Low Hot Water Pressure... Help!

19 Dec 2010
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United Kingdom
Hi, A bit of a story, but please bear with me.

Last year we had a new bathroom fitted, along with this was a mixer tap. We previously had 2 single taps and pressure on both was fine, when the mixer was fitted hot water pressure became very poor.

We have lived with this.

I am in the process of fitting a new kithen, and have again gone from 2 taps, which both the hot and cold pressure was good with, to 1 mixer tap, which the hot water pressure is poor with??

Help! The cold water is fed from the mains, the hot from the tank upstairs. any suggestions??
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Mains cold and gravity hot almost always results in poor hot water performance on basin mixers, although usually it's the cold backfeeding up the hot that's the issue rather than poor flow rate. It's not, however, normally an issue with kitchen taps as these generally mix outside the tap body.

I suspect your taps are fed by flexible metal braided pipes? These may possibly be of small diameter? If so then that's your problem - the pressure from the gravity hot system is insufficient to give you a good flow from the tap. The cure is either new taps with larger feeds, or a pump, although in a very few cases it may be possible to obtain copper tails of a larger bore that could help to alleviate the problem
most mixer taps are high pressure >1 bar, not suitable for gravity feed hot water. or give very poor flow.

i have fitted (not supplied) and seen so many.

all taps are rated at a recomened working pressure.
yes the tap is fed by 2 braided pipes, i suspected these may have been the problem!

excuse my ignorance, but are larger diameter pipes available? thinking out loud i guess the diameter going onto the base of the tap may be an isue then??
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You are correct in that, to a certain extent, you are limited by the size of the fitting in the base of the tap. Having said that, this will almost certainly be larger than the bore of the flexible connectors currently running into it, and therefore copper tails would improve things if available.

A working example - if you have 10mm outside diameter copper tails they will typically have a 0.7mm wall thickness. 2 × 0.7=1.4, 10mm - 1.4mm=8.6mm bore. By comparison 10mm O/D flexi's may have a 3mm wall, 2 × 3 = 6mm, 10mm-6mm = a measly 4mm bore, less than half of those on the copper tails and, if you do the maths, just 22% of the internal area of the copper tails. This is clearly going to restrict the flow quite considerably.

Now, before I get shouted down, most copper tails have 15mm connections ends to make connections, but they gradually reduce into a much smaller threaded fitting and are about 10mm in diameter by the time they get to the tap. The threaded fitting brazed onto the end is sometimes a ¼" MI BSP thread, sometimes a ⅜" MI BSP thread, and sometimes something else entirely. If you can find copper tails with the same thread size and pitch as your flexi's then making the upgrade will give your taps at least half a chance.

Hope this helps and I haven't blinded you with science!
ok, so i was blinded for a while, but thats what google`s for!

Now that i know what copper tails are.. the science makes perfect sense.

I`ll be on the case in the morning, im guessing, plumb centre or city plumbing would be the best to look?

Thanks for your help.
Yeah merchants will be best but if at all possible take the tap with you to check the tails fit properly. If you get no joy, but can find out the thread size on the existing flexi tails (it may well be metric, buy an M10 and an M12 nut from a hardware shop and see if either will screw onto the thread without crossing up). If it is M10 or M12 (or even M15) Lunns do tails to fit http://www.lunns.net/tapspares/ Use your browser to search the page for B546117, it'll take you straight to them
Help! The cold water is fed from the mains, the hot from the tank upstairs. any suggestions??

Be aware that you can get high or low pressure mixer taps, in your case you need low, as the hot water is gravity fed.

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