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Calcimator

Boiler Sizing Calculator

Size heating boilers based on building heat loss, domestic hot water load, pickup factor, efficiency, and altitude derating.

Inputs

BTU/hr

Typical residential: 30,000–100,000 BTU/hr; commercial: 100,000+ BTU/hr

BTU/hr
%
%

Standard boiler: 80–85%; ENERGY STAR: ≥90%; condensing: 90–98% per ASHRAE 90.1

ft

Results

Gross Input Required

175,882 BTU/hr

Required Boiler Rating

175,882 BTU/hr

Net Output Required130,000 BTU/hr
Fuel Consumption1.76 therms/hr
Est. Annual Fuel Cost$4,221.00
How to Use This Calculator
  1. Enter Building Heat Loss, Domestic Hot Water Load, and Pickup Factor.
  2. Set Combustion Efficiency and Altitude.
  3. Review Gross Input Required (BTU/hr) and Required Boiler Rating (BTU/hr).
  4. Use Net Output Required (BTU/hr) and Fuel Consumption (therms/hr) to inform your decision.

How the result changes with Combustion Efficiency

Combustion EfficiencyGross Input RequiredRequired Boiler Rating
64233,594 BTU/hr233,594 BTU/hr
74202,027 BTU/hr202,027 BTU/hr
85175,882 BTU/hr175,882 BTU/hr
95157,368 BTU/hr157,368 BTU/hr

What each input means

Building Heat Loss
Design heat loss of the building from Manual J (residential) or ASHRAE heat loss calculation (commercial). ACCA Manual J is the ANSI-recognized standard for residential load calculations.
Domestic Hot Water Load
Additional BTU load for domestic hot water if served by the same boiler (indirect tank).
Pickup Factor
Extra capacity for recovery from setback. Typically 10–25% for intermittently heated buildings.
Combustion Efficiency
Rated combustion efficiency. ASHRAE 90.1 Table 6.8.1 sets minimum boiler efficiencies: gas-fired ≥80% (AFUE) for residential; commercial boilers ≥80% combustion efficiency. High-efficiency condensing boilers: 90–98%.
Altitude
Elevation above sea level. Gas-fired equipment is derated ~4% per 1000 ft above sea level.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Building Heat Loss = 100000, Domestic Hot Water Load = 30000, Pickup Factor = 15, Combustion Efficiency = 85 = 5 input(s) provided
  2. Calculate Gross Input Required
    Gross Input Required
    175882 = 175882
  3. Calculate Required Boiler Rating
    Required Boiler Rating
    175882 = 175882
  4. Calculate Net Output Required
    Net Output Required
    130000 = 130000
  5. Calculate Fuel Consumption
    Fuel Consumption
    1.76 = 1.76

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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