Thermostat Operation and Wiring
Simple heating system with a single zone use a single electrical circuit through the thermostat to call for heat. Power for this circuit is provided either by a "millivolt" thermocouple next to the furnace's pilot light, or more commonly a 24VAC supply fed by a dedicated transformer. When the switch in thermostat closes, the furnace, electric heating element, and/or fan is started. When the thermostat opens again, power is cut and the heat stops.
3-wire Hydronic Zone Valves
Multi-zone hydronic (hot water) heating systems use a central boiler/circulator serving the entire building, with separate piping branches supplying each zone. Thermostats therefore need to both turn the boiler on or off and open or close a zone valve. Each valve consists of a valve body that opens or closes water flow through the pipe attached to a valve head containing a motor and switch contacts. The motor controllers generally use a system of limit switches to ensure that the valve is driven completely to the open or closed position when the thermostat cycles on or off, and have an auxiliary output for controlling the boiler. These outputs from all zones are connected in a wired-OR configuation to the boiler, so it will be started when the first zone calls for heat and turned off once the last zone closes.
2-wire systems are compatible with a simple SPST thermostat interface, and have self-contained control systems to drive the motor in both directions. Other systems have a 3-wire interface, and require an SPDT thermostat. The first two wires are the same 24V supply and call for heat signal as SPST systems have. Closing this circuit causes the valve to open and the boiler to start. The third wire is used to close the valve again, since the valve cannot close itself automatically when the open signal is disconnected. If a SPST thermostat it used with these systems, the valve will open once the termerature drops below the setpoint, but will not close again when the termperature rises.
Honeywell RTH2300B modification
The Honeywell RTH2300B
is an entry-level 5-2 day electronic programmable thermostat designed for 2-wire systems. It can be modified for use with 3-wire systems.