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

Ok, chust had a shower and a think...

A variation on my last suggestion - soz, I should have thought about it before.

Ok, turn off the mains inlet to the boiler, both CH valves and then open a single hot tap. The hot tap should not flow 'cos you've turned off the supply to the boiler. In which case we can surely discount it being a faulty bath/shower mixer unit feeding the boiler via the hot DHW pipe (because any flow-by from the cold side would then come out that open hot tap), and we can also discount it being a faulty PtoP exchanger or diverter valve, 'cos both these devices have had their source of water removed (cold mains into the boiler, and a cold feed via the hot DHW pipe).

So, the boiler itself would appear to be fine. I think.

Ok, assuming hot tap isn't flowing, now 'dump' the boiler pressure as before. Once the pressure has dropped to near zero, open a CH valve. I'm guessing the pressure will rise to 2.5 bar as before.

Ok, this time leave things like that (mains inlet closed, hot tap open, a CH valve open) and open the dump valve and hold it open.

One of two things should happen; the first is that the rad system pressure will deplete and the dumped water will come to a halt. At this point when you close the dump valve the pressure won't rise even when the dump valve is closed again - the pressure should remain at zero because the whole system is now 'empty' of pressure. This is what should happen if things were normal...

Or, water will continue to be heard coming in to the boiler via that CH pipe ad infinitum, and being dumped out the dump valve. Regardless of how long you keep the dump valve open.

In this case, you should have clear proof that mains water is replenishing the CH system when it simply shouldn't be.

Two possible causes of this; (1) the top-up valve is letting by, even if you have removed the handle, or (2) some almighty twit has plumbed your CH system into the mains (unlikely...)

There, solved.

I can go to bed now.

To hear I'm wrong on all counts when I wake up...

 

Cool - the hot tap did not flow.

Surely that means that the source of this mysterious water can't be from the 'hot' side - faulty mixer tap, etc?

(What do peeps think?)

And if water continued to flow out the safety discharge pipe even with the dump valve opened for some time, then surely that means your rad system is being constantly replenished? (Unless you didn't leave it open for enough time - which I doubt very much as you've been dumping the excess water numerous times. But, are you sure you gave it enough time to 'empty' the rad system?)

(Possibly it's only 2 bar now because your mains is lower at that time?)

So, where is this water coming from? Only 2 possibilities I can think of - weird plumbing (very unlikely, but not impossible - perhaps the bathroom fitter got confused when he chopped off a few pipes and mixed up a rad with a cold feed!)

Or, your filling loop is faulty.

I don't suppose your mains feed to your boiler also has an isolator before the filling loop? (In fact, is the filling loop internal? Is it isolated when you shut off the mains inlet valve to the boiler?)

 

Cool.

I chust can't think of anything else!

Water is coming in to the boiler from somewhere. You can hear it, and the pressure goes up.

Where is the mains inlet isolator on your boiler in relation to the internal filling loop? Is it before it? Ie - if you shut off that mains inlet, then the filling loop no longer has a supply? In that case, it can't be the filling loop that's letting by and causing this?

Jeepers.

Have we missed something?! Could it really be coming from the CH system?

Ok, ok, another wee test.

Shut off CH isolators as before. Leave everything else as it should be - mains inlet to boiler open.

Drain off pressure.

Open CH valve and see what pressure gets to.

Grain off a bit more see if it stays like that. I'm guessing it'll carry on going back up to the 2 or 2.5 bar as before (unless it really was a case of you not having drained off enough!)

Assuming that the water chust keeps on coming from the CH side and out your dump valve, so it's clear water is being replaced from the mains, try turning the MAINS stopcock off as it comes in to your house. NOW see if the boiler pressure keeps getting replenished by the CH system.

If you find that (a) your system pressure doesn't drop when you try the first task, and (b) the system pressure does now drop off fairly quickly when you turn off the incoming mains and it stays low, then surely that means your incoming mains is feeding your CH system somewhere else?!

Can you double-check that the boiler/CH installer didn't fit a separate filling loop somewhere?! It is quite common for them to do this as it fills the system more quickly than an internal loop.

 

Well, blimey, cor an 'at.

Yes, it's likely that the mains isolator on the boiler is before the filling valve, so that should remove it being a faulty valve from the list of possibilities.

So that seems to me to leave only one weird possibility, and that's that the CH pipework is being supplied by the mains water. And that is something quite incredible.

(I don't understand why only the CH's flow raises the pressure - I'd have thought that the flow and return are 'linked' through any number of radiators, so whatever is in one is in t'other. Unless the rads are all fully shut off.)

I can't think of anything else, but I'm not a plumber.

Have to hand this one over to others!

 

If you call out nPower again, sit the guy down and talk through everything you've already tried first. Make sure he understands all this or else he's going to start pulling things apart that he maybe shouldn't.

 

One more test!

Shut off the mains water and the boiler's mains inlet isolator. Leave the CH isolators open.

Bleed a rad until it stops. If you have an upstairs, check a few rads up there too to confirm water isn't being ejected under pressure in any of them (this will be different to the 'trickle' you'll likely get on downstairs rads - I'm just talking about releasing the system pressure.)

Ok, by the time you bleed the rads, the gauge should read zero. Does it?

Leave a hot tap open - nothing should come out as the boiler inlet isolator is shut off.

Now turn on the mains stopcock.

Does the system pressure start to climb? Does any water come out of the hot tap?

 

If the answers are 'yes' and 'no', then it might be worth trying to find out where the mains water is getting to the boiler's sealed system.

You could try shutting off the flow and return isolators on the boiler, and then undoing each connector there in turn, perhaps 'flow' first - it should be a simple brass nut. No need to pull it all away - just undo the nut and gently pull so's you know it's disconnected. Place a pan under it.

Turn on the mains stopcock. Does water start coming out? If so - surely there's your answer.

Now it's a simple case of finding out how the hell it's getting there...

 

Blimey.

Although a pain, it would - I think - be useful to know for sure that this water is coming from a CH pipe before you lift floors.

It shouldn't be a major task to undo each CH connector to the boiler in turn. But de-pressurise fully first!!!

Then turn on the mains stopcock (which you, er, turned off first...)

 

Result, if you hadn't been so tenacious who knows what could have transpired cost wise for starters.

 

Also thanks goes to DA for his relentless support.

 

Blimey! Well done for finding it!

Phew...

Now, I wonder if your pressure will now start to fall off very slowly over a few days?! I mean, did some twit of a plumber - or the house's previous owner - deliberately leave it on to 'cure' a pressure leak issue?!

At least you are covered by the contract. If your pressure falls away, call them out to sort it.

Also worth adding some fresh inhibitor to your system as all the old stuff will have been flushed away...

 

Och, thanks Jit...