Draining Back Boiler

Hello
I currently have a coal fire, with a back boiler / immersion heater system.
I am looking at having central heating installed with a combi-boiler and radiators, but keeping the coal fire as is.
Removal of the back boiler is not really an option as I have a fireplace which I do not wnt to move, therefore the only option is to drain the back boiler.
What is the safest way for this to be done? Would merely drilling many holes into it suffice? Would filling it with liquid cement be safer? I don't want the boiler to have a build up of gases and then to blow up when I have the coal fire lit.
Any comments greatly received.
Thanks in anticipation.
Jamie
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system.
I
Just cut the pipes off it and leave them open.
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