Hot water overflowing into cold water tank

[Dilster]

I have a plumbing problem and would be grateful of any help anyone can offer.

We had an attic conversion a year ago, and as part of it the builder moved the HW cylinder into a new airing cupboard in the attic. The header tank sits in the same cupboard at just over 3 feet above the top of the cylinder. The cold water tank is also in the cupboard, the bottom is probably 1 foot above top of HW tank. Never had any problems in old location but now have the following happening.

There is a pipe that runs from the CW tank into the HW tank, presumably to fill the HW tank? The problem appears to be that this is flowing the wrong way – hot water is flowing from the HW tank into the CW tank (I know this because if I put my hand inside the CW tank I can feel a flow of HW). This pipe has a valve which I’ve tried turning off, but HW then flows out of the cylinder vent pipe into the top of the CW tank. The net result is CW tank overflows through our external overflow pipe. The ball valve in the header tank is ok. Header tank water level is fine.

This happened once before and our plumber came out wasn’t sure what the problem was but it had stopped when he came out which didn’t help. Can’t get hold of him at all now, hence thought of asking the wider community. He did confirm that the heating element in the cylinder had not failed (not sure how, but he was good guy so I’m inclined to accept this). All was fine for 6 months or so, but we now have a problem again. Possibly linked to having the heating back on, but didn’t happen straight away so not sure. We have got TRV’s on most of the rads, if this is relevant (I think there is one without, but not at home and can’t 100% remember at moment)

Any ideas? Any help would be much appreciated.

Thanks
Phil

 

[seco services]

Sounds like you have cold mains passing to hot gravity ie through a mixer tap or shower valve.
The cold mains will push the hot backwards through the cylinder in to the cws.

Try isolating any mixer tap that has cold mains gravity hot see when it stops
ie kitchen mixer etc.

 

[Dilster]

Ah, that sounds logical. I'll have a look later and post the outcome

Thanks
Phil

 

[CBF]

i agree with seco, however you started well cylinder & cws tank then just confused matters by going on about hw tank, while we could suss out what you meant others may not

 

[Dilster]

Fair point CBF. For the avoidance of doubt to anyone who might find this post useful in future, my references to HW tank mean HW cylinder.

The solution, as you both suggested was a mixer tap. I've got an antique french bath/shower mixer in which hot and cold meet in a central ball and then flow to bath or shower depending on whether those taps are open (I've attached a pic for those interested as quite a strange beast). I usually close all taps off after use, but seems I'd left both hot and cold open and just shut off the bath and shower taps. As Seco said, this then created a flow from the cold (which I hadn't even realised was plumbed straight into the main) to the hot and back into the HW cylinder.

Thanks for your help, which is greatly appreciated.

Phil

Untitled

• Dilster
• 30 Oct 2012
• 1

 

[seco services]

Abit of a strange looking/setup mixer.

 

[twgas]

I once came across a kitchen tap with a similar idea, custard was ok with it but when they had visitors who turned the wrong tap off it gave exact same symptoms. Colleague ordered a new cylinder as he thought coil had failed. I changed cyl without double checking his diagnosis. When done still had same problem, took a while to sus, never seen a kitchen tap like it anywhere else - ever.

 

[TicklyT]

A check valve in the hot feed to the mixer would stop it happening again.

(And one in the cold supply to stop backflow and potential water supply contamination if the mains pressure drops)

 

[Dilster]

Yeah, agree, but sadly some way down the jobs to do list at moment. For now education has cleared up the symptoms if not the cause.