The answer to 'why is my water heater blowing cold air' is that there could be several common causes, including a malfunctioning thermostat, a broken heating element, or sediment buildup in the tank. The symptoms you may notice are the water heater producing cold or lukewarm water instead of hot water, and the unit may be blowing cool air instead of hot air.
Some potential solutions include checking and adjusting the thermostat, replacing a faulty heating element, or flushing the tank to remove sediment. However, if you're not comfortable troubleshooting the issue yourself, it's best to call a professional HVAC technician to diagnose and repair the problem.