I am trying to install the HW Model # RLV430A - My old T-Stat is round manual HW - the wiring in the wall consists of 4 wires: (1) each - red, green, yellow, white. The new has 2 black wires only - I need to know how to combine them and the instructions w/the new T-Stat was not helpful. Our heating system is propane forced air. Any advice??