Some hydronic heating systems use electrically operated valves to control the flow of water into each zone. In these zoned hydronic systems, the valves operate in conjunction with a single circulator. The construction details of a typical electric zone valve are illustrated in Figure 10-62.
When the room thermostat calls for heat, an electrical current is sent to the valve operator. The current flows through a normally closed switch and around a coil called a heat motor. The heat created in the heat motor causes a piston to move out and push against a spring-loaded lever that normally holds the valve closed. This action opens the zone valve. At the same time, the piston extends a bit further and trips an end switch that sends a signal through a relay back to the circulator, turning it on and sending water into the zone. The piston extends a bit further and reverses the sequence. This in-and-out movement of the valve piston will continue for as long as the room thermostat calls for heat.