Plumbing Question

Hot tap running cold when cold tap is on!

Thanks for the info guys and sorry for taking so long to reply, just got married so things have been busy!

I did some more testing this morning and have the following info.

Our hot water system is fed from a small header tank in the loft. Its a condenser boiler, not a combi.

The sink and bath are in the same room and when I run the hot tap on the bath AND the hot tap on the sink and then turn on the cold tap on the bath, the hot tap in the sink starts to run cold.

As soon as I turn off the cold tap on the bath, the sink gets hot again (within 5 secs). So I don't know if that pin-points anything?




Our hot tap provides lovely hot water, but as soon as I turn on the cold tap more than about 10%, the hot tap then produces cold water!

It's a mixer tap, and no, I am not being stupid and not realising that the cold water is lowering the temperature - the hot water really does come out cold.

If I run the hot tap on full, but the cold tap on about 10-20%, I only get cold water. Also, if I turn on the cold water for even 1 sec, the hot water then runs cold for about 2 secs, so it isn't even residual cold water in the tap.

does anyone have a clue why this is happening? Thanks in advance.

6 Answers

Best Answer

This is an extremely common issue and is always (well, 99% of cases) caused by the cold water pressure being higher than the hot water pressure and forcing the hot water beck up the supply pipe into the cylinder (and ultimately the tank in the loft). When you stop running the cold water, the hot water takes a few seconds to return.
The simple solution is to install a non-return valve as close to the tap as possible on the hot water pipe, ensuring that the cold water cannot force the hot water back along the hot water pipe.
I'll almost guarantee that this will rectify the problem.

Find yourself a plumber who will offer a free quote and get some prices.

We'd be looking at our minimum labour charge plus parts for this (so £60 plus £10 parts). If it didn't work, then we wouldn't charge you. Not all plumbers are the same though, so get quotes first!

Best of luck,

Dave - DrainMedic

Answered 1st Jan 2015


Member since 7 Nov 2014

you say a mixer tap is it mains fed cold with tank fed hot if so thats the problem. Could be a whole raft of reasons depending on what type of system you have but this answer I believe is the most common as a lot of DIYers (or bodgers) will fit a mixer tap system to unbalanced supplies causing this problem.

you have confirmed you have tank fed hot water and mains fed cold water on a mixer tap. When you try to mix hot and cold with the mixer tap the cold water preasure overpowers the hot water preasure and actually pushes the hot water back up the hot water supply pipe thus filling the supply pipe with cold water which is why the tap runs cold even when you turn off the cold water.

Answered 18th Oct 2012

kevin cassidy building contractors

Member since 16 Dec 2010

Unbalanced pressures at your taps cold mains pressure is forcing the low pressure hot back up the pipe and into your loft tank.
Either take a new low pressure feed from loft tank to the affected taps or fit pressure reducing valves or isolation valves turned way down to both colds bath and basin.
Regards James

Answered 8th Oct 2014

J P Plumbing and Heating

Member since 6 Oct 2014

Is it just the one tap or all the taps in the house. If its just the one tap then there is a fault witht the mixing unit and it needs replacing. If its all the taps it could be to do with the boiler

Answered 18th Oct 2012

London Lofts and Kitchens

Member since 16 Oct 2012

combi boiler? cant cope with demand and flow

Answered 10th Apr 2013

L.D Plumbing and Heating

Member since 10 Apr 2013

Check whether water feeds are inversed in error (hot water plumbed to cold and the other way around)

Answered 28th Nov 2014

Fix My Home

Member since 15 May 2014

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