It has been the practice of some to place them in the supply using a manifold just above the boiler; this is often in conjunction with the “pumping away” concept. Each zone valve is controlled by its own zone thermostat. End switches in each zone valve control the central circulator and the burner. On a call for heat, the valve is opened. When it reaches the fully open position, the end switch “makes” (makes contact), causing the aquastat relay to pull in, starting the circulator and the burner. If the circulator control and high limit switches are closed, the circulator and burner come on. All of the end switches in a system are wired in parallel. This ensures that the circulator and burner are powered when any zone calls for heat. The circulator and burner shut down when all the thermostats are satisfied. Note: The following troubleshooting procedures are intended to familiarize you with some basic types of systems. For detailed instructions on particular zone valves, refer to the manufacturer’s spec sheets. Also, try to move the valve stem assembly by hand to be sure that the paddle is free. Some exceptions will be where a plunger will have to be pushed down manually to see i...