| Area (ft²) | Height (ft) | Indoor (°F) | Outdoor (°F) | Insulation | ACH | Duct % | Safety % | AFUE % | Altitude (ft) | Expected Output (BTU/h) |
|---|---|---|---|---|---|---|---|---|---|---|
| 2000 | 8 | 70 | 20 | Average | 0.50 | 10 | 15 | 95 | 0 | ≈ 52,000–70,000 |
| 1500 | 9 | 70 | 10 | Good | 0.30 | 10 | 15 | 96 | 3000 | ≈ 40,000–55,000 |
| 2600 | 8 | 70 | 0 | Poor | 0.70 | 15 | 20 | 92 | 500 | ≈ 90,000–120,000 |
These are illustrative ranges; your specific envelope and infiltration drive actual results.
Temperature difference
ΔT = Tindoor − Toutdoor, design
Conduction (envelope) load
Qcond = A × C × ΔT × window multiplier
where A = floor area (ft²), C = overall envelope coefficient (BTU/h·ft²·°F) from preset or override.
Infiltration load
Qinf = 1.08 × CFM × ΔT, with CFM = (ACH × Volume) / 60, Volume = A × height.
Adjustments
Total sensible load: Qtot = Qcond + Qinf
Ducts: Qduct = Qtot ÷ (1 − duct%)
Safety: Qneed = Qduct × (1 + safety%)
Output required: Qout = Qneed
Input required at sea level: Qin,sea = Qout ÷ AFUE
Altitude derate: factor = 0.04 × altitude(kft); Nameplate input: Qnameplate = Qin,sea ÷ (1 − factor)
Suggested standard size = next size in {40k, 60k, 80k, 100k, 120k, 140k} ≥ Qnameplate.
This is an educational tool inspired by common load-calculation principles. For construction or permit work, perform a Manual J or consult a qualified designer.
- Enter floor area, ceiling height, and temperatures to set ΔT.
- Select an insulation level or enter a custom envelope coefficient.
- Set infiltration (ACH). Typical tight homes: 0.2–0.3. Older: 0.5–0.8.
- Add duct losses if ducts are in unconditioned areas.
- Set a safety margin for design uncertainty or intermittent setbacks.
- Choose AFUE and altitude to size the nameplate appropriately.
- Click Calculate, then export your results to CSV or PDF.
Is this a substitute for a Manual J calculation?
No. This estimates load using envelope coefficients and infiltration. It’s great for preliminary sizing and sanity checks. For permits, rebates, or complex homes, commission a Manual J from a qualified designer or energy professional.
What AFUE value should I enter?
Use the nameplate AFUE. Condensing units are typically 92–98%. Non‑condensing are often 80–90%. AFUE affects the required input capacity; the heating load (required output) is determined by the building and climate.
Should I oversize the furnace?
Avoid large oversizing. A modest margin (about 10–20%) can cover setbacks and extreme cold. Excess oversizing causes short cycling and noise. Consider two‑stage or modulating equipment to handle part‑load conditions comfortably.
How do duct losses affect sizing?
Ducts in attics, crawlspaces, or garages can lose 5–20% of heat. Increase the “Duct loss” percentage to reflect this. Air‑sealing and insulating ducts can reduce the load and may allow a smaller nameplate size.
Why does altitude matter?
Higher altitude reduces air density and usable input. Many gas furnaces require derating around 4% per 1,000 ft. The calculator inflates required nameplate input so delivered output still meets your design load.
Are BTU per square foot rules reliable?
Rules like “30–60 BTU/ft²” ignore insulation, windows, infiltration, and climate. Use them only as a quick check. This tool explicitly models those drivers, giving a more tailored sizing recommendation.
When should I consider a heat pump?
In moderate climates or tight, well‑insulated homes, cold‑climate heat pumps can meet most heating needs. Pair with a small furnace or electric backup if required, and compare lifecycle costs and comfort.