How Many BTU to Heat a House?

Plan on about 30 BTU per square foot in a moderate climate, so a 1,500 sq ft house needs roughly 45,000 BTU per hour of heat. The real number depends heavily on climate: about 20 BTU per sq ft in a mild climate and 45 or more in a very cold one, a range far wider than cooling. Insulation, ceiling height, windows, and air leakage all move it, so a Manual J load calculation is the accurate method for equipment you are going to install.

Roughly 20 BTU per square foot in a mild southern climate, 30 in a moderate/mixed climate, and 40-50 in a cold or very cold climate. Heating BTU per square foot varies about 2 to 3 times across US climate zones because it is driven by how cold your winter design temperature is, unlike cooling which stays in a tighter band. Use your climate zone, not a single national number.

About a 50,000-60,000 BTU input furnace in a moderate climate. The heat load is around 45,000 BTU, and a furnace is rated by input, so at 90% AFUE efficiency you need roughly 45,000 / 0.90 = 50,000 BTU input to deliver it, with a little margin. In a very cold climate the same house can need a 75,000-90,000 BTU furnace. Size to the load, not above it, because an oversized furnace short-cycles and wastes fuel.

Neither; size it to the load. An oversized furnace short-cycles, turning on and off too often, which wastes fuel, wears the equipment, and heats unevenly. An undersized furnace cannot keep up on the coldest design days. The right size comes from a Manual J heat-loss calculation, which is why the square-foot figures here are a starting estimate, not a final specification.

About 30,000 BTU in a moderate climate (30 BTU per sq ft), ranging from roughly 20,000 BTU in a mild climate to 45,000 in a very cold one. A 90% AFUE furnace to deliver the moderate-climate load would be about a 33,000-35,000 BTU input unit. Confirm with a load calculation, since a well-insulated 1,000 sq ft home needs far less than a leaky one.