If you read the text book, it tells you to always fit the TRV in the flow side the same as the on/off valve it replaces. Indeed if older TRV's are installed in the return side, because they were directional, they may make a loud noise. Newer valves needed to be mechanically "set" for flow direction before installation. Now as far as I know, all new TRV's can be fitted in either flow or return without adjustment.
I personally prefer to fit them on the end of the rad that is the most open to the room regardless of flow direction. My colleague meanwhile, for neatness, prefers to always install them on the right hand side of the rad.
I understood from my controls course that either will be ok but if fitted on flow side should be installed horizontally, if on return vertically, as higher temp on flow will cause valve to cut off early if sensor too near rad.
A mere leckie (who does boiler controls as a major part of my work) so I will bow to those with more up to date knowledge,
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