Moisture Control

Moisture control represents a critical function of HVAC systems, directly affecting occupant comfort, building envelope integrity, and indoor air quality. Proper moisture management prevents material degradation, controls biological growth, and maintains conditions within acceptable psychrometric ranges.

Fundamentals of Moisture Control

Psychrometric Relationships

Moisture exists in air as water vapor, quantified through multiple psychrometric properties:

Relative Humidity (RH): The ratio of actual water vapor pressure to saturation vapor pressure at a given temperature, expressed as a percentage. RH varies inversely with temperature when absolute moisture content remains constant.

Humidity Ratio (W): The mass of water vapor per unit mass of dry air, expressed in lb_water/lb_dry air or kg_water/kg_dry air. This absolute measure remains constant during sensible heating or cooling.

Dew Point Temperature: The temperature at which air becomes saturated with water vapor at constant pressure and humidity ratio. Surfaces below the dew point experience condensation.

Wet Bulb Temperature: The temperature indicated by a thermometer with a moistened wick exposed to moving air, representing the lowest temperature achievable through evaporative cooling at current atmospheric conditions.

Target Humidity Ranges

ASHRAE Standard 55 specifies acceptable humidity conditions:

Lower RH limits prevent static electricity, material desiccation, and respiratory discomfort. Upper limits control mold growth, condensation, and corrosion.

Humidity Control Strategies

Relative Humidity Management

Maintaining target RH requires coordinated control of temperature and absolute moisture content:

Proportional Control: Modulating equipment capacity based on deviation from setpoint provides stable control but allows persistent offset. A typical gain of 10-20% capacity change per 1% RH deviation offers responsive control without cycling.

Proportional-Integral Control: Adding integral action eliminates steady-state error but requires careful tuning to prevent oscillation. Reset time of 5-15 minutes works for most applications.

Humidity Ratio Control: Controlling absolute moisture content (W) eliminates the interdependence between temperature and RH, providing superior performance in applications with variable sensible loads.

Condensation Prevention

Condensation occurs when surface temperatures fall below the air dew point. Prevention requires either raising surface temperature or lowering dew point:

Surface Temperature Management:

• Insulation to maintain surfaces above dew point
• Radiant heating of susceptible surfaces
• Air circulation to prevent stratification
• Double-pane glazing with low-e coatings

Dew Point Reduction:

• Dehumidification to lower moisture content
• Ventilation with outdoor air (when beneficial)
• Vapor barriers to prevent moisture migration
• Source control of moisture generation

Critical surface temperature for condensation prevention:

T_surface > T_dewpoint + 2°F (safety margin)

Dehumidification Methods

Refrigerant-Based Dehumidification

Cooling air below its dew point condenses water vapor, reducing absolute moisture content:

Standard Cooling Coils: Air passes over coils at 45-55°F, condensing moisture. Sensible heat ratio (SHR) of 0.60-0.80 typical for comfort cooling. Lower SHR indicates greater latent capacity.

Deep Cooling with Reheat: Coils operate at 38-45°F for enhanced moisture removal, followed by reheat to maintain desired supply temperature. Increases latent capacity but consumes additional energy.

Apparatus Dew Point (ADP): The effective surface temperature of the cooling coil, determining moisture removal capability. Lower ADP increases dehumidification.

Moisture removal rate (lb/hr) = 4.5 × CFM × (W_entering - W_leaving)

Where W is humidity ratio in lb_water/lb_dry air

Bypass Factor: Fraction of air bypassing direct coil contact, typically 0.05-0.30 for finned coils. Lower bypass factor improves dehumidification effectiveness.

Desiccant Dehumidification

Solid or liquid desiccants absorb water vapor through chemical affinity or physical adsorption:

Solid Desiccant Systems: Rotary wheels impregnated with silica gel, molecular sieve, or activated alumina rotate between process and regeneration airstreams. Process air releases moisture to the desiccant; regeneration air (heated to 150-250°F) drives moisture from the wheel.

Advantages:

• Independent temperature and humidity control
• Effective at low temperatures where coils ice
• Deep dehumidification to 20-30% RH possible
• No condensate disposal required

Limitations:

• Regeneration energy requirement
• Cross-contamination between airstreams
• Higher first cost than refrigerant systems

Liquid Desiccant Systems: Lithium chloride or calcium chloride solutions absorb moisture from air. Solution concentration determines equilibrium humidity.

Typical performance: 15-40 gr/lb moisture removal at 95°F entering air

Heat Pipe Dehumidification

Passive heat pipes precool entering air, increasing coil latent capacity, then reheat leaving air using recovered energy:

Operation: Refrigerant in sealed pipes evaporates in the upstream section (cooling entering air), condenses in downstream section (reheating leaving air). No external power required.

Performance: Reduces supply air temperature 3-8°F before the coil, increasing moisture removal 15-30% without additional energy consumption.

Applications: High latent load environments, hot-humid climates, pool facilities, laboratories.

Subcooling with Hot Gas Reheat

Refrigeration system extracts heat during dehumidification, then uses hot gas or condenser heat for reheat:

Configuration: Air cools to 45-50°F across evaporator, removing moisture, then reheats using refrigerant-to-air heat exchanger before discharge.

Efficiency: Energy Factor (EF) of 2.0-3.5 L/kWh typical, superior to electric resistance reheat.

Control: Modulating hot gas valve maintains discharge temperature while maximizing dehumidification.

