Dual headed shower with equal pressure

[bcraig]

Hi guys,

Hoping I can get some advice for designing my shower plumbing.

I am going to have a dual headed shower. One fixed rose and a sliding shower head. They will each have their own mixer. I need to be able to run one or the other or both together without pressure loss.

I have my mains water in 20mm copper and hot water coming off my gas heater in 20mm copper.
Currently my shower, basin, bath etc are all feed from 15mm copper. So while operating the shower then the basin I get very noticeable pressure drop and preferred flow to one fitting over the other.

My understanding is that I need to increase the pressure with a greater diameter pipe to the fitting suffering pressure drop.
So If I want my two shower heads to run at equal and full pressure I am thinking I need to run from my 20mm hot & cold mains with 20mm feeds to these fittings with equal pipe lengths (to prevent unequal preferred flow) to each mixer and shower head.. I am sure I not using the correct terminology at all but I am hoping you will still get what I am on about.

I have attached an image to illustrate.


What I am unsure about is this.. The mixers and shower heads have 15mm connections. The pressure will drop between the 20mm feed and the reducer to the 15mm mixer connection. But then mix out back to 20mm where the pressure will once again increase right? Then the same as it passes through the shower head. Also I would need to consider that one head is fixed at about 1800mm off the floor while the other exits the wall at approx 1300mm off the floor. But it actually has to pass through another approx 1000mm of flexible shower hose.

Am I heading down the right path?

 

[Jadnashua]

The general rule of thumb is that your pipes must be able to supply more volume than the valve and showerhead can use to not exhibit lower output. NOte, it is not loss of pressure, it is lack of volume, as the actual pressure would be the same. If you only supply 10-liters of water to a head that can drain 15 effectively, it won't come out as fast...you need there to be a restriction...sort of like when you put your finger over the end of a water hose...the actual supply pressure is identical, but by creating a restriction, it can spray further...you didn't change the supply pressure at all. Running larger pipes or shorter runs with fewer fittings (or both) is the general solution.

Note, if there is a restriction in your water heater (is it a tank-type, or tankless design?) may mean the WH cannot supply a high volume of hot water, and the size of the pipe won't make any difference.