Heating BTU Calculator

🔥 Calculation Method

Airside Heating

Space Rule-of-Thumb

Boiler / Furnace

Supply CFM

Entering Air Temp (°F) Mixed air or OA entering coil

Leaving Air Temp (°F) Design supply air temperature

Elevation (ft) For altitude correction (0 = sea level)

📚 Heating Design Reference

Space Type	Rule-of-Thumb (BTU/hr/ft²)	Notes
Residential (new)	8–15	Well-insulated, sealed construction
Residential (older)	20–35	Pre-1980, minimal insulation
Commercial office	10–18	Depends on glazing ratio and climate
Warehouse / industrial	8–20	Large volume, high infiltration potential
Retail	15–25	High infiltration from frequent door openings
School / gym	12–20	High OA requirement inflates heating load

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📝 About This Calculator

This heating BTU calculator covers three common ways to size a heating load. The airside tab applies the sensible heat equation to a supply airflow and temperature rise with altitude correction. The space tab gives a quick rule-of-thumb estimate from floor area, construction quality, and design temperature difference. The boiler/furnace tab converts a required output into fuel input, an oversized capacity, and a fuel consumption rate.

Together these cover the three questions that come up early in a heating design: how much capacity an air handler delivers, roughly how much a space loses, and what fuel input a boiler needs. The rule-of-thumb and quick methods are for screening; final sizing should come from a detailed heat-loss calculation.

🔢 Formula & Method

Method	Equation
Airside sensible	Q (BTU/hr) = 1.08 × CFM × (LAT − EAT)
Altitude factor	1.08 × (1 − elev ÷ 145366)⁵⋅²⁵⁶¹
Space rule-of-thumb	Q = Area × base × climate × (ΔT ÷ 60)
Boiler fuel input	Input = Output ÷ Efficiency (AFUE)

The airside method is the sensible heat equation from ASHRAE Fundamentals; 1.08 = specific heat (≈0.244 BTU/lb·°F) × density (0.075 lb/ft³) × 60 min/hr, scaled by the barometric pressure ratio for elevation. Fuel consumption divides the fuel input (BTU/hr) by the fuel heating value — natural gas ≈1,020 BTU/ft³, propane ≈91,600 BTU/gal, #2 fuel oil ≈138,700 BTU/gal, electric 3,412 BTU/kWh.

❓ Frequently Asked Questions

How do I calculate airside heating BTU/hr?

Use the sensible heat equation: Q = 1.08 times CFM times the temperature rise across the coil, where the rise is leaving air temperature minus entering air temperature. For example, 2,000 CFM raised from 55 to 105 degrees F gives about 108,000 BTU/hr at sea level. The 1.08 factor applies to standard sea-level air; this tool reduces it for elevation using the barometric pressure ratio.

What boiler input do I need for a given output?

Fuel input equals required output divided by efficiency. A 150 MBH output at 92 percent AFUE needs about 163 MBH of fuel input. Then add a pickup or oversizing allowance, typically 15 to 30 percent, and round up to the next available boiler size. This tool reports both the input MBH and the oversized output, plus the fuel consumption rate for the selected fuel.

Why oversize a boiler or furnace?

An oversizing or pickup factor covers morning warm-up after a night setback, distribution and piping losses, and domestic hot water demand that the design heat loss does not include. A common range is 15 to 30 percent. Too much oversizing causes short cycling and reduced efficiency, so the pickup allowance is a balance rather than a safety blanket.

Is the space rule-of-thumb method accurate enough for design?

No. The BTU-per-square-foot method is a screening estimate that scales a construction-quality factor by a design temperature difference. It is fine for early budgeting and equipment pre-selection but should not size final equipment. Use a Manual J or an ASHRAE heat-loss procedure that accounts for the actual envelope, glazing, infiltration, and ventilation for design.

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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.

Heating BTU Quick Calculator