#8 If the boiler is in the garage then it would be simple to add another radiator in there maybe on its own zone so your not heating the hole house. Or install trace heating on the pipe work your worried about freezing. Would be more cost effective than trying to run an electric storage heater...
A relay is what is what your after with a the relay coil powered from a switched ignition source.
You idea sounds about right.
Make sure you've 100Mo relay correct wire rating ect.
Because they we've got a 32amp ring main. And appliances with wiring that are designed to handle 13amps max(often less) so we stick a fuse in the plug to protect the appliance, saves on copper.
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