The heat coming off the radiator is as a result of work done on the incoming air which is cool. What you actually want is the TRV sensing the incoming cold air, THAT is the measure of the room temperature, not the temperature of the outgoing air. So, having the TRV at the bottom, is nicely representative of air room air temp. Also, you don't want it to respond too quickly, this is bad for oscillation and hunting. The "inertia" of the response of a TRV prevents the valve snapping open and shut, as would happen in an underdamped system. It's "integral action", if you like.
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