CH Drain down

Assuming that you have one central heating boiler etc for both the hot water and the CH system then the same water heated by the boiler will pass through both systems. You will have to use the immersion heater until the system has been refilled.

Make sure that the boiler cannot fire up or the pump run while the system is drained.

That is correct. The radiators and the coil in the HW cylinder are all part of the *primary* circulation system - so if you have drained the radiators[1] you've no longer got an operational primary circuit.

Sounds good Roger. Never had such isolating valves. Perhaps I might add some next time I drain down. But then again the last time I had to drain down was because I had to replace the pump and it's isolating valves were useless.

They rarely last longer than a pump if not operated from time to time.

OP here, Another bit of advice please. One of the rads I have fitted is like this one

Cheers

John

Also make sure you do not have a Primatic cylinder as draining the CH would also drain the DHW!

I fitted my valves a couple of summers ago when I was in the process of removing each radiator in turn and washing it out - and then replacing the valves with a TRV one side and a drain-off lockshield the other side. I did this over a period of several weeks, and wanted to be able to use the boiler to heat the HW in the meantime.