Humidity significantly affects both your cooling needs and comfort. An AC sized for dry climates may struggle in humid regions.
Understanding Humidity and Cooling
Humidity is the amount of water vapor in air. It's measured as:
- Absolute Humidity: Actual water content (grams per cubic meter)
- Relative Humidity (RH): Water vapor as percentage of maximum possible (most common measurement)
Example: 50% RH means air holds 50% of the maximum moisture it could hold at that temperature.
How Humidity Affects Cooling Load
Humidity increases cooling load in two ways:
1. Latent Heat (Moisture Removal)
AC must remove moisture from air, not just cool it:
- Cooling air (sensible heat): Straightforward temperature reduction
- Removing moisture (latent heat): Requires additional energy
- In humid climates, latent cooling can be 20-40% of total load
2. Heat Index Effect
High humidity makes room feel hotter:
- 95°F with 30% humidity feels like 95°F
- 95°F with 60% humidity feels like 110-115°F
- People need more aggressive cooling in humid climates
Cooling Load Adjustments for Humidity
Adjust basic BTU calculation by climate humidity:
| Climate | Humidity Level | Adjustment |
|---|---|---|
| Dry (Desert) | 20-30% RH | -10% |
| Normal | 40-50% RH | No change |
| Humid | 60-70% RH | +10-15% |
| Very Humid | 75-85% RH | +20-30% |
Example: Same Room, Different Climates
300 sq ft room, basic load: 7,500 BTU
- Dry climate (Arizona): 7,500 × 0.90 = 6,750 BTU (0.56 ton)
- Moderate climate (Normal): 7,500 BTU (0.625 ton)
- Humid climate (Florida): 7,500 × 1.15 = 8,625 BTU (0.72 ton)
- Very humid climate (Coastal): 7,500 × 1.25 = 9,375 BTU (0.78 ton)
The same room needs 30% more cooling capacity in humid climates!
Why AC Struggles in Humidity
- AC must cool AND dehumidify
- Removes 1-2 gallons water/day from air
- This moisture removal requires energy
- In severe humidity, latent load exceeds sensible load
Comfort and Humidity
Ideal indoor humidity: 40-55% RH
Effects of High Humidity
- Room feels warmer (heat index effect)
- Mold and mildew growth risk
- Dust mites proliferate
- Musty odors
- Health issues (allergies, asthma)
Effects of Low Humidity (Below 30% RH)
- Dry skin and respiratory irritation
- Static electricity issues
- Wood furniture cracking
- Less comfortable despite lower temperature
Humidity Control Strategies
AC Alone (Limited)
- AC does remove moisture, but only as side effect
- Low-speed running helps, but reduces cooling
- Not ideal for dehumidification
Improved AC Operation
- Use lower cooling speed (more dehumidification)
- Set temperature 1-2°C higher than normal
- Longer run time = more moisture removal
- Inverter ACs better at humidity control
Supplemental Dehumidifier
- Removes moisture without heavy cooling
- Useful in humid but not hot climates
- Costs $100-300
- Runs continuously for best results
Moisture Reduction
- Shower with bathroom fan on
- Exhaust cooking steam outside
- Keep basement/crawlspace dry
- Reduce wet laundry drying indoors
- Ensure proper ventilation
Best AC Types for Humid Climates
- Inverter ACs: Better humidity control via variable speed
- Larger capacity AC: Runs longer, removes more moisture
- Dual-hose dehumidification: Some split ACs have better moisture removal
Conclusion
Humidity significantly impacts AC sizing and performance. In humid climates, add 10-30% to your cooling load. Use our humidity-adjusted calculator or consult an HVAC professional for accurate sizing in humid regions.