Pipe size effect

[mickbob]

Hi, I'm in the process of renewing some plumbing work from copper and compression fittings to pushfit plastic in my bathroom. It's a mixed system gravity fed hot and mains pressure cold, The hot water tank is only marginally higher than the bath taps at around 1.5 metres. Currently the Hot water pipe enters the bathroom at 22mm but is then reduced to 15mm for about 1 metre up to the hot bath mixer tap. In short my question is why would a plumber do this and what effect would changing it to 22mm all the way to the mixer tap be? As this is a practical issue practical answers rather than theoretical fluid mechanics please. Thanks guys

 

[muggles]

The height of your hot water cylinder in relation to your taps has no effect on flow or pressure. It is fed from a cold water tank in the loft, and it is the height of that which makes the difference.

Given that you're also changing to plastic, which has a smaller bore than copper and is more restrictive to flow, replacing with 22mm all the way to your bath tap will certainly help although you're unlikely to see a massive change over your existing system

 

[mickbob]

Cheers muggles it's the actually cold water tank that supplies the hot which is 1.5 metres above my taps, its not in the loft! I measured from the top of it and unfortunately it's not possible to move it. Noted about the smaller bore I'll replace with copper rather than plastic pipe. The thing I can't get my head around is everything that I have read fluid mechanics wise suggests reducing from 22mm to 15mm reduces the pressure and the flow to the hot tap in an already poor pressure/flow situation is that practically correct? If so why would the previous plumber do it, is there something I'm missing? I'm leaning towards changing it to 22mm even a marginal improvement in water coming out of the hot tap will help I just don't want to make it worse.

 

[terryplumb]

Altering the hot supply pipe diameter from 15 mm to 22 mm copper will not alter the pressure at all ,it will however improve the flow rate, albeit not dramatically. If the tap itself is ,internally ,restrictive to the 22mm flow ,or any in line isolation valves have smaller internal bore ( smaller diameter than 22 mm pipe),or flexible hose tap connectors are used ,they would all make the supply pipe diameter increase pointless.
ALL mixer taps are more restrictive to flow than two seperate bath taps,and really need both hot and cold supplied under mains pressure .