Good Evening all. Long time since i have been on here.
I'd like to know about open vented hot cylinders. How does the water flow from the tank in the loft to the cylinder? i know that the water enters the loft tank via the mains pipe and runs through a ball cock. I also know that the pipe that feeds the cylinder is also the vent which runs back to the top of the tank. Is there a valve or something to stop the flow when the cylinder is full? I was thinking about it the other day and came to the conclusion that there must be because the immersion heater heats the water in the cylinder and the hot water draw off is in the top of the tank. surely the water that is in the loft tank would get warm/hot too if the whole cylinder is being heated and there is no valve. Oh i don't know I'm confusing even myself now.
First of all, the pipe that feeds the cylinder from the cold water tank is NOT also the vent which runs back to the top of the tank. There should be a pipe leading from the bottom of the cold water storage tank feeding in at the bottom of the hot water cylinder. The vent pipe from the top of the cylinder is likely to come off the pipe that supplies your hot taps. It is the head of water in the cold storage tank that pushes the hot water from the cylinder when one or more taps is opened. If all the taps are closed, all the water just sits there quite happily, hot in the cylinder and cold in the storage tank.
For information, the cylinder is ALWAYS full. The water in the cold water storage tank does not get warm because no hot water should reach as far as the top of the vent pipe into the cold water storage tank, unless there is a system problem such as the water in the cylinder boiling due to the immersion heater failing to cut off for instance. Note that the vent should not dip into the cold water in the storage tank, otherwise a syphon is likely to be set up drawing cold water from this tank to the hot taps as well as hot water from the cylinder.
Have a look at the pictures of direct and indirect hot water systems on this site and you will see how the cylinder and cold water tank should be configured.
Further to my previous reply to you, I would make the following comment. Your lack of basic knowledge of such a simple system worries me, given all the advice you have recently offered to others! I know that you have included a disclaimer about not being a plumber or electrician, which is perhaps as well.
further to dave's response, if the cold water in the storage tank feeding the cylinder is getting warm it could be the sign of a serious and potentially very dangerous problem. the system should be shut down and professional help immediately sought.