HVAC - Thermal Comfort Notes

HVAC Introduction

• HVAC controls temperature, humidity, air quality, and air movement.
• Improper HVAC sizing leads to undesirable conditions, excess energy use, and high costs.
• HVAC system design includes load calculations, system selection, duct/pipe sizing, and equipment layout.
• Main goal: acceptable Indoor Environmental Quality (IEQ) including comfort, wellbeing, health, indoor air quality, thermal comfort, acoustics, and lighting.

Thermal Comfort

• Defined in ASHRAE Standard 55: satisfaction with the surrounding environment.
• Goal: achieve thermal comfort for 80% of occupants, assuming typical clothing for activity level.
• Engineers control temperature, humidity, and air speed; they analyze metabolic rate (met) and clothing insulation (clo).

Metabolic Rates

• People have different metabolic rates based on activity level ($1 \text{ met} = 18.4 \text{ Btu/h*ft}^2$).
• Heat generated (Btu/hr) is calculated by body area (AD) * met.
• $Btu/hr = AD * met$
• Example: For a 5’-8” male weighing 154 lb seated at rest:
  • $AD = 19.6 ft^2$
  • $Met = 1.0$
  • $Btu/hr = 19.6 ft^2 * 18.4 Btu/hr*ft^2 = 360 Btu/hr$

Clothing Insulation

• Clothing affects perception of thermal conditions.
• Engineers estimate clothing insulation level (clo) for space setpoints.
• $1 clo = 0.88 ft^2<em>h</em>F/Btu$

Comfort Zones

• Occupants have a common range of acceptable temperature and humidity.
• ASHRAE Standard 55 provides recommended ranges.
• Decrease temperature by $1^{\circ}F$ for each 0.1 clo increase.
• Decrease temperature by $2.5^{\circ}F$ for each 1.0 met above 1.2 met.

Temperature

• Human tolerance to humidity is higher than to temperature.
• Temperature control is the primary focus.

Humidity

• Relative humidity affects human comfort; maintain between 35-60%.
• Slightly higher in summer, slightly lower in winter.
• Around 50%, people feel better with fewer problems.
• Over 60%, space feels “stuffy” promoting bacteria/fungal growth.
• Below 30%, causes respiratory problems & static electricity.

Air Speed

• General recommendation: 10 – 50 feet/minute.
• 50 fpm in cooling, 30 fpm in heating.
• Diffuser/register selections based on 50 fpm velocity in the occupied zone, 6ft above the floor, 1ft from interior wall, 2.2ft from exterior wall.

Right Temperature

• $78^{\circ}F$: Optimal for bathing, disturbs sleep.
• $75^{\circ}F$: Warm, lethargic, sleepy, optimal for limited clothing.
• $72^{\circ}F$: Most comfortable year-round for sedentary people.
• $70^{\circ}F$: Optimal for mental work.
• $64^{\circ}F$: Physically inactive people shiver, active people are comfortable.

Ideal Conditions

• Temperature: $72^{\circ}F$
• Humidity: 50%
• Air Movement: < 30 feet / minute