10.3 Air Conditioning

10.3 Air Conditioning

Now that we understand how humidity affects comfort and how windows influence heat gain, we turn our attention to the systems designed to manage both: air conditioning.

Air conditioning is about more than just lowering temperature. A well-designed system simultaneously:

  • Cools the air (removes sensible heat)
  • Dehumidifies the air (removes latent heat)
  • Filters and circulates air for indoor air quality
  • Maintains relative humidity in the comfort zone (~40–60%)

Why Cooling Also Means Dehumidifying

When warm, humid indoor air passes over the cold evaporator coil inside an air conditioner, two things happen simultaneously:

  1. Sensible cooling: The air temperature drops as heat energy transfers to the refrigerant.
  2. Latent cooling: As the air cools, its capacity to hold moisture decreases. When the air reaches its dew point, water vapor condenses on the coil and drains away.

Key Insight

Cooling air always increases its relative humidity if moisture isn't removed. That's why air conditioners must condense and drain water—to actually lower humidity, not just temperature.

Real-World Observation

Have you ever noticed water dripping from the outdoor unit of a window AC or from a condensate drain line? That's the moisture removed from your indoor air. On a humid day, a typical residential AC can remove 10–30 gallons of water per day from your home.
Diagram of a refrigerant system described in the previous paragraph.
Refrigerant System
Credit: © Penn State is licensed under CC BY-NC-SA 4.0

The Comfort Zone: Targeting ~50% Relative Humidity

Human comfort depends on both temperature and humidity. Research shows most people feel most comfortable when:

Temperature: 72–78°F (22–26°C)

Relative Humidity: 40–60% (ideally ~50%)

Relative Humidity, Comfort, and Energy
Humidity LevelComfort ImpactEnergy Impact
< 30% RHDry skin, irritated respiratory passagesMay require humidification (adds energy)
40–60% RHOptimal comfort, effective evaporative coolingEfficient AC operation
> 60% RH"Sticky" feeling, reduced sweat evaporation, mold riskAC works harder; may need dedicated dehumidifier

Why 50%? At this level, your body's natural cooling mechanism—sweat evaporation—works efficiently. Higher humidity slows evaporation, making you feel warmer even at the same temperature. This is why a humid 82°F day can feel like 90°F.

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