Biomass Thermal Utilization Act of 2021 or the BTU Act of 2021 This bill expands the 30% energy tax credit to include open-loop biomass heating property and extends the residential energy efficient property tax credit for five years. The bill defines open-loop biomass heating property as any property that uses open-loop biomass to produce thermal energy in the form of heat, hot water, hot air, or steam and is used for space heating, air conditioning, domestic hot water, or industrial process heat, but does not include any boiler or furnace unless they operate at thermal output efficiencies of at least 75% and are installed indoors.