Introduction
Hypothetical case study — illustrative example, not a real project. The scenario below is entirely fictional, constructed to demonstrate how a cooling load calculation works end to end: gathering inputs, building the load component by component, converting units, and translating the result into an equipment decision. No real home, client, or contractor is involved. Work through it alongside your own numbers in the cooling load calculator to see the same method applied to your building.
The Scenario
Reminder: this is a hypothetical, illustrative example — not a real project. Imagine a fictional two-story, 2,100 sq ft home in a hot climate: 9 ft ceilings downstairs, 8 ft upstairs, a dark shingle roof, R-13 walls, R-30 attic insulation, and 280 sq ft of glazing — unfortunately concentrated on the west (120 sq ft) and east (90 sq ft) elevations, with the remainder north and south. The windows are older clear double-pane units (SHGC ~0.65). Four people live in the home, two working from home offices during the day. The outdoor design temperature is 101°F; the indoor setpoint is 75°F. The homeowners are replacing a 25-year-old 5-ton unit that short-cycled and left the house humid — a textbook symptom from our mistakes guide.
Building the Load, Component by Component
Following the method in our formula guide, each load component is estimated from the measured inputs:
| Load component | Basis | BTU/hr |
|---|---|---|
| Envelope conduction | Walls, roof, windows, slab at 26°F ΔT | 15,500 |
| Solar gain | 280 sq ft glass, west/east heavy, SHGC 0.65 | 11,000 |
| Internal gains | 4 occupants, 2 home offices, kitchen, lighting | 7,000 |
| Latent load | Occupants + humid infiltration | 4,500 |
| Infiltration | 1990s construction, original windows | 4,000 |
| Total | 42,000 |
The total is 42,000 BTU/hr — a full ton and a half below the 5-ton (60,000 BTU/hr) unit being replaced. The old system was oversized by roughly 40%, which explains decades of clammy afternoons.
Converting and Selecting Equipment
Tonnage: 42,000 ÷ 12,000 = 3.5 tons. Metric cross-check: 42,000 × 0.29307 ÷ 1,000 = 12.31 kW, and 3.5 × 3.51685 = 12.31 kW ✓. The sensible heat ratio is (42,000 − 4,500) ÷ 42,000 ≈ 0.89 sensible — a dry-hot-climate profile, so equipment with strong sensible capacity at design conditions is appropriate. In this fictional scenario, the homeowners also priced envelope upgrades using our materials guide: low-E film on the west and east glass (−4,000 BTU/hr) plus a radiant barrier (−1,500) would bring the total to about 36,500 BTU/hr — just over 3 tons, or roughly 10.7 kW. They chose to do the upgrades first and install a 3.5-ton two-stage system, giving them comfortable headroom with excellent part-load dehumidification.
What This Illustrates
- Orientation dominates: over a quarter of this home’s load came through west and east glass — the exact input a square-footage rule would miss entirely.
- The old nameplate lied: sizing from the existing 5-ton unit would have repeated a 40% oversizing error for another 20 years.
- Upgrades shrink equipment: modest envelope improvements moved the needle by half a ton, with proportional savings on equipment and energy.
- Verification is cheap: the homeowners’ own calculation, built with the techniques in our estimating guide, gave them the confidence to question a contractor’s initial 5-ton quote — the scenario our DIY vs. pro guide recommends.
How the Fictional Inputs Were Gathered
Still hypothetical — this illustrates the measurement process, not a real visit. In this scenario, the homeowners spent a Saturday working through the measurement routine from our estimating guide. They sketched the floor plan, marked compass orientations, and measured every window per elevation — discovering that the previous owner’s “great natural light” was really 120 sq ft of unshaded west glass. The attic inspection revealed R-30 batts (adequate) but a dark shingle roof baking the attic to an estimated 140°F on design days. The occupancy count was honest: four people, with two home offices running dual-monitor workstations (~500 W combined, or ~1,700 BTU/hr) through the hottest afternoon hours. The infiltration assumption reflected 1990s construction with original windows — middling, not terrible. Each assumption was written on a single sheet taped inside the air handler closet, following the documentation habit recommended in our pro tips. The total measuring time was under two hours, and the resulting 42,000 BTU/hr figure immediately explained why the old 5-ton unit had short-cycled for 25 years: it was satisfying the thermostat in under ten minutes per cycle, never running long enough to dehumidify.
The Contractor Conversation
Hypothetical dialogue, illustrative only. Armed with their number, the fictional homeowners invited three contractors to bid. The first proposed a like-for-like 5-ton replacement “because that’s what’s there” — and was dismissed on the spot, a textbook case from our mistakes guide. The second ran a proper Manual J and arrived at 41,000 BTU/hr, within 3% of the homeowners’ figure, then recommended a 3.5-ton two-stage system — instant credibility. The third came in at 47,000 BTU/hr because he had assumed six occupants instead of four and used the record high temperature rather than the 1% design temperature; walking through the assumptions together corrected both, and his revised bid matched the others. The homeowners chose the second contractor, scheduled the envelope upgrades first, and had the load re-verified at 36,500 BTU/hr before the equipment order was placed. The moral of this fictional story is not that homeowners should design their own systems — it is that an independent number turns a sales pitch into a technical conversation you can actually evaluate.
Related Guides
- Complete Cooling Load Guide — the full overview with a worked example.
- Cooling Load Formulas Explained — the math behind every component.
- Materials Guide — upgrades that shrink your load.
- Common Cooling Load Mistakes — errors this scenario avoids.
- Cooling Load Calculation Checklist — every input, in order.
Get Your Number
Your home has its own story — write it in numbers with the cooling load calculator and see which components dominate your result.
This case study is a hypothetical, illustrative example — not a real project, client, or installation. Figures are planning estimates for educational purposes only. Actual cooling loads depend on site-specific conditions — verify your results with a licensed HVAC professional before purchasing equipment, and consult your local building authority for any code-related decisions.
