open up a tap and goes back up as soon as you close it, which would indicate that somehow the water and heating are linked. Assuming you have had no major work in the house apart from the bathroom then the fault has to be on the boiler.
You stated that you have had a new plate heat exchanger, which is one point the two could cross, so lets eliminate that.
You had a new pressure guage fitted so we must assume its accurate.
The only other two points on the boiler where the crossover could take place would be the diverter valve or if fitted integral by pass valve.
May be worth getting your maintanace contract guys back in as to date the problems not been fixed, and it could be one of thoes two I mentioned.

 

...except the water appears to be coming from the rad system .

Greg, can you confirm something else for us. You open up the isolating valve to the rad system and you can hear the water come in and the pressure rises to 2.5 bar? Does water continue to come in - can you hear it?

You know the 'dump' valve you've used to de-pressurise the system? You probably know that's a safety valve designed to open at around 3 bar. I wonder if it's opening earlier - at 2.5 bar? Hence the pressure seemingly stabilising there.

When you open that dump valve, do you know where the water comes out? It should be via a 15mm copper pipe outside, pointing down at the ground. When you hear water coming in to the boiler, can you see water coming out that pipe at the same time?

And also to confirm you have no tanks - you turn on a hot tap and the boiler fires up to heat that water instantly? There is no hot water store?

 

The boiler details that have been provided are for a combi boiler, there is a preheat function available with this boiler, this can be deselected by pressing the ECO button.

 

Hmm - good possibility. Although 2.5bar is pretty pants (although not unusual).

Definitely no water trickling out the PRV pipe outside? Def no hot store tank?

(Soz - you've answered these Qs)

 

Ok, one more thing to try. Isolate the mains inlet to the boiler too - there's a shut-off valve there. You cannot isolate the DHW flow out from the boiler, but you can ensure no hot taps are on. Also shut off the CH flow and return as before, and then dump the excess to drop the boiler pressure.

Now see if the boiler pressure recharges only when you open a CH pipe (not the mains inlet).

If the pressure builds up only when the CH pipe is re-opened, then that must surely indicate that the water pressure is coming from the rad circuit, and not the mains directly into the boiler, and not via the hot water supply (eg from a faulty mixer tap)?

In which case, someone has done something really weird with a plumbing pipe!

(If what I'm saying is right...)

Of course the radiators and circuit will have some 'stored' pressure in them, but surely not enough to have recharged the boiler so many times, and without the top-up loop being used?!