Warning: Any work with high voltage equipment can be dangerous or even fatal. It should be performed by a qualified electrician. Please ensure that you comply with local electrical codes and regulations. Improper installation can lead to device failure and even cause a fire. Damage to thermostat due to improper installation is not covered under warranty.
There are two quick and easy methods to determine which voltage your heating system operates on. Please note that these methods are based on rules of thumb and apply to the majority of homes. Some homes have different setups and these rules of thumb may not be applicable.
Locate the breaker in your electrical panel that is connected to your thermostat. If you see a single breaker switch like breaker one shown in the picture below, you likely have 120V.
If you see a double breaker, like the pictures shown below, you likely have 240V.
Turn off the power to your thermostat before checking the wiring as high voltages can be dangerous and even fatal.
You can also look into the gang box of your existing thermostat. If you have black and white wires you likely have 120V. If you have black and red wires you likely have 240V.