Overflow likely source?

[olbasoil]

I have a constant overflow coming from the main tank in the loft. It builds from a trickle to a pour. The isolation valve on stopcock is off. The expansion from immersion heater hangs over the tank, but is dry. This tank is highest in loft, so only source must be mains coming back up through the bottom of the tank. Any ideas?

 

[rpm]

Have had iso valves in the off postion let water bye. What sort of valve is it, wheeled gate valve, lever turn valve and how old if known.

 

[olbasoil]

Have had iso valves in the off postion let water bye. What sort of valve is it, wheeled gate valve, lever turn valve and how old if known.

It's only a small 15mm screwdriver turn valve, but I can confirm no water is getting in that way. Not a drip from the ballcock while tank level is still rising. It must be coming up the downpipes, and it must be coming in at mains pressure. Could a mixer tap or washing machine be passing from cold mains to hot gravity?

 

[Chuck]

Any ideas?

Do you have a shower with the cold fed from the mains?

 

[rpm]

New kitchen mixer tap?

 

[olbasoil]

Do you have a shower with the cold fed from the mains?

There are two showers. I can't confirm where the cold feed comes from because Im looking after a toddler, but that's something to check when she goes to sleep.

 

[olbasoil]

There are two showers. I can't confirm where the cold feed comes from because Im looking after a toddler, but that's something to check when she goes to sleep.

To complete the thread, I called out a plumber who discovered a shower was unbalanced (mains cold, gravity hot) and this was causing the flowback.