8 Line Voltage Thermostats

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Line-voltage thermostats are available as either single-pole single throw (SPST) or double-pole single throw (DPST) switch types and are popular for use with baseboard heaters. They are relatively inexpensive and act as on/off switches to conduct full power to the heater load.

Diagram of a A 120 V SPST thermostat and a 240 V DPST thermostat.
Line-voltage thermostat contacts.

Used most commonly with multiple baseboard heaters, line-voltage thermostats must be installed in each room or heated area to allow precise control of heat.

For example, it is common to install a baseboard heater under the window in every room with an exterior wall and have one thermostat near the entrance to that room, either above or beside the light switch, to control that individual heater.  This ensures that the whole room will be heated before the thermostat clicks off, but that the unoccupied room next door does not heat up.

Since heating loads are energized at 240V for increased efficiency, thermostats controlling them are not allowed to have a marked “OFF” position unless they open all ungrounded conductors. This means that only a double-pole, single-throw (DPST) thermostat will have an “OFF” position, while a single-pole, single-throw (SPST) thermostat will instead be marked “LOW” or “MIN.”

Regardless of the type of thermostat used, NEVER use a thermostat as a disconnecting means before working on a baseboard heater. Disconnect the power at the source and verify it is dead before working on any voltage source.

Line-voltage thermostats must be for the voltage, current and power ratings of the loads they will be controlling. For example a thermostat rated at 240 volts and 2880 watts must only be used to control maximum 12 amps of load current.

[latex]12\text{ A}=\dfrac{2880 \text{W}}{240\text{ V}}[/latex]

A thermostat that has been taken apart. It has a dial that you use to set the temperature.
A standard SPST line voltage thermostat


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Basic HVAC by Aaron Lee is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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