Heat has 3 zones, going up and down (vertical system). Thermostat in front living room does whole front of the house (all the way to third floor) Kitchen and back floor bedroom and 3rd floor back (maybe dining room and middle bedroom as well) Sun room, office 1st thermostat: in Ted's room that does my office and sun room. 2nd thermostat: in hall near shared 2nd floor bathroom, covers kitchen regular back bedroom, third floor bedroom and possibly dining room and other bedroom. 3rd thermostat: front living room There is a valve for each section and every so often one of the valves goes bad.