ASHRAE 55: Complete Guide to Thermal Comfort Standards

ASHRAE Standard 55 specifies the combinations of indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to a majority of the occupants within the space. This standard is fundamental to HVAC design, building operation, and occupant satisfaction. Understanding ASHRAE 55 is essential for creating comfortable indoor environments, optimizing energy use, and ensuring occupant productivity and well-being.

The standard addresses thermal comfort through the PMV-PPD model, adaptive comfort approach, operative temperature, humidity limits, air movement effects, and local thermal discomfort. It provides both prescriptive requirements and performance-based methods for achieving acceptable thermal conditions. This comprehensive guide covers the thermal comfort models, calculation methods, acceptable ranges, design guidelines, and practical application examples.

Introduction to ASHRAE 55

Purpose and Scope

ASHRAE Standard 55 serves multiple critical functions:

Comfort Definition:

• Defines acceptable thermal conditions
• Establishes comfort criteria
• Provides measurement methods
• Enables comfort assessment

Design Guidance:

• HVAC system design criteria
• Temperature setpoint selection
• Humidity control requirements
• Air movement design

Performance Evaluation:

• Building performance assessment
• Occupant satisfaction evaluation
• System operation optimization
• Energy efficiency balance

Regulatory Compliance:

• Building code requirements
• Green building standards
• Workplace regulations
• Health and safety standards

Scope of Application

ASHRAE 55 applies to:

Occupied Spaces:

• Office buildings
• Residential buildings
• Educational facilities
• Healthcare facilities
• Retail spaces
• All occupied indoor spaces

Environmental Conditions:

• Air temperature
• Radiant temperature
• Humidity
• Air speed
• Personal factors
• Clothing and activity

Thermal Comfort Models

PMV-PPD Model

Predicted Mean Vote (PMV):

PMV predicts the mean thermal sensation vote of a large group of people:

Where:

• $M$ = Metabolic rate (W/m²)
• $L$ = Thermal load on the body (W/m²)

Thermal Load Calculation:

Where:

• $W$ = External work (W/m²), typically 0
• $f_{cl}$ = Clothing area factor
• $t_{cl}$ = Clothing surface temperature (°C)
• $\bar{t_r}$ = Mean radiant temperature (°C)
• $h_c$ = Convective heat transfer coefficient (W/m²·K)
• $t_a$ = Air temperature (°C)
• $p_a$ = Partial pressure of water vapor (kPa)

Predicted Percentage Dissatisfied (PPD):

PPD predicts the percentage of people who will be dissatisfied with the thermal environment:

PMV Scale:

Acceptable PMV Range:

• PMV: -0.5 to +0.5
• Corresponding PPD: ≤ 10%

Adaptive Comfort Model

Applicability:

• Naturally ventilated buildings
• Occupant-controlled environments
• Buildings without mechanical cooling
• Operable windows present

Acceptable Operative Temperature:

Where:

• $t_{op,accept}$ = Acceptable operative temperature (°C)
• $t_{rm}$ = Running mean outdoor temperature (°C)

Running Mean Temperature:

Where:

• $\alpha$ = Weighting factor (typically 0.8)
• $t_{rm,prev}$ = Previous running mean (°C)
• $t_{od,prev}$ = Previous day's mean outdoor temperature (°C)

Acceptable Range:

• 80% acceptability: ±2.5°C from $t_{op,accept}$
• 90% acceptability: ±1.5°C from $t_{op,accept}$

Environmental Parameters

Operative Temperature

Definition: The uniform temperature of an imaginary black enclosure in which an occupant would exchange the same amount of heat by radiation and convection as in the actual non-uniform environment.

Calculation:

Where:

• $h_r$ = Radiative heat transfer coefficient (W/m²·K)
• $h_c$ = Convective heat transfer coefficient (W/m²·K)

Simplified (for typical conditions):

Typical Values:

• $h_r$ ≈ 4.7 W/m²·K (for seated person)
• $h_c$ ≈ 3-5 W/m²·K (for low air speed)

Air Temperature

Acceptable Ranges:

Vertical Temperature Gradient:

• Head-to-ankle difference: ≤ 3°C (5.4°F)
• Measured at 0.1 m and 1.7 m above floor

Mean Radiant Temperature

Definition: The uniform temperature of an imaginary black enclosure in which an occupant would exchange the same amount of heat by radiation as in the actual non-uniform environment.

Calculation:

Where:

• $F_{p-i}$ = View factor from person to surface $i$
• $T_i$ = Absolute temperature of surface $i$ (K)

Simplified (for rectangular rooms):

Where $A_i$ and $t_i$ are area and temperature of surface $i$

Relative Humidity

Acceptable Ranges:

Dew Point Limits:

• Maximum dew point: 16°C (61°F) for typical applications
• Prevents condensation on surfaces
• Maintains comfort

Air Speed

Acceptable Air Speed:

Air Speed Correction:

For elevated air speeds, operative temperature can be adjusted:

Where:

• $t_{op,adj}$ = Adjusted operative temperature (°C)
• $v$ = Air speed (m/s)
• Valid for $v$ = 0.15-0.8 m/s

Personal Factors

Metabolic Rate

Metabolic Rate Values:

Metabolic Rate Calculation:

Where:

• $M_{activity}$ = Metabolic rate per unit area (W/m²)
• $A_{DuBois}$ = DuBois body surface area (m²)

Where:

• $W$ = Body weight (kg)
• $H$ = Body height (m)

