Total building/process heating load in MBH (1 MBH = 1,000 BTU/hr). Typical range: 100-10,000 MBH for commercial buildings. If omitted, the tool cannot run; this field is required.

Boiler type/fuel combination. Options: condensing_gas (high-efficiency condensing), electric_resistance, firetube_gas, firetube_oil, watertube_gas, watertube_oil. If omitted, defaults to 'condensing_gas'.

Boiler redundancy configuration. 'N' = single boiler sized for full load, 'N+1' = N boilers for the load plus one standby, '2x50' = two boilers each sized for 50% of load. If omitted, defaults to 'N' (no redundancy).

Site altitude in feet above sea level. Combustion-based boilers are derated 4% per 1,000 ft above 2,000 ft due to lower air density. Typical range: 0-10,000 ft. If omitted, defaults to 0 (sea level).

Natural gas cost in dollars per therm (1 therm = 100,000 BTU). Used for gas-fired boiler annual cost estimates. Typical US range: $0.50-$2.00/therm. If omitted, defaults to $1.00/therm.

Electricity cost in dollars per kWh. Used for electric boiler annual cost estimates. Typical US range: $0.08-$0.25/kWh. If omitted, defaults to $0.12/kWh.