Hot water in cold tank

[garagerob]

This is too confusing!
Which tank in your loft is overflowing?
Are you getting hot water from your cold tap or cold water from your hot tap?
Are either of the ball valves running on your tanks?
Is the "pipe hanging over the tank" passing water?
Does tthe "pipe hanging over the tank" go to the big or small tank?
Has anything changed with the central heating pump?

 

[rona61]

Which tank in your loft is overflowing?
The large, main cold water storage tank

Are you getting hot water from your cold tap or cold water from your hot tap?
The cold taps have run lukewarm water a few times in the last couple of days

Are either of the ball valves running on your tanks?
The ball valves have all been replaced and appear to be working perfectly

Is the "pipe hanging over the tank" passing water?
No. The hot water is entering the cold tank from an inlet below the water line very near the bottom of the tank (which is actually meant to be an outlet, not an inlet, I'm assuming)

Does tthe "pipe hanging over the tank" go to the big or small tank?
The big main cold tank

Has anything changed with the central heating pump?
Not that I am aware. I had a new hot tank put in yesterday but they didn't replace the pumps or the thermostat.

 

[garagerob]

From these answers, as seco services said, it sounds like your getting cold mains water passing back through your kitchen Mixer tap (the mains pressure on the tap overcoming the tank fed pressure) and into the tank.
You need to run both hot and cold feeds from each mixer until they are normal temp's then shut the taps and feel the pipes to see if the hot pipe goes cold or the cold pipe goes hot.
Most likely to be the tap...

 

[rona61]

Many thanks to you and also especially to seco for heroically unpicking what I was saying. Boyfriend is away and I am completely at a loss here.
Will do as you suggest (or get engineers to, if they turn up) and post back to let you know outcome.

 

[Goldspoon]

Can you explain what 'passing' means in this context?

Basically the kitchen mixer has "high pressure mains cold" and very "low pressure hot from your cylinder". If something is wrong with the tap then the cold high pressure mains water can makes its way through the tap and up through the hot pipe (unequal pressures so cold wins). In doing so it pushes hot back into the cylinder. The cylinder water is pushed back up the feed pipe and into the cold water storage cistern in the loft (hence hot water coming in from the bottom). The level in this cistern rises and goes out the overflow.

This would seem the most likely scenario although it does seem strange that you can stop the problem happening by turning off the boiler and running the hot taps.

Can you turn off your kitchen tap using isolation valves?

 

[rona61]

Thanks for very clear explanation, Goldspoon. Meanwhile I have called another heating company who are coming out soon I hope. Will post after visit.

 

[diy]

are you getting hot or cold water from your kitchen mixer tap

 

[rona61]

Thanks for all your suggestions. It definitely wasn't the mixer tap. The (2nd) heating engineer who came out had no instant solution, says it could be any of a number of things and warns that it will probably be a slow process of elimination to find out. Which he has begun... at least I have confidence in his expertise, unlike the last lot.
But it's gonna be a cold, cold christmas all right.

 

[kevnurse]

Seems to me that the new hot water cylinder is not properly plumbed in. the symptoms are consistent with the pump and boiler being connected directly into the cylinder itself rather than the heater coil. "hot water from the taps is hotter than expected". "Problem stops when boiler (and pump) switched off", "cold tank filling up with hot water under pressure through the outlet pipe". "problem stops when the taps run off the hot water", which will be the boiler water being pumped directly through the taps rather than going up into the loft.

I'm not a plumber by trade, but thats my best guess...