Introduction
Hypothetical case study — illustrative example, not a real project. The scenario below is fictional: the house, the homeowner, and the numbers are invented to demonstrate how furnace sizing works end to end. No real property, client, or contractor is described. Follow along to see the full process — measuring, calculating, converting through AFUE, comparing quotes, and making the final decision — applied to a concrete example.
To run the same process on your own home, use the Furnace Size Calculator alongside this walkthrough.
The Scenario (Fictional)
The Hendersons (fictional homeowners) live in a 1,900 sq ft (176.5 m²) two-story colonial built in 1987 in a cold-climate region with sustained sub-freezing winters. Their 30-year-old 120,000 BTU/h input, 80% AFUE gas furnace still runs, but heating bills have climbed and the blower is noisy. They want to replace it before it fails mid-winter. Their goals: a correctly sized replacement, 95%+ efficiency, and no oversizing.
Step 1: Measuring the House
Following the room-by-room method from the estimating guide, they measure:
- Heated area: 1,900 sq ft (176.5 m²) — they exclude the 450 sq ft unheated garage, which a first-pass guess had wrongly included.
- Ceilings: 8 ft (2.4 m) throughout — factor 1.00.
- Insulation: original 1987 construction, ~6 inches of attic fiberglass, double-pane original windows, some noticeable drafts — rated average-to-poor; they run both.
- Climate factor: cold zone, 45 BTU/h per sq ft (between the 40–55 band).
- Age: 1987 build — factor ×1.00.
Step 2: Calculating the Heating Load
Base load: 1,900 × 45 = 85,500 BTU/h. Metric cross-check: 176.5 m² × 484.4 BTU/h per m² = 85,500 ✓ (45 BTU/h per sq ft × 10.7639 = 484.4 per m²). In kilowatts: 85,500 ÷ 3,412 = 25.1 kW; check: 176.5 m² × 142 W/m² = 25,063 W ≈ 25.1 kW ✓.
Insulation scenarios:
- Average (×1.00): 85,500 BTU/h (25.1 kW)
- Poor (×1.25): 106,875 BTU/h (31.3 kW)
The 21,000 BTU/h spread — a full equipment size — comes entirely from the insulation judgment call. This is the moment the Hendersons decide to get a professional assessment rather than guess, and the contractor’s blower-door test and attic inspection confirm the house is closer to average than poor: some air sealing was done by a previous owner. They proceed with 85,500 BTU/h as the planning load.
Step 3: Converting Load to Furnace Size
They want 95% AFUE condensing equipment. Required input = 85,500 ÷ 0.95 = 90,000 BTU/h. The adjacent standard sizes: 80,000 input (delivers 76,000 — about 11% under the planning load) and 100,000 input (delivers 95,000 — about 11% over). Dead between sizes — the classic borderline case described in the formula-explained guide.
Step 4: The Professional Load Calculation
The contractor’s room-by-room Manual J comes in at 78,000 BTU/h — about 9% under the rule-of-thumb figure, a typical result since the rule of thumb is deliberately conservative. At 95% AFUE: 78,000 ÷ 0.95 = 82,105 BTU/h input required. The 80,000-input unit (delivering 76,000) is now only ~2.5% under the professional load — well within tolerance — while the 100,000-input unit would be ~22% oversized. Decision: 80,000 BTU/h input, 96% AFUE, two-stage, variable-speed blower.
Note what didn’t happen: nobody copied the old 120,000 BTU nameplate. The old unit was roughly 50% oversized — the likely reason for its short-cycling, noise, and high bills over the years. The common mistakes guide covers exactly this trap.
Step 5: Quotes and Costs
Three written quotes for 80,000-input, 96% AFUE, two-stage units (illustrative figures):
| Quote | Installed price (illustrative) | Load calc method | Notes |
|---|---|---|---|
| A | $6,200 | Manual J provided | Includes PVC venting, condensate drain, permits |
| B | $5,400 | “30 years experience” | No load calc; venting upgrade priced as extra |
| C | $7,800 | Manual J provided | Recommends 100,000 BTU “for safety” |
The Hendersons eliminate B (no calculation, vague scope) and C (oversized recommendation contradicting its own load calc — the upsell flagged in the cost guide). They choose A, and the contract specifies the Manual J result, venting scope, permits, and commissioning measurements.
Step 6: Installation and Verification
Permit pulled, rough-in and final inspections passed (see the code compliance guide for the typical sequence). Commissioning report records: temperature rise within manufacturer spec, gas manifold pressure correct, static pressure acceptable, and room-by-room airflow balanced. First-winter result (illustrative): even temperatures, quiet operation, and heating bills down roughly 25% versus the old unit — the combined effect of right-sizing and the 80%→96% efficiency jump. Because the two-stage unit spends most of its time on low fire, the house also feels more comfortable: fewer drafts, steadier room temperatures, and none of the old furnace’s roaring startup cycles.
Lessons From the Walkthrough
- Measure honestly — excluding the garage removed a phantom 20,000 BTU/h on the first pass.
- Don’t guess borderline inputs — the insulation question was resolved with testing, not optimism.
- The rule of thumb was conservative — Manual J came in 9% lower, which resolved the between-sizes tie downward.
- The old nameplate lied — 120,000 BTU of input for a ~78,000 BTU/h load was ~50% oversized.
- Quotes are comparable only with a number in hand — the Hendersons’ homework made the oversell in Quote C obvious.
Before your own project, run through a full pre-purchase verification of measurements, quotes, and permits.
Related Guides
- Complete Guide — the sizing method this walkthrough applies.
- Estimating Guide — the measurement walkthrough used in Step 1.
- Cost Guide — budgeting and quote comparison.
- Formula Explained — the math behind Steps 2–4.
- Sizing Checklist — verify everything before you sign.
Get Your Number
This fictional project followed the same path your real one should: measure, calculate, verify borderline calls professionally, compare honest quotes, and confirm the installation. Start your version with the Furnace Size Calculator — the Hendersons’ Step 1 and 2 take about twenty minutes.
Disclaimer: This is a hypothetical, illustrative case study — not a real project. Figures are invented for demonstration. Calculator results are planning estimates; verify your final furnace size with a licensed HVAC professional, and consult your local building authority for permit and code decisions.
