The answer to 'why is my boiler blowing hot air' is that your boiler's pressure relief valve may be stuck open, causing the boiler to release hot air instead of heating your home. This can happen due to a buildup of sediment, a faulty valve, or other mechanical issues with the boiler.
Some common symptoms of a boiler blowing hot air include the boiler running constantly, the home not getting warm, and higher than normal energy bills. To fix this, you may need to clean or replace the pressure relief valve, bleed the system to remove any air pockets, or have a professional technician inspect and repair the boiler.