The answer to 'why is my furnace blowing hot air' is that your furnace is likely operating as intended and heating your home. This can happen when the thermostat is set to 'heat' mode and the temperature in your home drops below the set point, causing the furnace to turn on and blow warm air through the vents.
Some common causes for a furnace blowing hot air include a malfunctioning thermostat, a clogged air filter, or issues with the furnace's heating components. If the hot air is accompanied by unusual noises, strange smells, or the furnace is not cycling on and off as expected, it's best to have a professional HVAC technician inspect the system.