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Do we need a second hot water system as mains water pressure too low for combi?

We have a powerful combi which several plumbers have told us would be capable of running two showers but the mains pressure is so low we don't have enough to run two hot taps at once off the combi. Have an electric shower in an en-suite and want to replace that with a wet room and proper shower as the mains pressure is too low for an electric or any instant heater to provide a decent flow.

Am I looking at something like a megaflow in the attic? Will that still mean we can only use one hot water source at a time as the pathetic mains pressure will be mixed with the megaflow HW and will also be trying to supply HW via the combi? Is there any way to get two good water pressure showers working at once?

3 Answers from MyBuilder Plumbers

Best Answer

If you have measured the mains pressure and had a reading with no taps running ( known as standing pressure ), and this reading was suitably poor, then you may be , what is know in the trade as, Stuffed.

A Megaflow needs good flow and pressure, so does a combi to use multiple outlets simultaneously.

The good old fashion water tank and negative head pump combination will work well in most applications.

It does sound as if you may need to rethink your plumbing options from scratch, as tanks and pumps would mean going back to a traditional set up (hot water cylinder).

On a positive note, the combi may, (refer to manufacturers instruction), be used as a heat only boiler, therefore you could possibly keep it if you decide to go traditional.


Answered 7th Aug 2012

Agree with pjb, megaflow needs the same pressures as a combi to work well. How about installing a cold water storage tank with a pumped supply to the exisitng combi, if you've got the space for this will save on costs as your not changing the whole system.
(You cannot install a pump on the mains, against water regs and you'll leave neibours with little to no water).


Answered 8th Aug 2012

get a qualified plumber to sort this out for may have to go back to a conventional plumbing system with pumps,water board are only obliged to provide water at a minimum of .7 of a bar,to take a proper reading of your water main do it at a time of high local usage ie 6am to 9am and 4pm to 9pm.


Answered 21st Aug 2020

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