draining system

I have a radiator that has rusted and is leaking from the bottom by the valve (it may actually be leaking from the valve and running down to the bottom, hence the crust on the valve and rust on the rad but not sure)

Anyway, I wanted to put a new radiator on and replace the valves at the same time, so I am going to need to drain down the whole system. I am fairly adept with DIY, and have installed garden taps (not self cutting) using compression fittings and also plumbed in my icemaker/chilled water dispenser. We don't have spare cash available at the mo to afford a plumbers charge to drain and fit, so having read up on everything, I plan to do it myself.

My question really is about draining the system. I have read articles that said I need to completely turn off the boiler. Makes sense as there will be no water in it. However, I'm not sure where it actually draws its electricity from (the programmer is plugged into the wall just beneath the boiler, does it draw from that?) and also the article said to switch off the gas supply. Where? Sorry if these sound like dumb questions, but unless you have ever had cause to work on central heating systems before most people wouldn't know.

Please help. I need to do this this weekend

Thanks

You don't say if it's a sealed system or open vent.

You can do the work without full drain down.

But as a [recaution turn off the boiler no need to shut off the main gas emergency control valve if boiler is electrically isolated). Double precaution make sure any thermostats timers are set to off.

1/ Open vent? Put 22mm John Guest push fit cap over open vent pipe, get a winesaver with lever and two rubber rings stop feed pipe with that.

2/ Attach hose to to drain off open drain off and wait until water stops flowing. Shut drain off.

3/ put towels and dedcorators roller tray under pipework to rad, do work, no water should come out but towel is there as a precaution. Undo big nut from valve and allow any water to empty into decorators tray. Then when no water flow you can remove whole valve for changing.

Sealed system, start from 2.

afterwards repressurise sealed system (search under filling loop for how to do this. open vent just reverse 1/.

Work required? get junior hacksaw, remove old rad valve hacksaw through old olive until almost to pipe, put electrical test screwdriver in slot you made and prize apart olive, remove, paste up pipe (boss white or jet blue is my recomendation), apply new nut and olive paste up olive, fit new fitting tighten up until you feen olive bight, continue half a turn. Nip up on test.