Hot water tank in loft overflowing

[sahd]

Many thanks to all th helpful suggestions to the problem. My plumber says it cannot be the coil because my system is not set up like that. He has lowered the water level in the tank and moved the perspex cover to an angle to provide ventilation as it is causing condensation, which he thought, yesterday, could be the problem but again today (early morning) it overflowed 'till I ran off some hot water. He has checked the thermostat on the boiler and that's ok. Please anymore suggestions I'm beginning to despair, especially as I am on a water meter.

[gasman]

The only things left then is the head calculated for the expansion pipe may be incorrect and may simply require the expansion pipe to be lifted higher to prevent the expanding hot water from rising up the expansion pipe and filling up the header tank. Or the pump may be set too high and needs switching to a lower speed so as to not force the water into the header tank.

[scoob101]

In my poinion as it only happens early in the morning I would say it is a faulty ballvalve which is passing slightly during the night when the mains pressure is at its maximum, and there is no one drawing any hot water off.
Get a decent part 2 ballvalve and float re fitted and set correctly, the recommended water level is a minimum 2 inch air gap from top of water to overflow outlet.