Humidity Control & AC Sizing Guide

The Hidden Half of Cooling Load

Your AC has two jobs: remove heat (sensible load) and remove moisture (latent load). Most homeowners—and many contractors—only calculate sensible load, leading to cold, clammy homes that feel uncomfortable even when thermostats show 72°F. In humid climates, latent load can be 30-50% of total cooling needs.

Our BTU calculator includes humidity factors, but understanding latent load helps you choose the right equipment type and avoid oversizing pitfalls.

Understanding Latent Load

What is Latent Load?

Latent load is the energy needed to remove moisture from air. When humid outdoor air infiltrates your home (through doors, windows, cracks, or ventilation), your AC must condense that moisture on the evaporator coil and drain it away.

Sensible load is the energy needed to lower air temperature—what most people think of as "cooling."

Why Latent Load Matters

• Comfort: Humans feel cooler at lower humidity. 78°F at 45% humidity feels as comfortable as 72°F at 65% humidity.
• Health: High humidity (>60%) promotes mold, mildew, and dust mites.
• Efficiency: Removing moisture requires longer AC runtimes—short-cycling systems can't dehumidify effectively.

Latent Load by Climate Zone

The Oversizing Problem in Humid Climates

Oversized AC systems are disastrous for humidity control. Here's why:

Short-Cycling Prevents Dehumidification

An AC removes moisture when air contacts the cold evaporator coil long enough for water vapor to condense. This takes time—typically 10-15 minutes of continuous runtime.

Oversized systems cool the air too quickly, satisfying the thermostat in 5-8 minutes. The compressor shuts off before the coil gets cold enough or runs long enough to condense significant moisture. Result: cool but muggy air (70°F, 65% humidity).

Case Study: Tampa Home

• Actual load: 24,000 BTU (60% sensible, 40% latent)
• Installed system: 36,000 BTU (3 tons) - 50% oversized
• Problem: System cools to 72°F in 6 minutes, shuts off with indoor humidity at 68%
• Symptoms: Clammy feeling, musty smells, mold on bathroom ceilings
• Solution: Replaced with 24,000 BTU (2-ton) two-stage system → 15-20 min runtimes → humidity drops to 48%

Calculate your proper capacity with our BTU calculator to avoid this scenario.

Equipment Solutions for Humidity Control

1. Two-Stage or Variable-Speed Compressors (Best)

How it works: Compressor runs at low speed (50-60% capacity) most of the time, ramping to high speed only on hottest days.

Benefits:

• Longer runtimes at low speed = better dehumidification
• Quieter operation
• Better efficiency (20-30% lower energy use)
• Fewer temperature swings

Cost: $1,500-3,000 more than single-stage

Payback: 6-10 years in energy savings, but comfort improvement is immediate

Best for: Zones 1-3 (humid climates), homes with high latent loads

2. Oversized Evaporator Coil + Smaller Compressor

How it works: Larger coil surface area stays colder longer, condensing more moisture per cycle.

Benefits:

• Better moisture removal with single-stage equipment
• Lower coil temperatures = more condensation

Cost: $300-800 more than matched coil

Tradeoff: Slightly lower efficiency (coil creates more airflow resistance)

3. Standalone Dehumidifier

How it works: Separate appliance removes moisture without cooling; AC handles sensible load only.

Benefits:

• Decouples cooling and dehumidification
• Can run independently in spring/fall (mild but humid)
• Whole-house models integrate with HVAC (90-150 pints/day capacity)

Cost:

• Portable (30-50 pints/day): $200-400
• Whole-house (90-150 pints/day): $1,500-2,500 installed

Operating cost: $150-300/year electricity for whole-house unit

Best for: High-humidity climates (Zones 1-2) or homes with persistent moisture issues (basements, coastal properties)

4. Enhanced Cycling Mode (Budget Option)

How it works: Smart thermostat runs fan for 10-15 minutes after compressor shuts off, draining condensate from coil.

