Advice needed for pipe sizing and pump selection for shower

[Chillibeans]

Im installing a thermostatic shower unit and 8 inch square celing mounted head. The cold water tank only sits about a foot above the shower head so im going to be needing a pump but im lost at what pump to go for and also what pipe size to use.
The shower head has a 15mm connection, all inlets and outlets on the shower unit are 22mm, cold water tank is 50 gallon and the hot water cylinder is 117 litres.
I want to place the pump in the airing cuboard next to the hot water cylinder which is almost directly below the cold water tank. The pipe run from the pump to the shower unit is roughly 9 meters under floorboards then up the wall, the pipe run from the shower unit to the shower head will be roughly 3 meters up the wall and along the loft space.
The shower unit can take any presure from 0.1 bar to 4 bar.

How do i work out what pump i need and also the pipe sizing i need from,

1. Pump to the shower unit
2 Shower unit to the shower head

I know i will have to drop to 15mm to connect to the shower head but do i drop from 22 to 15 directly after the shower unit or just before the shower head??

 

[Richard C]

How do i work out what pump i need

A Stuart Turner Monsoon Standard would be my 1st choice; they are well built & very quiet compared to some.
http://www.stuart-turner.co.uk/products/monsoon/s20-bar-twin
A drencher head needs a high flow rate for good effect &, personally I wouldn’t fit anything below 2 bar but go for the 3 bar if there is any likely hood of ever fitting another shower. With the shower head 300mm below the bottom of the cws tank, a positive head pump should be OK. ST specify a minimum of 100mm with a minimum gravity flow rate of 0.6 l/min at the shower head or a more expensive negative head (Universal) pump will be required; I use 1 l/min as a safe margin as different pump makes vary.

Also be aware that it’s not just the vertical distance from the cws tank to the shower head that will dictate if you need a positive or negative head pump, up & over loops also have the same effect. These should be avoided where at all possible as their vertical distance of below the cws tank also determines the need for a positive or negative head pump, as will long horizontal runs as they reduce the gravity flow rate; if it’s less than 0.6l/min, the pressure switches won’t kick in & the pump won’t activate. Loops are also prone to air locking & vents must be provided at the highest point; never fit inverted loops in the pump supply pipe work.

and also the pipe sizing i need

Pump to the shower unit

Always run 22mm from the tanks to the pump inlets; some drop down to 15mm from the pump to the mixer valve but to maintain flow rate, I always run in 22mm right up to the mixer.

Shower unit to the shower head

15mm will be OK.

I know i will have to drop to 15mm to connect to the shower head but do i drop from 22 to 15 directly after the shower unit or just before the shower head??

As above.

The HW cylinder is on the small side but should be OK for if only single showers are taken & you don’t spend too long in there . Only fit full bore (gas type) isolation valves; most gate valves restrict flow & reduce performance.