Hello, I'm going to be renewing my bathroom and switching from a bath to a separate bath and a separate shower cubicle. I have noticed that the bath is fed by 22mm pipe. Am I best to run 22mm to the shower or do I get better pressure running 15mm pipe. The shower say it need at least 1.5 bar to run and I have a combo boiler with no tank. Any advice would be appreciated...
do 22mm pipe give you better water pressure?
- Thread starter donkey1976
- Start date
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gaswizzard
The pressure won't change whether you run 15mm or 100mm diameter pipework.
You need to understand the difference between static pressure and what you might call dynamic pressure. Static pressure is, as the name suggests, the pressure you measure when nothing is moving. Unless you have a pump in the system it depends only on height. As gaswizzard says, pipe diameter is irrelevant.
Where pipe diameter does matter is when water starts moving because you have to use some of that pressure up to push water through a pipe. This is the pressure drop of the pipe and it increases with flow rate.
In order to fill a bath within a reasonable time, you need a high flow rate. That's why bath taps are big; 3/4" instead of 1/2" in old money. Now a mains fed cold tap can be plumbed in 15mm pipe because you have plenty of static pressure to start with and you can afford to lose some as pressure drop along the pipe. A hot tap fed from a header tank doesn't have much static pressure to play with so you need to lose as little as possible. All other things being equal, the pressure drop along a 22mm pipe is less than a quarter that of a 15mm pipe.
But you have a combi boiler anyway so your hot water is also at mains (static) pressure. You should use equal diameter pipes for both hot and cold to avoid balance problems and I can't imagine a shower needing anything bigger than 15mm.
Where pipe diameter does matter is when water starts moving because you have to use some of that pressure up to push water through a pipe. This is the pressure drop of the pipe and it increases with flow rate.
In order to fill a bath within a reasonable time, you need a high flow rate. That's why bath taps are big; 3/4" instead of 1/2" in old money. Now a mains fed cold tap can be plumbed in 15mm pipe because you have plenty of static pressure to start with and you can afford to lose some as pressure drop along the pipe. A hot tap fed from a header tank doesn't have much static pressure to play with so you need to lose as little as possible. All other things being equal, the pressure drop along a 22mm pipe is less than a quarter that of a 15mm pipe.
But you have a combi boiler anyway so your hot water is also at mains (static) pressure. You should use equal diameter pipes for both hot and cold to avoid balance problems and I can't imagine a shower needing anything bigger than 15mm.
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Thanks for the advice, I really appriciate it...
in lay mans terms another analogy less techno speak. Imagine the flow rate or amount of water displaced over niagara falls per second (both horseshoe and bridal veil falls) Now if you could put allo that water, per second, into a pipe 1" in diameter you could probably hit the man in the moon! 
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