So, I've go an older house with a header tank type hot water cylinder. Power company can turn off the supply to the cylinder during peak power usage times, standard set-up for older houses in NZ.
An hour ago I had a shower and the water was quite cold, after the shower I check the meter box and the cylinder is heating. We've had some bitterly cold weather here so the cylinder's probably been turned off by the power company hence the coldish shower.
Now the header tank is overflowing. I've had a beer to many to go climbing into the roof space to check it but it is possible that the ball cock is not turning off properly.
I wander if it is possible that the cold water (coming in from the street at about 7 deg C (guess)) being heated to 60 or 65 deg C is expanding and pushing back up to the header tank and making it over flow?
How much does water expand when you heat it 50 or so deg C?