Benefits:

• Free (just thermostat setting)
• Extracts residual moisture from coil

Tradeoff: Adds back ~10% of sensible cooling (evaporating moisture re-warms air slightly), so modest improvement

Sizing for Humidity Control

Design Day Conditions

Standard load calculations (like Manual J) use 99% summer design temps—the hottest 1% of hours per year. Example: Atlanta's 99% temp is 92°F.

Problem: Peak sensible load occurs at 92°F, but peak latent load occurs at 80-85°F with 80% humidity (after rainstorms). Your AC might be sized perfectly for 92°F but undersized for dehumidification at 82°F.

Solution: Don't Oversize

Resist the temptation to add 20-30% "safety factor" for hot days. An AC sized for 99% design conditions will run longer cycles on typical 85-88°F days, giving better dehumidification. The 1% of hours above 99% temp? Home might reach 75-76°F instead of 72°F—acceptable discomfort for better year-round moisture control.

Use our BTU calculator without adding extra capacity. Trust the numbers.

Special Case: Tight Homes

Energy-efficient homes with good air sealing have lower latent loads (less outdoor air infiltration). These homes may need slight downsizing (5-10%) to avoid short-cycling while still hitting target temperatures.

If you've done air sealing or spray foam insulation, note this in the BTU calculator inputs and compare results before/after insulation upgrades using our insulation impact guide.

Thermostat Settings for Humidity

1. Lower Fan Speed = Better Dehumidification

Most systems offer fan speed settings (low/medium/high). Lower speed = less airflow = colder coil = more condensation. Trade-off: slightly warmer supply air, but drier overall.

Recommendation: Use medium or low speed in humid climates, high speed in dry climates.

2. "Auto" Fan Mode (Not "On")

Running fan continuously ("On" mode) re-evaporates moisture from the coil back into your home. Use "Auto" mode—fan runs only when compressor runs.

Exception: If your thermostat has enhanced dehumidification mode (runs fan after compressor stops to drain coil), enable this feature.

3. Set Humidity Targets (If Available)

Some smart thermostats (Honeywell, Ecobee, Nest) allow humidity targets. Set to 45-50% for comfort, 50-55% for efficiency. Thermostat will extend cooling cycles to reach humidity target even if temperature target is already met.

Calculating Dehumidification Capacity

Typical AC Moisture Removal Rates:

• 1 ton (12,000 BTU): Removes ~1.5-2 pints/hour at optimal conditions
• 2 tons (24,000 BTU): ~3-4 pints/hour
• 3 tons (36,000 BTU): ~4.5-6 pints/hour

Note: This assumes continuous runtime. Short-cycling cuts actual removal by 40-60%.

Estimating Your Dehumidification Need:

A typical 1,800 sq ft home in Zone 2 (hot-humid) generates ~40-60 pints/day of moisture from:

• Infiltration: 20-30 pints/day
• Occupants: 10-15 pints/day (breathing, cooking, showers)
• Indoor sources: 5-10 pints/day (plants, pets, laundry)

A properly sized 2-ton AC running 8-10 hours/day removes 24-40 pints—matching load. An oversized 3-ton unit running 4-5 hours/day removes only 18-30 pints—falling short despite higher capacity.

When to Add Supplemental Dehumidification

Scenarios Requiring Standalone Dehumidifier:

1. Coastal homes (within 5 miles of ocean): Constant high humidity from sea breeze
2. Basements: Below-grade spaces always have moisture infiltration
3. Spring/fall humidity: 70°F days with 75% humidity—too cool to run AC, but need moisture removal
4. Existing oversized AC: Can't justify replacing working equipment, but need better humidity control
5. Indoor pool or hot tub: Generates massive latent load (100-200 pints/day)

Calculate total moisture load to determine if standalone dehumidification is needed alongside your AC.

Humid Climate Best Practices