Biomass Thermal Utilization Act of 2015 or the BTU Act of 2015

Amends the Internal Revenue Code to include 30% of qualified biomass fuel property expenditures made in taxable years beginning before 2021 in the residential energy efficient property tax credit. Defines "qualified biomass fuel property expenditure" as an expenditure for property which uses the burning of biomass fuel (a plant-derived fuel available on a renewable or recurring basis) to heat a dwelling used as a residence, or to heat water for use in such dwelling, and which has a thermal efficiency rating of at least 75%.

Allows: (1) a 15% energy tax credit until 2021 for investment in open-loop biomass heating property, and (2) a 30% credit for boilers or furnaces that operate at thermal output efficiencies of at least 80% and provide thermal energy.