At a pneumatic building the diaphram heads were either leaking or the stems to the 3-ways on heat side of the 4 pipe were sticking in their "proportioned controll-stroke" position.
Bank with years of one outstanding maintenance co as a whole, and 6 more years of the co ousting them, only finding unusable controls in 4 of 24 univent fancoils and air filters with black mold, and a 1.1/2 compressor and 20 gal air tank on a 5 gal x usu 1/2 hp compressor load ( if all leaks reduced to a couple or none)...

Your only as good as your last tech, right? GO FOR IT ! check ask know and go t29 !