Calculator Inputs
Example Data Table
| Scenario | Area | Height | Indoor / Outdoor | Shell | ACH | AFUE | Estimated Result |
|---|---|---|---|---|---|---|---|
| Compact insulated house | 1,500 ft² | 8 ft | 70°F / 25°F | Modern insulated | 0.45 | 95% | About 40,000–50,000 BTU/h input |
| Standard family home | 2,400 ft² | 8.5 ft | 70°F / 20°F | Standard insulated | 0.60 | 92% | About 80,000–100,000 BTU/h input |
| Older drafty structure | 3,200 ft² | 9 ft | 70°F / 10°F | Older drafty shell | 1.00 | 80% | About 160,000–200,000 BTU/h input |
These sample ranges are illustrative. Actual results depend on exact inputs, air leakage, altitude, and duct performance.
Formula Used
This calculator uses a practical simplified sizing method that combines shell heat loss, infiltration heat loss, duct loss allowance, equipment efficiency, and altitude correction.
1) Envelope load
Envelope Load = Area × Base Coefficient × ΔT × (Ceiling Height ÷ 8) × Multipliers
The multipliers account for insulation, glazing quality, window ratio, wind exposure, and foundation condition.
2) Infiltration load
Infiltration Load = 0.018 × Volume × ACH × ΔT
Volume is in cubic feet, ACH is air changes per hour, and ΔT is the indoor-outdoor design temperature difference in °F.
3) Gross building load
Gross Load = Envelope Load + Infiltration Load
4) Required delivered output
Required Output = Gross Load × (1 + Duct Loss %) × (1 + Safety Factor %)
5) Recommended furnace input
Recommended Input = Required Output ÷ (AFUE × Altitude Derate)
For altitude above 2,000 ft, the tool applies a 4% derate per 1,000 ft above that level, with a minimum floor for practicality.
How to Use This Calculator
- Choose imperial or metric units.
- Enter conditioned floor area and average ceiling height.
- Set indoor and outdoor winter design temperatures.
- Select the shell, insulation, glazing, exposure, and foundation conditions.
- Enter window percentage, air changes, duct loss, altitude, and furnace AFUE.
- Add a reserve percentage if you want extra sizing margin.
- Click Calculate Furnace Size.
- Review the output size, nearest nominal furnace, chart, and cost estimate.
- Use the CSV or PDF buttons to save the results.
Frequently Asked Questions
1) What does this furnace sizing calculator estimate?
It estimates the furnace input size needed to satisfy a building’s heating demand after considering shell losses, infiltration, duct losses, altitude, and equipment efficiency.
2) Is this the same as a Manual J calculation?
No. It is a structured preliminary estimator. Manual J is a room-by-room method with finer envelope, orientation, and ventilation detail. Use this tool for planning, comparison, and early budgeting.
3) Why does air changes per hour matter so much?
In cold weather, outside air entering through leakage must be reheated. Draftier buildings can require much larger furnace capacities, even when floor area stays the same.
4) Why does altitude affect furnace sizing?
Higher altitude reduces air density. Many combustion appliances lose available output unless they are specially adjusted. The calculator accounts for that derating when recommending furnace input.
5) Should I always add a safety factor?
A modest reserve can cover uncertainty in inputs and weather swings. Oversizing too much, however, may reduce comfort and efficiency. Small reserves are usually more reasonable.
6) Is furnace size based on input or output?
Furnaces are often marketed by input BTU/h. Actual delivered heat depends on AFUE and site conditions. That is why the calculator shows both required output and recommended input.
7) Can I use metric values directly?
Yes. Select metric units, then enter square meters, meters, Celsius, and meters elevation. The calculator converts values internally and still reports furnace size in familiar BTU/h and kW.
8) What input values should I verify carefully?
Outdoor design temperature, ACH, duct loss, AFUE, and shell condition are the most influential entries. Better field data will produce more dependable furnace recommendations.