Low water pressure after a new bathroom fit? If the plumber swapped your 22mm pipes for 15mm, that is a big part of it, but it is not the whole story.
The short answer: a gravity-fed system cannot supply taps and showers designed for mains pressure, no matter how the pipework is run. Narrower pipe and lots of bends make it worse, but the fix is either an unvented cylinder or a shower pump.
Darren wrote in after a new bathroom left him with a dribble and a bath that takes 30 minutes to fill. In this one we go through how much head height a loft tank actually gives you, why raising the tank does not rescue it, and the two ways out with rough costs for both.
KEY TAKEAWAYS
Pipe diameter and bends matter more than people expect. Going from 22mm down to 15mm and adding bends piles on friction, and on a low pressure system there is no headroom to absorb it.
Gravity pressure comes from head height, and one bar is 9.8 metres of it. The drop from a cold tank to a shower head might be three feet. Fine for old low pressure fittings, nowhere near enough for anything specified for one bar. Raising the tank into the apex of the roof buys you very little.
An unvented cylinder puts the house on mains pressure and solves it properly. Budget around a thousand pounds for the cylinder and another three to four hundred for the install. It cannot create pressure that is not already coming into the house.
A shower pump is the cheaper route, and the space under the hot water cylinder is usually the right place for it. It needs its own dedicated cold feed off the tank rather than sharing the one already feeding the cylinder, plus a proper hot tapping off the top of the cylinder with a Surrey flange.
Keep the pump as low as you can so it stays flooded. Run it off the existing hot supply and it will pull air in through the vent pipe, and that is what makes a shower splutter and lose pressure as the cylinder empties.
WATCH NEXT
Increase Water Pressure to a Shower or the Whole House
https://www.youtube.com/watch?v=5srvu_TxFEQ
00:00 Darren’s problem, 22mm swapped for 15mm
00:34 Why modern taps and showers need mains pressure
01:02 How little head a loft tank actually gives you
01:36 One bar is 9.8 metres of head
02:13 Option one, an unvented cylinder and what it costs
02:45 Option two, fitting a shower pump
03:17 Dedicated cold feed and tapping the cylinder
04:15 Getting the pump into the supply pipework
04:47 Why a badly fitted pump splutters
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