The answer to 'why is my water heater blowing hot air' is that your water heater may have a serious issue, such as a faulty thermostat, a broken dip tube, or a malfunctioning heating element. This can cause the water heater to blow out hot air instead of heating the water properly.
Some common symptoms include the water not getting hot enough, the tank making strange noises, or the pilot light going out frequently. In some cases, the water heater may be leaking or the pressure relief valve may be releasing steam. If you notice any of these issues, it's best to call a professional HVAC technician to diagnose and repair the problem.