Whether your replacing a cylinder or fitting a new tap the draining of the domestic hot water is the same procedure and very simple to do.
Drainning The Hot Water
Turn off power to boiler. (boiler spur switch).
Turn off the valve on the coldfeed. (Located on the pipework coming down from the tank above/in loft to the bottom of the cylinder.)
Open all hot water taps and wait until the water stops running.
Connect a hose pipe to the bottom of the cylinder drain off valve with a jubilee clip.
Put other end of the hose pipe to a drain outside or other suitable drainage point and drain by opening up the drain off valve.
If you just want to drain the pipework and not the full cylinder, just drain from the pipework your working on. You do this by connecting your hose to the drain off valve on that pipework.
If working on taps or the pipework, you may also need to drain off by connecting to an available drain off point. Or draining off into a container like a bucket.
If your wanting to drain the full lot, including the water storage tank in the loft, just turn off the water supply to the tank and leave the valve on the coldfeed open.
Faulty Drain Off Cock.