Vaillant EcoTec 937 Pressure Problems - Advice Please

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First post, so please be gentle.

I have the above boiler, 3 years old and, as the pros will know, it is a combi with a 15 litre pressure storage vessel inside. It serves 3 zones, upstairs, downstairs and a conservatory with UFCH.

Before having it fitted, all the existing (12 year old) and new pipework was pressure checked to 4 bar, flushed, etc. It has had regular Fernox, is bled with no airlocks, no leaks at rads and we now have all new floors, so I'm desperate not to go leak hunting.

From day 1, it would happily hold steady pressure at 1.1-1.2 bar but without warning would suddenly drop out and need a top-up every 4-6 months. Now, it can hold pressure for several days, but will drop to 0.0-0.2 bar overnight and trip out. It has had a dose of Fernox F4 sealer and I am confident there are no leaks old or new.

Strangely, the amount I have to top up is very small, only a few seconds on the tap, but after doing so, once running the pressure can shoot up from 1.2 bar to over 2.5 bar and I have to drain a cup-full off to bring it back down again. Once it has gone in to the red zone on the dial and I had to panic a bit.

It is definitely NOT the pressure switch, because the gauge on the front concurs with the electronic reading.

I'm suspecting the boiler, but does anyone have any ideas?

I'm loathe the call out the joker who fitted it 'cos we didn't part on good terms after he hiked the bill
 
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Sounds like your expansion vessel needs re-pressurised, mine is giving me problems at the moment also. Read your manual to find out how to do this (isloate and empty boiler and pressurise using a car pump) Also it sounds as though your boiler is emptying through the pressure release valve (pipe that goes outside), you will need to check that it is not leaking otherwise it will need replacing.
 
Many thanks for the super quick reply.

I have the boiler isolated, turned off and standing at pressure, so I'll see if there is any change over the day. Fortunately, it's not too cold, so we can put up with it!
 
No Probs. I should have said before, if the pressure release is dripping you may be able to tighten it again rather than replace (search PRV leaking to see other forum posts). If you have isolated the boiler put a balloon over the pipe outside to catch any water that may be leaking out of it.
I'm taking my vaillant boiler apart tonight to have a go at repressurising the EV
 
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Good luck with your own job, let me know how you go on!

I'm pretty confident that the pressure relief valve is okay - when I first started with problems I set a marker there and there have been no events since. I suspect it would also spot drips, but I may be wrong.

I've stood the boiler all day with the heating circuit completely isolated. After 8 hours, both the circuit and the boiler were rock steady at 1.2 Bar. But as soom as I put things back into operation the pressure jumped to 2.2 Bar.

It looks like it might be when the store is being charged so there's a good chance you might be right about the problem.
 
Well i emptied my boiler then started pumping the valve and more water started flowing out of the drain valve. Empied about 5l out of my expansion tank. Have refilled boiler to minimum and it seems to be working much better (0.5 bar rise with thermostat turned high). Will be annoyed if ive fixed it though because the plumber told me he did this job when he did the service. It was a bit messy (valves leaked when i turned them) but i didn't see much evidence of that when he did it. Makes me wonder if he did a half a**ed job, either not emptying boiler first or not leaving the drain valve open. Its pretty text book stuff though. Just need to keep an eye now to see if pressure slowly drops like before. If so at least i can be pretty sure its a fault with the EV now rather than worrying about a hidden leak as i was initially.
 

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