Pressure increase in unvented hot water cylinder

[Rad1o]

We have a copper hot water cylinder connected to a solar system. For a while we have had the odd overflow open up on the cylinder and lost pretty much all the hot water.
The cylinder has a safety pressure release and temperature release valve set at 3 bar and I think 70°C.
I messured the pressure in the expansion vessel and found it to be between 3 - 4 bar at times.
The water board said good pressure, just put a pressure reducing valve in to limit it. I did only to then find that very close to the cylinder we had a PWC 2.1bar valve anyway which I had forgotten and still we are loosing water.
Today I checked the system.
It was not the temperature - 25°C in the morning.
The inlet pressure was double checked with 2 PRVs.
So I thought that the only other possibility could be the outlet for the hot water into the house.
We have two shower mixer valves connected and a few hot and cold taps.
I isolated all the water supplies individually and only when a downstairs showerroom is on, the water pressure in the cylinder rises above release pressure.
So I assumed that there is backward flow through the mixer from cold to hot.
The mixer is a grohetherm 3000 19663.
Can this mixer not handle different pressures (2.1bar and 3 - 4bar) on cold and hot water, is the mixer cartridge for hot and cold broken or do I need another presure reducing valve for the cold supply in the downstairs shower room?

 

[swbjackson]

Any form of outlet that mixes hot and cold is liable to give problems with unbalanced supplies. Easiest solution is to limit the cold as well to give equal pressures. Ideally the shower should be fitted with single check valves on the inlets as well.

 

[matt1e]

It's likely to be a mixer passing as Swbjacksons post above, but regarding your expansion vessel you also seem to be measuring system pressure not precharge, you cant measure the precharge correctly while it is connected to the system without isolating it first

Matt

 

[Agile]

And quite apart from all that inappropriate advice, you should not be touching or modifying your system because you dont hold the appropriate G3 qualification!

Tony

 

[Rad1o]

Hi Agile,
I have heard of competent person schemes for electrics and gas but water on the house supply???

Am I not allowed to put service valves in to isolate taps and measure pressure on expansion vessels on a schrader valve (car tyre valves)?

I am not touching the hot water cylinder if that is what you are concerned about. I will change the inserts of faulty taps and mixers if they are leaking.

 

[simond]

Put a PRV on the incoming cold. That way you are not touching the unvented cylinder.

A better but probably more difficult solution would be to run your cold from after the unvented combi valve, thus guaranteeing balanced use at all times. If you have the dcoumentation for your cylinder (2.1bar - suggests OSO or a Mitsubishi heat pump Gledhill type??) it will show this.