The Lochinvar & A.O. Smith 100208835 For boilers and water heaters in particular, thermostat control is an essential part of HVAC systems. By tracking the present temperature, this control mechanism maintains the system's temperature at a consistent level. The thermostat controls the heating or cooling system to keep the interior temperature at the appropriate level when the outside temperature varies from the setpoint. The heating components or burner are turned on and off as needed to accomplish this. The thermostat maintains ideal comfort levels and maximizes energy savings by precisely managing the temperature.