This could turn out expensive as you might end up putting a frost stat in your garage to turn the boiler on incase the temp drops below freezing . The rad will freeze up if its below 0 degrees in the garage and a frost stat bypasses all timer controlled circuits and turns the boiler on iff the temp drops below 5 degrees c . Im doing this in my garage too just to be on the safe side . Best thing is its free for me as im a sparky and have pretty good plumbing knowledge and fitted central heating systems before except the gas side of things . Its easy to fit a frost stat so pm me if you need any advice .