Advice needed - Loft conversion water pressure issues

4 Dec 2015
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United Kingdom

I would be very grateful for any help/advice anyone can offer me on this problem.

We have recently had a loft conversion to extend our 4 bed property. The loft now has a bedroom and en-suite bathroom, adding to our exiting en-suite, and main bathroom already in the house.

We replaced the original cold water storage tank with an new unvented system on the advice of the loft conversion company, but the water pressure is now much worse than it was with the water tank.

Whereas previously you could easily run both showers at the same time with no loss of pressure it is now not possible to even turn on a tap without the shower turning to a drizzle.

I have been told by the plumber who fitted the unvented system this is because we now only have one source of cold water, coming in from the mains, and with no additional tank to back it up we will lose pressure more easily.

Is there a way to solve this problem?

Also, in the opinion of the professionals on here, were we right to have been told we needed an unvented system in the first place?

Many thanks.
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You did need an unvented cylinder as there will have been no where on a floor above to relocate the cold water tank.

But the nupty plumbers who fitted the unvented should have checked the dynamic flow rate available from the mains.

But you are obviously confusing the pressure with the flow.

The usual solution is an upgraded supply pipe from the street.

But before doing that the dynamic flow rate needs to be checked. There could also be a restriction in the mains supply pipework too.

A dynamic pressure and flow test should have been performed as part of the system design prior to confirming that an unvented system was appropriate.

Need to have them performed and confirm the mains supply/pipework sizing is up to the job. I'd be passing that one back to the installers to correct.
I was advised the supply from the mains is 12 litres per minute, which is a maximum that can be achieved. From what I have read you cannot put a booster pump on to the mains supply, is this correct?

Thank you for your help
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As I said before, you don't understand pressure and flow.

To be usable for showers etc. you need flow with remaining pressure in the system to eject the water under pressure through the shower jet holes.

Merely measuring the open pipe flow rate is no use. An open pipe flow of 12 litres per minute would often be just 6 li/min dynamic @ 1.0 bar.

To start to have the benefits of an unvented cylinder you need a minimum of 22 li/min @ 1.0 bar.

I am not sure how much you can blame nupty plumbers working for builders, perhaps not a lot. They probably did not even have the G3 qualification required and did not notify to Building Control or complete the Benchmark Certificate.

If it was my house and I was a nasty person, I would require a new mains supply pipe to be installed at the builder's cost and for the cylinder to be reinstalled by a qualified person.

If your entire loft has been converted, a normal vented cylinder wouldn't have worked any more as there would be nowhere to put the loft tank, so I can see why they suggested an unvented but as it's run directly off the mains you're never going to get more water out than you put in. If you're only feeding it at a maximum of 12 litres a minute, that's all you're going to get, and at a lower pressure as you'll get losses through the valves and pipework. With that kind of flow rate, you'd have been better off with a combi boiler.

Do you know what your static pressure is? If that's very high, but your flow rate is very low, then there are ways of rescuing the situation but it will involve putting your hand deeper into your pocket I'm afraid. I suspect that the loft conversion guys will tell you that they've given you a system that works; if they made no promises about performance, then you have no comeback. If, on the other hand, they told you that there would be an improvement, then you may have a case for going back to them

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