Room BTU calculator

This is a steady-state estimate for one room at the design temperatures: every surface contributes U x A x temperature difference, ventilation contributes its air changes, and the cooling path adds occupant, appliance, and window solar gains. It does not model orientation or shading beyond the flat solar input, thermal mass, intermittently heated rooms, or whole-house interaction, and it is not a substitute for a code heat-loss calculation. Heating sizes are conservatively quoted without internal gains, and a 10 percent margin is already included in the recommended figure.