2025-08-17 00:00:00 This application is a must for any plumber, builder or home DIY enthusiast, in fact anyone who wants to find out what radiator or AC BTU's are required for a particular room size. Whether you have existing radiators (or Fan Coil Units - FCU) and want to check that they are the correct size or if you are adding a new radiator (FCU) to a room - this is the application for you.