No reason at all, if it can be done it'll be easy.
Normally towel rails aren't as wide as the radiators they're replacing and some towel rails are bottom entry. To minimise the amount of pipework on show, then the system would probably have to be drained.
the towel rail is bottom feed and return, so i will need to convert it to the side. i was thinking an electric element t-connector and plug the bottom, or maybe an elbow? the towel rail is slightly smaller than the rad i'm replaceing, but the pipe work is micro-bore, so think i can make it fit.
You can buy TRV's for micro-bore from any good plumbers merchants. If you intend changing the valves, then yes you will have to drain the system. Would be worth re-doing the pipework at this stage. You could only really get away without draining the system if your Towel rad were exactly the same size as replacement boiler, and have compatible valves. If you want to save some time whendraining down, lock off all the other radiators and drain the system from the system drain plug.
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