Is it harmful to turn the furnace on and off with the power swithch instead of the thermostat?

Yes, turning the furnace on and off with the power switch instead of using the thermostat can be harmful. This practice can lead to wear and tear on the furnace components, as they are not designed for frequent power cycling. Additionally, it may disrupt the furnace's cycling and efficiency, potentially leading to overheating or other safety issues. It's always best to use the thermostat for regular operation to ensure proper control and maintenance.