Cold water tank in loft overflowing

[Fireblade9]

hi this has us a bit baffled our heating systems is cold water tank and little f e tank in the loft immersion cylinder in airing cubard and boiler in kitchen
The problem was the overflow to the cold tank keep dripping so I replaced the ballcock turned mains on watched it fill up and cut off nicely job done
Next day noticed overflow dripping went in loft looked in cold tank ball valve under water so turned isolating valve on mains in ran a few taps so water level dropped left it isolated over night the next day the water level was right up again to the overflow
The cold feed pipe from the bottom of the cold tank goes into bottom of cylinder now that pipe gets hot all the way back up to tank can even see in the tank swirling like it’s back filling with hot but run any hot tap and the cold feed pipe goes ice cold again for about a hour the heats back up
Iv had two plumbers look and are not sure
All taps are mixer type no taps or showers have been changed in last 4 years
Any help or ideas much appreciated I apologise for long post just trying to give as much details

 

[Best]

Yes, looks like it isn’t your heating system mixing with your water.
Just keep an eye on heating tank level though.
Probably your kitchen mixer would be my first check

 

[Fireblade9]

Yes, looks like it isn’t your heating system mixing with your water.
Just keep an eye on heating tank level though.
Probably your kitchen mixer would be my first check

Thanks going to try what you suggested turn hot water gate valve off in airing cupboard then disconnect hot feed to kitchen mixer tap then should I try running the cold side of the tap if it leaks out of were the hot flexi screws into the bottom then
I’ll just replace for a new tap
Does that sound ok or have I missed something

Thanks

 

[Best]

Yes, that’s more or less all you need to do, but the gate valve in airing cupboard that turns your hot off might break inside leaving you without hot water, so be prepared for that.
If it is a valve that has been used recently without trouble, then maybe you will be ok.
We plumbers prefer full flow large lever valves

 

[heatservice]

I think all cold taps are mains fed how could I check to see if mine are bi flow or single
F E tank been isolated 24 hours now no real movement 3mm up last night then back down today could this rule out immersion coil split/hole

A bi-flow tap mixes the hot & cold water once it has left the spout, single flow taps mix the water in the spout so a blockage could result in backflow as the mains cold water is at a higher pressure.