First this will be unrelated to the heat pump itself. It is just a heat source.

Two things affect flow through the DHW cylinder. Assume it is an unvented cylinder?

First the water supply flow rate to the cylinder. Was this checked and static and dynamic pressure recorded? These should be on the commissioning certificate

If your dynamic (flowing pressure) mains water pressure at peak times is not a minimum of 1.5 bar and 20 l/m flow, levels of performance may not be acceptable!

Do you have a reduced flow if you turn on two taps? Try with two hot and two cold.

What is your stored water temperature?

If you have high main water pressure, this is reduced at the cylinder to 3 bar. To stop issues you either used a balance flow connection the cylinder or add a similar pressure reducer to the mains near the stop cock.

Did they use a balanced flow to the shower? If they didn't there will be two different pressures going into the mixer tap on the shower, this will cause similar flow related issues.

thank you for responding. I have responded in red with more info.

two things affect flow through the DHW cylinder. Assume it is an unvented cylinder?

yes, it's an unvented cylinder.

First the water supply flow rate to the cylinder. Was this checked and static and dynamic pressure recorded? These should be on the commissioning certificate

according to the comms certificate, the calculated flow rate is 14 l/min. 

incoming static cold water supply pressure of 3 bar

If your dynamic (flowing pressure) mains water pressure at peak times is not a minimum of 1.5 bar and 20 l/m flow, levels of performance may not be acceptable!

was measured at around 6 bar pressure during survey.

Do you have a reduced flow if you turn on two taps? Try with two hot and two cold.

 I turned on 2 taps and the shower and the flow reduced to a trickle. I'll try 2 hot and 2 cold.

What is your stored water temperature?

46°C but even when the water temp is >60°C (disinfection cycle) poor flow at the shower

If you have high main water pressure, this is reduced at the cylinder to 3 bar. To stop issues you either used a balance flow connection the cylinder or add a similar pressure reducer to the mains near the stop cock.

Did they use a balanced flow to the shower? If they didn't there will be two different pressures going into the mixer tap on the shower, this will cause similar flow related issues.

nothing was added at the stop cock I don't believe.

So based on this then, the cold will have 6 bar and the hot will be at 3 bar? 

I'll need to check if a balanced flow connection at the cylinder. I'm not sure to be honest, there is a combination valve on it as the engineers considered this may be causing the issue??

@jacs

Have you checked the filter on the shower unit if there is one?

@derek-m yes I did, I have replaced the shower cartridge unit and it made no difference.

@johnmo

thank you for responding. I have responded in red with more info.

two things affect flow through the DHW cylinder. Assume it is an unvented cylinder?

yes, it's an unvented cylinder.

First the water supply flow rate to the cylinder. Was this checked and static and dynamic pressure recorded? These should be on the commissioning certificate

according to the comms certificate, the calculated flow rate is 14 l/min. 

incoming static cold water supply pressure of 3 bar

If your dynamic (flowing pressure) mains water pressure at peak times is not a minimum of 1.5 bar and 20 l/m flow, levels of performance may not be acceptable!

was measured at around 6 bar pressure during survey.

Do you have a reduced flow if you turn on two taps? Try with two hot and two cold.

 I turned on 2 taps and the shower and the flow reduced to a trickle. I'll try 2 hot and 2 cold.

What is your stored water temperature?

46°C but even when the water temp is >60°C (disinfection cycle) poor flow at the shower

If you have high main water pressure, this is reduced at the cylinder to 3 bar. To stop issues you either used a balance flow connection the cylinder or add a similar pressure reducer to the mains near the stop cock.

Did they use a balanced flow to the shower? If they didn't there will be two different pressures going into the mixer tap on the shower, this will cause similar flow related issues.

nothing was added at the stop cock I don't believe.

So based on this then, the cold will have 6 bar and the hot will be at 3 bar? 

I'll need to check if a balanced flow connection at the cylinder. I'm not sure to be honest, there is a combination valve on it as the engineers considered this may be causing the issue??

@jacs

What happens to the pressure in the hot water cylinder when you increase the flow rate by opening more taps?

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

Posted by: @jacs

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cold will have 6 bar and the hot will be at 3 bar

If that's the case it's a winder you get a hot shower at all.

Posted by: @jacs

↑

there is a combination valve on it as the engineers considered this may be causing the issue??

They are mandatory, the balanced flow port for cold water is on the combination valve. Extremely unlikely to the an issue with the combination valve.

So other things come up

If the balanced flow has not been used 

You need a check valve on the hot water outlet of the cylinder to stop reverse flow, you will also need a prv on the cold water main sett to a max of 3 bar.

@johnmo please can you explain why I'd struggle to get a hot water shower with those water pressures?

if I turn in the stop clock to reduce flow, could this help?

Posted by: @jacs

↑

please can you explain why I'd struggle to get a hot water shower with those water pressures?

Your mixer is designed to see almost equal pressures on cold and hot sides. Most shower mixers will self adjust to maintain set flow temperature.

If one side is 6 bar and the other 3 bar, the 3 bar will not break in to the 3 bar. So I suspect the mixer will wind back cold water flow as far as it can, to achieve the target flow temperature. As a consequence you have rubbish flow.

Try

Switch shower on, open one or more cold taps, see what happens. If you are getting big pressure differences that should help identify.

@johnmo thanks a lot, I'll give it a go tonight and report back.