Mold Prevention

Growth Requirements

Mold requires four conditions simultaneously:

1. Organic nutrients (building materials, dust, oils)
2. Appropriate temperature (40-100°F, optimum 77-86°F)
3. Oxygen (always present)
4. Moisture (RH > 60% at surface for extended periods)

Critical Moisture Threshold: RH at surface must exceed 60% for 24-48 hours to initiate growth. Extended exposure to 70-80% RH produces rapid colonization.

Surface vs. Air RH: Surface RH differs from ambient air RH due to temperature variation. Cold surfaces concentrate moisture, creating favorable microenvironments.

Prevention Strategies

Humidity Control: Maintain space RH below 60%, preferably 40-50%. Lower targets required for cooler surfaces or higher-risk materials.

Air Movement: Prevent stagnant conditions that allow localized moisture accumulation. Air velocity of 25-50 fpm across susceptible surfaces disrupts boundary layer.

Temperature Management: Maintain uniform temperatures to prevent condensation. Minimize temperature swings that drive moisture into materials.

Drainage: Ensure positive drainage from cooling coils, drain pans, and condensate lines. Standing water supports microbial amplification.

Filtration: MERV 8-13 filters remove mold spores from airstream, reducing colonization potential.

UV-C Irradiation: Germicidal UV lamps in AHUs or ductwork inactivate airborne spores and surface growth. Dosage of 100-200 μW·s/cm² effective for surface treatment.

Dew Point Management

Measurement and Control

Dew point control offers advantages over RH control:

• Independent of temperature variations
• Direct indication of condensation risk
• Simpler control logic in variable temperature environments

Measurement: Chilled mirror sensors (±0.5°F accuracy), capacitive sensors (±2-3°F accuracy), or calculated from dry bulb and wet bulb temperatures.

Control Strategy: Modulate dehumidification equipment to maintain dew point below target. Typical commercial building target: 50-55°F dew point.

Critical Applications

Building Envelope Protection: Maintain interior dew point below insulated wall cavity temperature to prevent interstitial condensation.

Cold Surface Protection: Ensure space dew point remains 5°F below coldest anticipated surface temperature (windows, thermal bridges, uninsulated pipes).

Process Applications: Many industrial processes specify absolute moisture limits expressed as dew point (electronics: -40°F, pharmaceuticals: 35-45°F).

Ventilation Strategies for Moisture Control

Outdoor Air Management

Outdoor air ventilation introduces or removes moisture depending on relative moisture content:

Enthalpy Comparison: Compare outdoor and return air enthalpy to determine moisture impact. Outdoor air with higher enthalpy adds latent load.

Economizer Lockout: Disable economizer when outdoor dew point exceeds indoor target by 2-3°F to prevent moisture introduction.

Energy Recovery: Employ energy recovery ventilators (ERV) with latent effectiveness of 50-70% to reduce outdoor air moisture impact. Reduces dehumidification load 40-60% in humid climates.

Exhaust Strategies

Source Capture: Exhaust moisture at generation points (kitchens, laundries, showers) before dispersion. Capture efficiency >90% requires hood placement and airflow design per ASHRAE guidelines.

Pressurization Control: Maintain slight positive pressure (0.02-0.05 in. w.c.) in occupied spaces relative to outdoors to prevent infiltration of humid outdoor air.

Displacement Ventilation: In high-ceiling spaces, low-velocity supply air near floor rises as heated, carrying moisture upward for ceiling-level exhaust.

Humidification Methods

Steam Humidification

Injects pure steam into airstream, providing rapid, hygienic moisture addition:

Types: Electrode boilers (15-100 lb/hr), gas-fired generators (50-500 lb/hr), steam-to-steam converters (using building steam).

Performance: Nearly 100% efficient energy conversion, precise control, minimal maintenance.

Distribution: Steam manifolds with multiple injection points ensure uniform dispersion. Absorption distance: 3-8 ft per manufacturer data.

Evaporative Humidification

Water evaporates from wetted media, absorbing latent heat and cooling air:

Media Types: Rigid cellulose pads, rotating drums, spray nozzles with eliminator plates.

Efficiency: 50-95% saturation efficiency depending on media contact time and surface area.

Limitations: Evaporative cooling (10-20°F typical) requires downstream heating in cold weather. Water quality critical to prevent mineral buildup.

Ultrasonic and Atomizing Humidifiers

Generate fine mist (<10 micron droplets) that evaporates rapidly:

Ultrasonic: Piezoelectric transducers vibrate at 1.6 MHz, creating fog. Low energy consumption (50-100 W per lb/hr).

Atomizing: Compressed air or high pressure (500-3000 psi) atomizes water. Requires reverse osmosis water to prevent white dust from minerals.

Applications: Clean rooms, laboratories, museums requiring precise control without thermal input.

Integration and Control

Coordinated HVAC Control

Moisture control integrates with overall HVAC strategy:

Cooling Priority: When space requires both cooling and dehumidification, cooling coil operates at lower temperature to enhance latent capacity, with reheat as needed.

Heating Priority: During heating season, humidification compensates for low outdoor absolute humidity.

Sequence of Operation: Typical priority: temperature control primary, humidity control secondary with deadband to prevent simultaneous heating/cooling.

Monitoring and Verification

Sensor Placement: Locate humidity sensors in representative locations, away from supply diffusers, exhaust grilles, and moisture sources. Multiple sensors average out spatial variation.

Calibration: Verify sensor accuracy quarterly using calibrated reference. Drift of 3-5% RH annually typical for capacitive sensors.

Trending: Monitor humidity, dew point, and outdoor conditions to identify patterns and optimize setpoints seasonally.

Effective moisture control requires understanding psychrometric principles, selecting appropriate equipment for climate and application, and implementing control strategies that balance energy efficiency with envelope protection and occupant comfort.