depends if you need to remove the pipe work or are able to leave in place
if you can leave the pipe work in place then simply turn the valves down,ie close them undo the nuts to the radiator and drain out the water from the rad,remove rad, fit unit,may need to cut some of the unit to fit
second option if you need to remove pipe work then do as above but in addtion will need to turn of the supply of water to the central heating system this will depend if its a combi(simply drain water out from the rad valves)and refill from the loop fill on under the boiler and re pressurise or if its a a vented system(small water tank in loft tie up ballcock or stop water entering tank,may be a isolator valve on the water supply pipe just before the tank ) or failing this turn off the water mains stopcock usually under the sink
find out if the pipe work is 15 or 10 mm cut back the pipe to the desired location ,go out and buy two speed fit stop ends and push them on to the pipe work
this should do you
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