Draining down heating system containing 3 port valve

I have to replace a 3 port valve on my Honeywell Y plan heating system. I cannot find where the drain cock is on the system and I need to drain the system down below the valve. I have made up a fitting with a drain cock that I intend to fit onto a radiator valve and drain the system from this point. The faulty valve is jammed in the mid position and the faulty pipework is 8mm fed from a manifold. Can anyone anticipate any problems? Thanks.
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