The answer to 'why is my water heater blowing hot air' is that your water heater's pressure relief valve may be malfunctioning. The pressure relief valve is designed to release excess pressure and prevent the tank from exploding, but if it is stuck open, it can cause hot air to be blown out. This can be caused by a buildup of mineral deposits, a faulty valve, or high water pressure in the system.
If your water heater is blowing hot air, you should first check the pressure relief valve to see if it is stuck open. You can do this by gently lifting the valve lever and seeing if it closes properly. If the valve is stuck open, you may be able to fix it by flushing the tank or replacing the valve. However, if the issue persists or you are not comfortable working on the water heater, it is best to call a professional HVAC technician to diagnose and repair the problem.