Furnace and Air Conditioner size for a Semi-Detached 2-storey House Semi-detached House square footage *Furnace Output [BTU/hr] up to 1500 sq ft40,000 BTU/hr50,000 BTU/hr 1500 to 1800 sq ft50,000 BTU/hr55,000 BTU/hr 1800 to 2200 sq ft55,000 BTU/hr60,000 BTU/hr 2200 to 3000 sq ft60,000 BTU/hr70,000 BTU/hr 2 more rows