valliant high pressure

10 Mar 2014
Reaction score
United Kingdom

since topping up pressure on my valliant 831 ecotec plus the pressure has gone over 3 bar. This occurs even when the boiler is not in use. In order to maintain constant 1bar I have turned the water feed to the boiler. The taps have been checked, o-rings replaced in the braided filler loop that connects the taps.

The boiler was rising to 3.5bar and spilling alot of water from around the release pressure pipe although there was dripping from that pipe outside. The pressure builds up in 2 hours regardless of whether the boiler is in use. Any clues as to where the water is getting into the system please? :confused:
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Yes couldnt stop water coming through so thought valves may be faulty. Plumber put new o-rings in hose but couldnt stop water flowing to start with but after hour it played up again.
New o-rings won't stop the water flowing, you need new valves. Should be possible to turn the ones you have off with a screwdriver though, the grey plastic handles wear out after a while but can be unscrewed and removed
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Ok, I queried about faulty valves but told would need new heat exchanger as only thing left. Boiler was installed 2006. I moved in December and no problems until topped up pressure as was a tad low. With water feed into boiler turned off, pressure runs about 1.9 without any leakages. Ok until need hot water, but doesnt increase at all for the short period I turn water back on to get hot water supply.

I was puzzled that pressure remains constant whilst heating is operating and main water valve to boiler was off. would this mean new heat exchanger?

Turn the water off to the boiler, then open a hot tap. Does the boiler pressure drop?
You need to replace both filling valves. One is incorporated into the flow Heating isolation valve, the other into the cold isolation valve. Common fault I'm afraid, not cheap.

To double check, with the heating system pressurised and the cold supply on to the boiler, make sure both filling taps are closed. Disconnect the small nut on the end of the flexi on the heating flow isolation valve (LHS). There should be no water drip from the flexi tube or the valve body. In your case I would predict that you will see dripping from both. Replace both valves.
As above its a very common fault, an u have the best vaillant engineer in the country (probably) on your doorstep, give him a clue
Thanks. I tried hot tap whilst water off and pressure stayed 0.9. It has remained that level whilst cold now that water supply isolation valve is off.

I will seek help to try the suggested tests made on the valves as it may be too technical for me and wouldn't want risk incorrect assembly.

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