Trevman's description reminds me of only one system that I've dealt with. This Transportation Office had only one loop to heat the whole building. A heat exchanger in the back used modutrols to control recirc. and make-up air dampers. These modutrols have pots in them that reference the wiper to the vane position. Another Modutrol, on the water side, controls a proportional valve to control how much water goes through the heating coils Vs. around them, before going to the rest of the loop. This valve also has position feedback via a pot in its Modutrol.
I doubt that this is what's in Attic's basement.
Joe
