Turn your furnace on and twist thermostat to heat mode and turning up the thermostat to a higher temperature. Check if hot air is coming from vents. Check for dirt in or around the thermostat by lifting the thermostat cover. It’s also important for some older thermostats to be level on the wall, so check that the thermostat is perfectly level. If there is excess dirt or corrosion, replace the thermostat or seek a professional to replace it for you.