Hot water goes cold, when cold water taps turned on?

Hi there,
I got the same problem as Pete. Got a old boiler, potterton puma 80 and the problem started recently. Hot water on it's own is piping hot and runs for hours - but the boiler stops the instant the cold water taps / mixing tap is turned on. Can't really shower is boiling water so it's pretty annoying. Any good ideas how to fix it? Is it to do with the cold water pressure?

I live in London.

Erik
June 2009

It seems unlikely but are you sure that your Main water stop-cock is FULLY OPEN? -I've been caught out by that before! V. embarrasing

Boiler repairman - Ipswich
May 2009

Thanks Guys
Right, live in Portsmouth in a house, get 1.0 bar from the hot taps, and 1 bar from the cold separately.
Turning any cold tap on in the house will cut the boiler out. So if the bath is running (just hot) and you turn the kitchen tap on the boiler cuts out. We've got four mixer taps, two sink mixers. one shower/bath mixer tap and a thermastatic mixer shower ( new ). (The problem started before the new one was fitted.)
It definnitely seems like a cross flow issue, have tried isolating the mixers, but not all of them are on isolating valves. The plumber thinks that the cold and hot pipes are connected directly, but if that was the case why would the system have worked for so many years without this problem. I don't really want to start ripping up floors (ceramic) to check the pipe work if there isn't a need to!

Pete
April 2009

Agree with yer man in Ipswich, but to add: Could it be a cross flow problem?

Do you have a mixer shower? Is the problem more noticeable at that shower and the taps in the bathroom?

What *may* have happened is that the valve in the mixer shower has gone faulty. As the hot water going through the boiler is at fractionally lower pressure than cold straight from the mains (only a tiny bit lost through the boiler) then the cold water forces itself past the faulty shower valve, up the hot water pipework, and eventually as far as the flow switch in the boiler, knocking it back off.

Bunslinger
April 2009

What is the Flow rate & Water pressure at a Hot tap, with both hot & Cold taps running, as if less than 0.5Bar it is likely that the boiler will drop out as there will be insufficient flow, as for the cause, you don't say which area you live in, but if you are in a city, and live in a block of flats, it is possible that the water company have reduced the Mains pressure (quite common in London)

Boiler Repairman - Ipswich (Very high water pressure)
April 2009

Hi Thanks, but have checked the pressure with a pressure gauge and am getting 1 bar for both hot and cold. The hot water flows at 9.5 Litres a min, it's only when the cold water is turned on that the boiler cuts off.

Pete
April 2009

SOUNDS LIKE A DROP IN PRESSURE IN THE COLD SUPPLY, WHICH WILL REDUCE THE AMOUNT OF LITRES PER MIN YOUR BOILER PASSES CAUSING THE BOILER TO OVER HEAT AND CUT OFF

STE
April 2009