Line Voltage System
Line voltage or high voltage thermostat systems run off your home's 120 Volt or 240 volt AC power and are not particularly common as they are primarily used for electric resistance heating systems. These thermostats are not as sensitive as the low voltage types and may take up to 7°F of temperature change to register a change. As a result expect wider temperature fluctuations with these thermostats.
A way to check if you have a line voltage thermostat is to look at the wires. They will have thicker electrical wires like you would see going to a light switch or outlet, 14Gauge or 12Gauge as seen in the above photo. Low voltage wiring looks like old doorbell or telephone wiring or speaker wire.
Also you can check for voltage listing inside the cover of the thermostat or on the heat pump, furnace (inside the access door), or on the boiler or on the electrical baseboard unit itself.
Line voltage thermostats are typically hooked up to a 120 volt or 240 volt electric resistance heating system.