Posted by: @jacs

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@derek-m not sure on the cylinder pressure but the flow drops massively when I open multiple taps, almost to a stop.

When you say that the flow drops massively, I assume this is at the shower head. It would appear that you have some form of restriction that is limiting the flow from or to the hot water cylinder, such that opening more taps will mean that this limited flow is being shared over more 'openings'. Because the shower head is higher than the taps it is more likely that this will lose flow first.

Try turning the shower 'cold' and open some cold taps. If the flow through the shower head remains constant then the problem is not with the shower, but is some form of restriction in the 'hot' pipework.

I would suggest that you check the pipework from the shower head back to the hot water cylinder for partially closed valves or thinner pipework. Also check the pipework from the mains supply to the hot water cylinder.

@jacs

If, as Johnmo suggests, the problem is due to a pressure imbalance at the shower unit, you should ask your installer to fit a pressure reducing valve on the cold water feed to the shower unit. It should then be possible to set both the hot and cold feed to 3 bar.

@derek-m I've checked the shower flow by turning on two cold taps and the shower flow/pressure didn't change.

@johnmo Hi, please can you confirm what balance flow is?

I turned the shower to cold and opened the cold taps and the shower flow didn't change.

Most mixers uses a balanced flow i.e. the flow pressure from hot and cold is the same. This is how it is normally achieved via the combined valve at the UVC 

If you don't have this setup, you really need a check valve on the hot outlet of the cylinder to prevent reverse flow.

The alternative to the balanced flow connection on the combi valve is a prv at the cold water entry to the house set at 3bar or less. Mine is after stop cock, and strainer, then prv then to the rest of the house.

Your cold water test shows it's not a cold issue. So all that is left is the hot water supply. Testing hot on its own isn't really possible because there may always be a mix of cold going into the hot side of the mixer.

Problem resolved - there was a reducing valve under the bathtub restricting flow.

@jacs brilliant. Thanks for letting us know, and glad you’ve resolved it.

Our newly fitted shower system was regularly running to cold if paused. The excellent plumber fitting the ashp solved it by fitting a pressure control valve on the inlet cold feed.

Different symptoms but same root cause the mixer shower needed balanced pressure feeds. Why the original plumber didn’t fit one is a good question.