Mixer tap causing overflow, does this sound plausible?

[harve]

Hi, I've been having an issue recently and wondering if I've figured it out, perhaps someone could help...

I've recently noticed that the cold tank in the loft (the big tank, not the one connected to the heating) has had a regular dribble from the overflow. It seems to be worse when a mixer tap in the bathroom (which was installed about a year ago) is run. At its worst its a constant stream that lasts for about half an hour after the taps are run.

I thought it could be the stopcock so went up in the loft to have a look. Its definitely not that as I turned it off. There is a pipe that runs from the hot water cylinder to the bottom of the cold tank in the loft. I guess this is the cold supply to the hot water cylinder as it connects to the bottom of the HW cylinder. This seems to be filling the CW tank and gets hot after a while which shouldn't be right surely?

So what I'm thinking is if the mixer tap is letting cold water into the hot supply possibly, then its causing the HW tank to overflow pushing hot water into the cold tank? The hot water to the mixer tap is rubbish. It seems to be way overpowered by the cold supply, so I guess something has gone wrong somewhere? Should the plumber have fitted a non return valve to the hot supply to the mixer?

Or could it be something else? I've read that this could be caused by a split coil in the HW cylinder? It seems to have got worse over the last few weeks which might support this possibly. The boiler is pretty old and is kettling quite badly (I'm getting it replaced soon) so I dont know if this could be causing the issue as well?

Any help would be great.
Thanks

 

[SimonJohns]

very comman issue,

easily fixed by installing DOUBLE CHECK valves on the feeds to tap

 

[Guest123]

Also common with Thermostatic Mixing Valves and as Simon says, fit double check/non return valves and the problem is cured.

 

[jtsplumbing]

Or it's a cheep rubbish tap from china and not WRAS aproved !!

 

[harve]

Thanks, I'll try the valves and see what happens. Its wasn't a cheap tap - it's a hansgrohe something or other.

 

[harve]

Next problem - I can only find 15mm compression fit double check valves. Is there anywhere that does one that has a screw thread? The pipes are chased into the wall with only a 15mm screw thread on the isolation valve. Preferably in chrome!
Thanks

 

[SimonJohns]

Do you have any photos?

Maybe easier to make a suggestion

 

[Bernie2]

It is possible the overflow is caused by a split cylinder coil perhaps, but an old system like that is probably more likely to discolour the water as well. It depends what sort of system you have. Its also common for the mixer to be fed off a low pressure hot supply and a mains pressure cold. If its not a bi-flow mixer then the high pressure cold will force the hot back up the cold feed and into the tank. Although its more correct to fit a none return valve close to the fitting, its also possible to trace the supplies and fit them where you can access the pipework and buy standard none return valves.
Make sure you know where your putting them though and the cylinder is left with a free flowing vent. Make sure you put them in the right way around. Try BES Plumbing they are usually a bit pricey but quick and good.