HW cylinder over pressure

12 Mar 2011
Reaction score
United Kingdom
Am having an problem with my sealed HW system pressure.

Recently I noticed water dripping through the tundish from the relief valve on the cyclinder. What seems to be happening is that the pressure in the HW cylinder rises to 6-7 bar and the pressure relief valve (7 bar) starts to drip/vent via the tundish. The cylinder is meant to operate at 3.5 bar and has a pressure reducing multibloc on the inlet. I have two pressure gauges fitted one on the multibloc and the other on the cylinder expansion vessel both read 6-7 bar.

The symptoms are that as soon as I pressurise the system by turning on the mains, the pressure immidiately raises to 3.5 bar (as one would expect) but then slowly rises to 6-7 bar over a period od 10-15 minutes. Interestingly the expansion vessel gauge rises to 6 bar first then a few minutes later the inlet (multibloc) catches up. This happens whether the boiler is on or not or whether the water in cylinder is hot or not. I am assuming is the mains pressure is 6-7 bar.

My first thought was the pressure reducing multibloc was faulty or not seating properly. I have replaced the cartridge in the multibloc and cleaned it. I have checked the expansion vessel and its pre-charge pressure (3bar). The CH and cyclinder coil is pressurised to 1bar.

The only other thing I can think of is that somehow water is bypassing in the multibloc body but having replaced the cartridge it seems unlikely as it has no other moving parts than in the cartridge but the whole unit will be a nighmare to replace and costs almost £200 so I do not want to do it until I am sure this is the problem.

Any other ideas as to how or why the pressure could be rising in the cylinder?


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Do you have a thermostatic mixer valve? if so then that will be the problem, if it is fed with mains cold water.
It should be fed from the balanced cold water connection off the pressure reducing valve,
Thanks. I do have thermostatic mixer valves but they connected to the balanced cold which is supplied by the same pressure reducing valve. It is a thought through and I will explore what other systems are supplied from the unbalanced mains.

Thanks again and any other ideas.
If the discharge water is cold or at about the storage temp, then the probalem will almost certainly be with a component on the cold water inlet part of the system

Check and clean valve/valve seating, if theres a strainer fitted check its working. If you've done this already and it hasn't solved the issue replace the pressure control device.
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Do you have a thermostatic mixer valve? if so then that will be the problem, if it is fed with mains cold water.
It should be fed from the balanced cold water connection off the pressure reducing valve,

Thank you very much buck51. It turns out you were right and I eventually traced the fault due to an open valve pressurising the balanced circuit (strange plumbing!) and back pressure through thermostatic mixer.

All sorted now.

Hi - I seem to have a similar problem to gilestrotter - I was interested in the advice given by buck51 .

My system appears to have the cold feed on the pressure valve by the presurrised cylinder capped off so presumably all my taps are fed from the mains ?

There is a constant drip into the tundish.

Could anyone give me advice on the best way forward.

I have already changed all the valves.

From reading the previous posts = am I right in thinking that all my cold water system should be fed from the valve near the cylinder ?

Is there a simple way to sort this out ?
This is quite a common problem but often not well diagnosed by plumbers!

Of course its all because the cold feeds are not taken from the balanced output.

Only the cold going to thermostatic or joystick type valves where any leagage would cause over pressurisation of the cylinder.

A bit of a bodge is to fit a pressure reducing valve to the whole house supply, set to about 3.5 Bar if thats the pressure the cylinder is designed to work at.

Why not just balance system at source to what the whole of the property requires (3 bar), as I did because of same water loss ?
Its fairly unrealistic to have all property outlets taken from the monobloc as retrofit to someone home? Unvented cylinder is not always close to mains for redirection of supply through monobloc.

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