Clothing Insulation

Clothing Insulation Values (clo):

Clothing Insulation Calculation:

Where $I_{cl,i}$ = Insulation of clothing item $i$

Typical Values:

• 1 clo = 0.155 m²·K/W
• Typical office: 0.5-1.0 clo (summer-winter)

Acceptable Thermal Conditions

PMV-PPD Acceptable Range

Standard Conditions:

• PMV: -0.5 to +0.5
• PPD: ≤ 10%
• Operative temperature: 20-26°C (68-79°F)
• Relative humidity: 30-60%
• Air speed: ≤ 0.15 m/s (30 fpm)

Extended Conditions:

• PMV: -0.7 to +0.7
• PPD: ≤ 15%
• Requires enhanced controls
• May require occupant adaptation

Adaptive Comfort Acceptable Range

80% Acceptability:

• Operative temperature: $t_{op,accept} \pm 2.5°C$
• Relative humidity: 30-80%
• Air speed: ≤ 0.8 m/s (160 fpm)

90% Acceptability:

• Operative temperature: $t_{op,accept} \pm 1.5°C$
• Relative humidity: 30-80%
• Air speed: ≤ 0.8 m/s (160 fpm)

Local Thermal Discomfort

Draft

Draft Risk:

Where:

• $DR$ = Draft risk (%)
• $t_a$ = Local air temperature (°C)
• $v$ = Local air speed (m/s)
• $Tu$ = Turbulence intensity (%)

Acceptable Draft Risk:

• DR ≤ 20% for 80% acceptability
• DR ≤ 15% for 90% acceptability

Vertical Temperature Difference

Acceptable Limits:

• Head-to-ankle difference: ≤ 3°C (5.4°F)
• Measured at 0.1 m and 1.7 m above floor

Discomfort Risk:

• Increases with larger temperature differences
• Affects comfort perception

Radiant Temperature Asymmetry

Acceptable Limits:

Floor Temperature

Acceptable Limits:

Design Guidelines

Temperature Setpoints

Recommended Setpoints:

Energy Optimization:

• Wider deadband: 2-3°C (3.6-5.4°F)
• Night setback: 2-3°C lower
• Occupancy-based control
• Adaptive setpoints

Humidity Control

Humidification:

• Minimum: 30% RH
• Prevents dry skin, eyes
• Reduces static electricity
• Typical setpoint: 35-40% RH

Dehumidification:

• Maximum: 60% RH
• Prevents mold growth
• Maintains comfort
• Typical setpoint: 50-55% RH

Air Movement

Design Air Speed:

• Typical: 0.15 m/s (30 fpm)
• Maximum: 0.25 m/s (50 fpm) for sedentary
• Can increase to 0.8 m/s (160 fpm) with elevated temperature

Air Distribution:

• Avoid direct drafts on occupants
• Uniform distribution
• Proper diffuser selection
• Consider local control

Practical Application Examples

Example 1: Office Building

Conditions:

• Office space, sedentary activity
• Metabolic rate: 1.0 met (58 W/m²)
• Clothing: 0.5 clo (summer)
• Air temperature: 24°C
• Mean radiant temperature: 24°C
• Relative humidity: 50%
• Air speed: 0.15 m/s

PMV Calculation:

• Operative temperature: 24°C
• PMV ≈ 0 (neutral)
• PPD ≈ 5%
• Acceptable

Example 2: Retail Space

Conditions:

• Retail space, light activity
• Metabolic rate: 1.2 met (70 W/m²)
• Clothing: 0.5 clo (summer)
• Air temperature: 23°C
• Mean radiant temperature: 25°C (solar gain)
• Relative humidity: 55%
• Air speed: 0.20 m/s

PMV Calculation:

• Operative temperature: 24°C
• PMV ≈ +0.2 (slightly warm)
• PPD ≈ 6%
• Acceptable

Example 3: Naturally Ventilated Building

Conditions:

• Naturally ventilated office
• Running mean outdoor temperature: 22°C
• Operative temperature: 24°C
• Relative humidity: 60%
• Air speed: 0.30 m/s

Adaptive Comfort:

• Acceptable operative temperature: 24.6°C
• Range (80%): 22.1-27.1°C
• Current: 24°C
• Acceptable

Best Practices

Design Practices

System Design:

• Provide temperature control
• Maintain humidity in range
• Avoid drafts
• Minimize temperature gradients
• Consider local control

Energy Optimization:

• Use wider deadbands
• Implement night setback
• Occupancy-based control
• Adaptive setpoints
• Consider adaptive comfort

Operation Practices

Setpoint Management:

• Follow recommended ranges
• Adjust for season
• Consider occupancy
• Monitor comfort complaints
• Optimize for energy

Monitoring:

• Track temperature
• Monitor humidity
• Measure air speed
• Assess occupant satisfaction
• Adjust as needed

Conclusion

ASHRAE Standard 55 provides essential guidance for achieving acceptable thermal comfort conditions. Key aspects include:

Comfort Models:

• PMV-PPD model
• Adaptive comfort model
• Local thermal discomfort

Environmental Parameters:

• Operative temperature
• Humidity
• Air speed
• Personal factors

Acceptable Conditions:

• Standard ranges
• Extended ranges
• Application-specific

Design and Operation:

• Setpoint selection
• System design
• Optimization

By understanding and applying ASHRAE 55, engineers and facility managers can create comfortable indoor environments, optimize energy use, and ensure occupant satisfaction and productivity.