A thermostat with a pre-selected temperature will effectively control the indoor temperature regardless of the HVAC system turned on/off. Those not having a thermostat should turn on and off the appliance manually. Coming to its working procedure, this device can measure and maintain the appropriate temperature. It gets connected to the HVAC system through proper wiring. After setting the temperature to it, in any case, if the temperature crossed the range, then it sends an electric signal to the heat pump and other appliances that trigger consequently.