hot water backing to header tank even when water turned off

[PEGASUS]

we have a hot water system that seems to have its own secret water supply! hot water cylinder is in a cupboard on the first floor, with a header tank in the attic directly above it. The problem is that warm water is backing from the hot water cylinder,up to the header tank. Although this in itself is annoying, as the over flow is directly outside my bedroom window, it is not the main problem. I have drained the header tank, and turned off the mains supply to it, and turned off the boiler.... the water is still backing up from the cylinder, filling the header tank and overflowing. I have just now turned of the mains to the house, I thought; there is no water coming in for the taps or the loo cysterns, yet, yet, water is still backing up back to the header tank from the cylinder! Is it likely that the hot water cylinder is fed from any where other than the header tank, since in normal times, as we use water, the ball cock in the header tank allows it to refill?
incedentally we have a condensing boiler which is serviced every year, and was passed as good, last week. I don't get it!

[plumbbob]

Various ideas spring to mind, but the answer needs information you haven't given. How many header tanks are in the loft? One two or more? If more than one, which one is overflowing? And by overflowing you mean pouring on to the ground outside the house type overflowing?

Is your hot water high pressure or low pressure?

Do you have single block mixer taps anywhere in the house?

[KEITH MARSHALL]

PEGASUS, I HAD SIMILAR PROBLEM, TRACED IT TO THE INTEGRAL CHECK VALVE IN OUR GRAVITY FED MIXER SHOWER UNIT.

THIS HAD FAILED AND WAS ALLOWING THE COLD WATER (MAINS) TO FILL THE HEADER TANK THROUGH THE OUT FLOW FROM THE TANK, VIA THE HOT WATER SYSTEM. RESOLVED THE PROBLEM BY FITTING A SEPERATE CHECK VALVE ON THE COLD WATER FEED TO THE SHOWER.

PRIOR TO FINDING THE SHOWER AS THE CAUSATIVE FACTOR, I CHECKED THE ONLY TWO OTHER MIXER TAP UNITS IN THE HOUSE BY ISOLATING ALL BUT ONE UNIT VIA THEIR ISOLATING VALVES AND LETTING THE SYSTEM RUN UNTIL THE HEADER TANK STOPPED FILLING BY THE BALL VALVE, AND THEN WATCHING TO SEE IF THE TANK CONTINUED TO FILL. REPEAT FOR EACH MIXER UNIT. HOPE THIS HELPS :?: