When to use: Use to size a boiler or heating plant for commercial or residential hydronic systems. Start with a heat loss calculation (from the Heating Load Estimator) and add domestic hot water (DHW) loads. The calculator applies piping losses, diversity, and redundancy factors to determine boiler output (MBH), hot water flow rate (GPM), and estimated annual fuel cost. Use ASHRAE 90.1 for minimum efficiency requirements.