Fortic tank is overflowing

[Pipeandthings]

Back pressure happening somewhere could be the cause. Basically cold pushing hot out of the way. Turn main stopcock off and see if overflow stops.

Thanks. No-one seems to say thermostat is the problem which is what plumber said needed replacing. Will try main stopcock.

 

[Deleted member 120897]

Thanks. No-one seems to say thermostat is the problem which is what plumber said needed replacing. Will try main stopcock.

That’s because a thermostat is an electrical rod that enters the immersion heater assembly, naff all to do with any form of additional water. This isn’t your setup, but the image is simple enough to understand, where it states heater coil is where the thermostat would sit:

 

[Pipeandthings]

That’s because a thermostat is an electrical rod that enters the immersion heater assembly, naff all to do with any form of additional water. This isn’t your setup, but the image is simple enough to understand, where it states heater coil is where the thermostat would sit:

View attachment 77343

Thanks. That was my thoughts. I assumed if water was overheating cos of stat then it would come through vent pipe not as is. So for some reason increased volume in tank as you say.