Single-pole and double-pole thermostats are commonly used for baseboard heating. Either can be used as a line-voltage thermostat, controlling one or more 220-volt heaters. From the outside of the box they might look the same, but double-pole thermostats provide an inherent safety feature that single-pole thermostats do not, because single-pole thermostats interrupt current flow but keep voltage on the heater.
Low-Voltage vs. Line-Voltage Thermostats
Thermostats control heating devices. In the home, when the temperature at the thermostat goes down to a set temperature, a circuit closes in the thermostat, so electricity flows through a circuit that includes a heating element. The heating element gets hot, warming the air around it; the room gets warmer. When the temperature reaches the thermostat's setpoint, the thermostat opens the electrical circuit; since electricity is no longer flowing through the heater, it cools down. The air in the room gradually cools to the lower temperature setpoint, and the cycle repeats.
Low-voltage thermostats send current to a low-voltage relay; the relay opens and closes a switch in the line-voltage circuit. Single-pole and double-pole thermostats are line-voltage thermostats, and the switch inside either opens and closes the 220-volt (line-voltage) circuit itself.
Household Circuit Voltage
Circuit breaker boxes (or, in older houses, fuse boxes) provide AC (alternating current) electricity in two voltages: 110V or 220V. This is done by having two buses, each 110V from ground, but, in effect, on opposite sides of ground, so they are 220V from each other.
220V circuit breakers connect across the two buses, so there are 220 volts between the "hot" red and black wires in a 3-wire 220V circuit. (The white wire is grounded.)
Power is proportional to voltage, and to the square of current in a circuit. Baseboard heaters are almost always connected to 220V circuits between a red and a black wire. Remember, both red and black wires are hot.
If a thermostat (or other switch) opens either one of the hot wires, no current flows in the circuit. The heater is not heating but it is still energized by the other hot wire (120 volts to ground).
Single-Pole vs. Double-Pole Thermostats
A single-pole thermostat opens the circuit by cutting current through one or the other of the hot wires. Since the power is "off," someone might try working on the heater and get electrocuted by connecting any part of the circuit to ground.
Double-pole thermostats open both hot wires. The circuit is energized only between the breaker box and the thermostat, and there should be no risk in working on the heater. Of course, to be safe, always shut off power at the breaker before working on any part of any circuit .
Turning the Thermostat Off
There is another difference between single-pole and double-pole thermostats. Single-pole thermostats can be set to a low setting but cannot be turned completely off. If the temperature falls below that setting, the thermostat will turn on the heater. Double-pole thermostats can be turned off so no matter how cold the room gets, they will not turn the heater on.