Boiler Sizing Calculator

They're the same calculated value shown under two labels: both represent the raw BTU/hr input rating you'd look for on a boiler's nameplate or spec sheet, after Net Output Required has had Pickup Factor's extra capacity added on top and the result divided by Effective Efficiency to account for combustion losses. There's no separate calculation behind each — treat them as the same figure described two ways rather than independent cross-checks.

Since Gross Input Required equals Net Output Required divided by Effective Efficiency, raising Combustion Efficiency directly and proportionally lowers the required gross input for the same building load — moving from an 80%-efficient standard boiler to a 95%-efficient condensing model reduces the required BTU/hr input by nearly 16%, which can mean a materially smaller (and cheaper) boiler for the same heating job.

Gas-fired combustion equipment is derated at higher elevations because thinner air reduces the oxygen available for combustion, roughly 4% less effective per 1,000 ft of elevation in this calculator's model. At 5,000 ft, for example, Effective Efficiency drops by about 20% relative to sea level, which raises the Gross Input Required for the same building heat loss — installers in mountain locations typically need to select larger boilers than a sea-level heat-loss number alone would suggest.

Pickup Factor adds extra capacity on top of Net Output Required to account for a boiler having to recover quickly from a temperature setback, such as an overnight thermostat drop or a building that sits unheated during the day — but that extra capacity flows into Gross Input Required (the burner input rating), not into the displayed Net Output Required figure itself, which reflects building heat loss and DHW load only and stays fixed regardless of Pickup Factor. A typical range is 10-25% for intermittently heated buildings; buildings that are heated continuously and never allowed to cool down need less pickup allowance than ones with aggressive setback schedules.