Design Inputs
Enter summer design conditions and project values. All dimensions use U.S. customary units.
Example Design Data
| Input | Example value | Purpose |
|---|---|---|
| Conditioned floor area | 1,800 ft² | Sets the occupied building scale. |
| Outdoor / indoor temperature | 95°F / 75°F | Creates a 20°F sensible design difference. |
| Window area and SHGC | 280 ft² / 0.30 | Estimates glass conduction and solar heat. |
| Infiltration rate | 0.35 ACH | Converts building leakage into airflow load. |
| Outdoor ventilation | 60 cfm | Adds required fresh-air sensible and latent load. |
| Duct gain / margin | 10% / 5% | Accounts for distribution losses and design uncertainty. |
Formula Used
The calculator estimates humidity difference from dry-bulb temperature and relative humidity. It then combines envelope, solar, internal, outdoor-air, infiltration, duct, and design-margin loads.
How to Use This Calculator
- Measure only conditioned floor area and average ceiling height.
- Use local summer design weather, not a mild daily average.
- Enter U-factors from verified assemblies or product documentation.
- Use a separate solar gain value for the selected glazing exposure.
- Estimate realistic peak occupancy, lighting, appliances, and fresh-air flow.
- Set infiltration from a blower-door result when possible.
- Review sensible, latent, total load, and sensible heat ratio together.
- Confirm final selection using equipment performance at design conditions.
Residential Cooling Load Planning
Residential cooling equipment should match the home’s real peak demand. Floor area alone cannot reveal that demand. Window orientation, insulation, roof exposure, air leakage, and household use all matter. A room with large west-facing glass can need much more cooling than another room of equal size.
Begin with local summer design conditions. Use an outdoor temperature and humidity level that represents a demanding normal season. Then choose an indoor temperature and humidity target. The difference between outdoor and indoor conditions drives the sensible and latent portions of the calculation.
Envelope loads pass through walls, roofs, and windows. Lower U-factors reduce conduction. Solar gain through glass is separate. It can become the largest daytime load in sunny rooms. Shading, overhangs, screens, and low-SHGC glazing can reduce this component.
Internal loads come from people, lights, cooking, electronics, and appliances. Electricity used indoors eventually becomes heat. People also add moisture. That moisture creates latent load. A system must remove enough moisture while it removes heat.
Ventilation and infiltration are often underestimated. Outdoor air carries heat and water vapor. Leakage adds air even when mechanical ventilation is low. The calculator converts infiltration from air changes per hour into cfm. It then estimates sensible and latent effects from the selected conditions.
Ducts located in hot attics or garages can add meaningful load. A modest duct allowance helps early planning. It does not replace duct testing, insulation checks, or verified leakage measurements. Use a small design margin carefully. Excessive margins can encourage oversizing.
Oversized equipment may cool rapidly but cycle too often. Short cycles reduce moisture removal and comfort. They can also increase wear. Undersized equipment may run continuously during severe weather. The preferred choice considers load, airflow, latent performance, and the manufacturer’s capacity data.
Check capacity at the actual indoor and outdoor rating point. Variable-capacity systems produce different output at different speeds. Match the indoor coil, outdoor unit, blower setting, and refrigerant line design. These details affect delivered capacity, efficiency, sound, and humidity performance. This avoids peak surprises.
This tool is useful for early budgeting, renovation comparisons, and design discussions. It is not a substitute for a room-by-room residential load procedure. A qualified designer should verify final equipment size, distribution airflow, code requirements, and local climate assumptions before installation.
Frequently Asked Questions
1. What does a cooling load represent?
Cooling load is the rate of heat and moisture a system must remove at design conditions. It is commonly shown in Btu/h or tons. It is not simply the home’s floor area.
2. Why are sensible and latent loads separate?
Sensible load changes air temperature. Latent load removes moisture. Air conditioners must address both. A home with high humidity may need equipment that delivers stronger moisture removal, even when total cooling capacity appears adequate.
3. What is a ton of cooling?
One ton of cooling equals 12,000 Btu/h. Nominal equipment tonnage is a planning label. Actual delivered capacity changes with outdoor conditions, indoor airflow, refrigerant settings, and equipment configuration.
4. How should I choose outdoor design temperature?
Use a recognized local cooling design value, not the hottest recorded temperature or a casual forecast. A local HVAC professional, weather design table, or code resource can help identify an appropriate summer design condition.
5. What U-factor should I enter?
Use the assembly or product U-factor whenever available. Lower values mean better resistance to heat flow. Do not confuse U-factor with R-value. U-factor is roughly the inverse of total R-value.
6. What is SHGC?
Solar heat gain coefficient describes how much solar energy passes through glazing. Lower SHGC usually reduces solar cooling load. The best value depends on climate, orientation, shading, and heating needs.
7. Why does infiltration increase cooling demand?
Leaking outdoor air brings both heat and moisture indoors. Infiltration therefore raises sensible and latent load. Air sealing can reduce demand and may improve comfort beyond what a larger air conditioner can achieve.
8. Should I add a large safety factor?
Usually no. Large arbitrary margins can oversize equipment. Oversizing may worsen humidity control and short cycling. Improve the input data first, then use only a modest allowance for known uncertainty.
9. Does this calculator size each room?
No. This version estimates whole-home load. Final duct design needs room-by-room loads because solar exposure, exterior surfaces, glass, occupancy, and airflow needs vary by room.
10. Can I use this result to buy equipment?
Use it as a planning estimate. Before purchase, verify the result with a recognized residential load method and manufacturer capacity data. Confirm that the selected system meets sensible and latent needs at local design conditions.
11. Why may actual operation differ from the estimate?
Actual loads vary with weather, solar direction, occupant behavior, window coverings, equipment condition, duct leakage, and thermostat settings. Design calculations describe a selected peak condition, not every hour of the year.