There might be a time when you will need to drain the cold water system on a short notice and rather than having to run around, trying to figure out how to do this at the time, I will now tell you the correct way of doing it. Bear in mind that it is advisable to locate the valves that I will mention in this article, as soon as you have moved into a new home. This is so that you can save on time and minimise damage, should there ever be a need for you to turn off the system quickly.
So, when it comes to indirect systems there may be two drain-valves to deal with. You should be able to find the first one just above the mains stop-valve and the other one can be found on the actual feed that goes to the hot water cylinder.
The First Drain-Valve
The first one of these valves is not particularly useful when it comes to an indirect system, as once you have switched off the mains stop-valve, you can drain most of the water in the rising main by turning on your kitchen tap. There will however still be some water left at the end of the last pipe but this only ever has to be completely emptied if you are doing some work to that part of the pipe. You will find that you will get a better result by using the second stop-valve and this is also the only way to drain the hot water cylinder.
The Second Drain-Valve
So, you can drain the cold water cistern by closing off the mains stop-valve and opening your bath and basin cold water taps. If the reason you want to close the water off is to do some work to the pipe that runs between the cistern and the taps themselves, try shutting off the gate-valves before opening the taps. This should then drain the pipe in question. If there is no servicing-valve and the mains stop-valve does not switch off the water supply leading to the cistern, you should try tying up the ball-valve. The easiest way to do this is by removing the cistern lid and placing a piece of wood across the cistern. Tie a piece of string to the ball-valve arm as this should help to keep it closed. If you have to do this, I would advise you to leave the piece of wood and the string attached to it, next to the cistern so it can be used in the future, should you so need to. I find this way of draining the cistern preferable to the other way as it allows you to still utilise cold water through your kitchen sink.