7kw electric shower - 4mm2 too risky?

[Yonny24]

Rules to Follow to Avoid Pump Problems | Valin - https://www.valin.com/resources/blog/rules-follow-avoid-pump-problems

The pump should be located close to where it draws suction from the cold water tank to ensure the minimum Net Positive Suction Head Available NPSHA of 2m h2o Is exceeded. If a long thin pipe run provides the water feed to the pump inlet there is the risk of the NPSHA dropping below 2m because of friction in the pipe, its bends and joints and turbulence acting to reduce head of water pressure.

The cold water tank also provides a large reservoir of water to provide a short term high flow rate to the shower which is why I do not recommend connecting the pump to your mains water pipe directly. At a high flow rate the water pressure in the mains pipe coming into your home will decrease and thus could fall below minimum inlet head of water of the pump.

Note also that you should avoid sharp bends near the inlet and outlet of the pump. Have long arcs of pipe work so the water flow is not overly disturbed.

Brilliant thanks for all the detailed information.
I'll keep the pump set up as it is.
I do believe there was some air in the piping the other day after changing a non return valve on the immersion heater tank on Friday though. Appears to be functioning better on the upper floor bathrooms now. Still the hot water pressure is not great from the upper floor immersion heater. I wonder if it needs an additional smaller booster pump in the airing cupboard where the immersion heater is. That non return valve I believe causes the issues initiating the flow sensor

 

[Yonny24]

Just a quick update. Discovered why we were having issues with the flow sensor on the pump. Pump was fine. Odd discovery with the non return valve and the flexible pipe fitting used.
The hot water was leaking slightly back into the inlet pipe. Noticed the inlet pipe was hot!
Swapped out the flexible pipe on the inlet valve.
One had a protruding washer v shape almost that seemed to be putting some pressure on the internal valve.
The replacement has a flat washer and has solved issue. No backwards leakage and the pump kicks in as it should when opening the cold water tap!
What an unusual discovery.
Left one was the one causing the issues with the valve. Right shows an older one I had hanging around with a flat washer

 

[marconi]

Nice investigative work and solution.

 

[marconi]

I just wondered where you have reached with this shower project?

 

[Yonny24]

I just wondered where you have reached with this shower project?

Unfortunately nowhere yet - been so busy.
Been working on CCTV around the exterior and other bits and bobs that needed more attention.

Also the actual cold water inlet piping could be very messy due to the location of the mixer for the current shower unit, without breaking down the walls and installing a new cold water pipe within the wall it could look unslightly with adaptors and a pipe on the oustide of the wall.

So really on hold for now as I'm undecided.

But as always appreciated all the assistance and replies.