I asked a guy about this and he mentioned the airflow issue and draughts. He also said with todays well insulated windows, it's not a problem. You could put radiators wherever you wanted but out of habit, builders put them under windows. I'm always having to tuck the curtains under the rads and it looks unsightly but it's better than a cold house.
Whenever i get to build, i'll use underfloor heating. Solves that issue!
ollie
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