Four-port valves
The Illustrated Guide to Mechanical Building Services, Third Edition (BG 31/2017), by David Bleicher, published by BSRIA in 2017, states: ‘Four-port valves are control valves. They can be used in a similar manner to 3-port valves in underfloor heating systems, but have lower flow resistance. Also used in air conditioning systems to switch between cooling and heating on fan coil units.’
--BSRIA
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