Cold water tank mysteriously filling

[Thecasementkid]

Hello. Wonder if someone can point me in the right direction?

I have a Sundial Y-plan sealed system at home, with a three-port valve. Been here 30 years and it's regularly serviced and maintained. Never really had issues. The boiler is a Potterton Profile 60e also serviced yearly.

A couple of weeks ago, water was flowing outside onto the floor from the cold water tank overflow pipe. Easy, I thought, must be the tank valve needing replacing, which I did.
Still leaked, not just a drip but a stream. Flushing the middle or top loo's cured it for a while.
I went up again and bent the lever arm to make sure pressure on the valve was enough to stop the flow.
Still leaked. I then noticed the valve was completely shut off when full so was working ok. Strangely, the tank water was warm. I'm now assuming there's back pressure from one of the two 22mm pipes that come out the bottom of the tank and thats filling it with warm water. There's also a 22mm u-bent pipe that discharges into the tank, but this is dry and not the issue. I taped a plastic bag on the end and it's dry.
During the day when the hot water/heating is off everything is fine but in the evening/early morning when both are on, it occurs.
Anyone have experience of this? I'm guessing it's a heating engineer's job which I'm happy to employ but would like to know what may be the cause.

Many thanks.

 

[picasso]

A tap or shower fitting probably passing cold mains into the hot water pipework.

 

[Madrab]

If it's not doing it all the time & the vent is dry, does the pressure in CH system drop and needs topping up frequently?

Do you have a modern mixer/mono bloc tap that has mains cold and hot from the cylinder - kitchen sink possibly?

 

[Adam the boiler man]

If hot water cylinder is off an immersion its common if stats gone.
Same as cylinder stat being broken or left dangling.
When heating up the level rises a fair bit so set ball valve a good 6 inches or so from overflow.

 

[dal5band]

i.e. If the hot tank is over-heated it can set up a convection current to the cold tank depending on the pipe runs. This would explain the warmed cold tank.

 

[Bodgedbuild]

If you do have hot water backing up into the cold water tank make sure you address the fault VERY urgently and check your tank is not thermo plastic. I say this because people have been killed by such faults. In many cases these tanks cannot take any amount of heat and collapse sending the entire tank of hot water through the ceiling.

 

[Elkato531]

Is your F&E cistern higher than your cold water cistern? Could be the coil inside the cylinder has started leaking and water from the heating system is backfeeding into the cold water storage. This would result in the stored water warming up. Normally its the F&E that overflows as its usually lower but I've had a couple of cases of both over the years.