Today on the 30th July 2021 I reviewed this bog and I felt it is as relevant today as it was in 2017. I received two questions from Graham Downey who was looking to buy a linked pair of Shower Power Boosters (15th September 2017).
I probably have a about 1 -1.5m head, both hot and cold are gravity fed and I reckon I’m getting about 3.5 litres/min flow. Do you think I can expect to see an increased flow and to what sort of extent?
Also, both of the hot and cold pipes feeding the shower are 15mm. I know that the pump comes with adaptors for 15mm pipes, but will the smaller diameter pipes reduce the potential maximum flow rate once the pumps are fitted?
Increase in Flow
There is a simple rule of thumb when predicting what flow increase you will get and that is if you double the pressure you get a 50% increase in flow. For an upstairs shower with our Shower Power Booster you will get four times the pressure and double the flow. Some will get a bit more, some a little bit less, and I will explain this further in the post below but put simply. If you started with 3.5 litres a minute then expect around 7 litres a minute.
On a personal note we are just having our shower and bathrooms upgraded in our home in Norwich and we purchased the mixers from reputable manufacturers. Both showers come with ‘Micky Mouse’ 8mm low flow shower hoses and only one has a decent shower head. The 8mm shower hoses will need to go in the bin and one of the shower heads will need to be replaced with a WrightChoice shower head. It is silly to get the pressure sorted only for the flow to be killed with a poor shower head and/or hose. Double the pressure and increase the flow by 50% breaks down as a rule of thumb if the maximum you can force through a new shower head is 5 litres a minute.
Head Loss Through Pipes
The head loss through a 10mm pipe delivering 7 litres a minute to a shower is very low. The head loss through a 15mm pipe is even lower and changing to 22mm will give no discernible benefit.
A more detailed explanation is given below which I only include because many plumbers hold a view that you need to put in bigger pipes than you need:-
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For a flow of 7 litres a minute the head loss in a 10mm pipe is very low which is why we recommend a high flow shower hose with a minimum diameter of 10mm. A shower hose of 8mm or less should be binned, a shower hose of 11mm diameter or more give little benefit over a 10mm shower hose.
The head loss in a 15mm pipe is half that of a 10mm pipe. The head loss is determined by the speed of the water flowing through your pipes which is determined by the cross sectional area of your pipe. The area of flow within the pipe is proportional to the square of the radius of the pipe. The area and flow increases in the following ratios:-
8 mm, ratio 64
10mm, ratio 100
15mm, ratio 225
22mm, ratio 484
The science tells us that if a 10mm pipe is ok then the head loss is so very low in a 15mm pipe. Unless you want to feed a bath or multiple taps at the same time, changing from a 15mm to a 22mm pipe to feed a shower is just plain silly.
Click on the link below to see the latest ShowerPowerBooster Information Manual.
