7541 Steam space heating systems (0) Radiation and convection heating systems using steam as the heating medium for space heating, complete with condensate return system 7542 High temperature hot water space heating systems (1) Radiation and convection heating systems using high temperature hot water as the heating medium for space heating 7543 Medium temperature hot water space heating systems (3) Heating systems using medium temperature hot water as the heating medium for space heating 7544 Low temperature hot water space heating systems (3) Heating systems using low temperature hot water as the heating medium for space heating 7545 Low temperature hot water underfloor and ceiling heating systems and chilled water ceiling cooling systems (8) Underfloor and ceiling heating systems using low temperature hot water as the heating medium Ceiling cooling systems using chilled water