Cooling Load Calculation: The Complete ASHRAE Guide
What Is a Cooling Load?
The cooling load is the amount of heat energy that must be removed from a space per hour to maintain a desired indoor temperature. It's measured in BTU/h (or watts in metric). An accurate cooling load calculation ensures your AC system is properly sized — not too big, not too small.
The cooling load has multiple components:
- Conduction: Heat flowing through walls, roof, floor, and windows
- Solar radiation: Sunlight through windows (often the largest component)
- Infiltration: Outdoor air leaking through cracks and openings
- Ventilation: Deliberate outdoor air introduction for air quality
- Internal gains: People, appliances, lighting, cooking
ASHRAE CLTD/SLF Method
ASHRAE Fundamentals Handbook Chapter 18 defines the Cooling Load Temperature Difference (CLTD) method for simplified residential and light commercial calculations:
Qconduction = U × A × CLTD × SC
- U = overall heat transfer coefficient (BTU/h·ft²·°F)
- A = area of the surface (sq ft)
- CLTD = Cooling Load Temperature Difference (°F)
- SC = Shading Coefficient for windows (1.0 = no shade)
CLTD Values by Surface Type
| Surface | Construction | U-Value | CLTD (°F) | BTU/h·ft² |
|---|---|---|---|---|
| Wall | Wood frame, R-13 | 0.07 | 20 | 1.4 |
| Wall | Brick veneer, R-13 | 0.06 | 18 | 1.1 |
| Roof | Flat, R-19 | 0.05 | 30 | 1.5 |
| Roof | Pitched, R-30 | 0.03 | 25 | 0.75 |
| Floor | Slab on grade | 0.10 | 5 | 0.5 |
| Window | Single pane | 1.10 | 14 | 15.4 |
| Window | Double pane, clear | 0.55 | 14 | 7.7 |
| Window | Double pane, low-E | 0.35 | 14 | 4.9 |
Note: Windows are often the dominant cooling load component. A single 3×5 ft window with single-pane glass adds 231 BTU/h just from conduction — plus solar gain.
Solar Heat Gain Through Windows
Solar heat gain is calculated separately using the Solar Heat Gain Factor (SHGF) and Shading Coefficient (SC):
Qsolar = A × SHGF × SC
| Window Orientation | SHGF Peak (BTU/h·ft²) | Peak Time | SC (no shade) |
|---|---|---|---|
| North | 20–40 | 12:00 PM | 1.0 |
| East | 150–200 | 9:00 AM | 1.0 |
| South | 100–150 | 12:00 PM | 1.0 |
| West | 200–250 | 3:00 PM | 1.0 |
| Horizontal (skylight) | 250–300 | 12:00 PM | 1.0 |
Infiltration and Ventilation Load
Qinf = 1.08 × CFM × (Tout − Tin)
- 1.08 = specific heat of air at sea level (BTU/h per CFM per °F)
- CFM = infiltration rate (use 0.5 ACH for tight construction, 1.0 ACH for average)
- Tout = outdoor design temperature (ASHRAE 0.4% or 1% value)
- Tin = desired indoor temperature (typically 75°F)
Latent load (humidity): Qlat = 0.68 × CFM × (Wout − Win), where W is humidity ratio (gr/lb).
Internal Heat Gains
| Source | Sensible (BTU/h) | Latent (BTU/h) | Total |
|---|---|---|---|
| Adult (seated) | 250 | 200 | 450 |
| Adult (light work) | 400 | 400 | 800 |
| 60W equivalent LED bulb | 10 | 0 | 10 |
| Computer / desktop | 300–500 | 0 | 300–500 |
| Oven (cooking) | 2,000–3,000 | 1,000 | 3,000–4,000 |
| Dishwasher | 600 | 400 | 1,000 |
Complete Cooling Load Example
Scenario: 1,800 sq ft home in Atlanta (Zone 3), outdoor design 93°F, indoor 75°F.
Roof: 1,800 sq ft pitched, R-30 → 1,800 × 0.03 × 25 = 1,350 BTU/h
Walls: 1,200 sq ft (net of windows), R-13 → 1,200 × 0.07 × 20 = 1,680 BTU/h
Windows: 200 sq ft double-pane, south-facing → Conduction: 200 × 0.55 × 14 = 1,540 BTU/h + Solar: 200 × 125 × 1.0 = 25,000 BTU/h
Infiltration: 0.7 ACH × 1,800 × 8 / 60 = 168 CFM → 1.08 × 168 × 18 = 3,266 BTU/h
Internal: 4 people × 450 + appliances = 2,500 BTU/h
Total: 1,350 + 1,680 + 26,540 + 3,266 + 2,500 = 35,336 BTU/h
Result: Need a 3-ton (36,000 BTU/h) system.
Common Cooling Load Mistakes
- Ignoring solar gain: West-facing glass can add 200+ BTU/h per sq ft of window at peak. A 4×6 ft west window adds 4,800 BTU/h from sun alone.
- Using rule-of-thumb for everything: 20 BTU/sq ft is a starting point, not a design tool. Actual loads vary 2–5× based on construction and orientation.
- Not accounting for duct losses: Ducts in unconditioned attics can lose 20–30% of cooling capacity. Include a duct loss factor.
- Oversizing for "worst case": Use the ASHRAE 0.4% design temperature (not the all-time record). The system should handle 99.6% of hours.
Standards Reference
- ASHRAE Fundamentals 2021 — Chapter 18 (Nonresidential Cooling and Heating Loads)
- ASHRAE Fundamentals 2021 — Chapter 19 (Residential Cooling and Heating Loads)
- ACCA Manual J — Residential Load Calculation (8th Edition)
- ASHRAE Standard 90.1 — Energy Standard
Frequently Asked Questions
How do you calculate cooling load for a room?
Use the ASHRAE CLTD method: Q = U × A × CLTD for each surface (walls, roof, windows), add solar heat gain through windows (Q = A × SHGF × SC), add infiltration (Q = 1.08 × CFM × ΔT), and add internal gains (people, equipment). Sum all components for total cooling load in BTU/h.
What is a good cooling load per square foot?
Residential cooling loads typically range from 12–30 BTU/sq ft depending on climate zone, insulation, windows, and orientation. Well-insulated homes in moderate climates: 12–15 BTU/sq ft. Average homes: 18–22 BTU/sq ft. Poorly insulated homes in hot climates: 25–30+ BTU/sq ft.
What is the difference between sensible and latent cooling?
Sensible cooling removes heat and lowers air temperature (measured by dry-bulb thermometer). Latent cooling removes moisture and lowers humidity (measured by wet-bulb or dew point). In humid climates, latent load can be 30–50% of total cooling load. The SHR (Sensible Heat Ratio) for typical residential is 0.70–0.80.
How does insulation affect cooling load?
Insulation reduces the U-value of surfaces, directly lowering conduction heat gain. Upgrading from R-13 to R-21 walls reduces wall heat gain by 38%. Upgrading from single-pane (U=1.1) to double-pane low-E windows (U=0.35) reduces window conduction by 68%. However, solar gain through windows often dominates regardless of insulation level.
What outdoor temperature should I use for cooling load calculation?
Use the ASHRAE 0.4% cooling design temperature for your location (the temperature exceeded only 0.4% of annual cooling hours). For Atlanta, that's 93°F. For Phoenix, it's 112°F. Don't use the all-time record — that's extreme and would oversize the equipment for 99.96% of hours.