What side of central heating system does drain down valve go on?

The Drain down valve should be located on the flow side of the radiator unless TRv's are fitted in which case it does not really matter. However, it is important to fit the drain down valve/'s at the lowest point in the system, preferable discharging through to the exterior of the building. The best case scenario is to fit an integral drain down valve on each radiator for ease of individual radiator maintenance.