WELL Building Standard for HVAC Systems

The WELL Building Standard represents a performance-based system measuring, certifying, and monitoring features of the built environment that impact human health and well-being. For HVAC professionals, WELL certification demands rigorous attention to air quality, thermal comfort, moisture management, and ventilation effectiveness—requirements that often exceed conventional building codes.

WELL Air Concept Requirements

The Air concept forms the foundation of WELL’s HVAC-related requirements, establishing stringent thresholds for indoor air quality parameters.

Ventilation Design Parameters

WELL mandates minimum outdoor air ventilation rates based on ASHRAE 62.1 with enhanced requirements for specific space types:

Minimum ventilation effectiveness:

$$\varepsilon_v = \frac{C_e - C_s}{C_r - C_s} \geq 0.9$$

Where:

• $\varepsilon_v$ = ventilation effectiveness (dimensionless)
• $C_e$ = exhaust air contaminant concentration (ppm)
• $C_s$ = supply air contaminant concentration (ppm)
• $C_r$ = room average contaminant concentration (ppm)

WELL Feature A01 requires minimum outdoor air rates of:

Air Filtration Standards

WELL Feature A02 establishes minimum MERV ratings significantly higher than conventional practice:

Particle removal efficiency:

$$\eta_p = 1 - \frac{C_{down}}{C_{up}}$$

Where:

• $\eta_p$ = particle removal efficiency (fraction)
• $C_{down}$ = downstream particle concentration (particles/m³)
• $C_{up}$ = upstream particle concentration (particles/m³)

The pressure drop across high-efficiency filters requires careful fan selection:

$$\Delta P_f = K \cdot v^2 \cdot \left(1 + \alpha \cdot m_d\right)$$

Where:

• $\Delta P_f$ = filter pressure drop (Pa)
• $K$ = clean filter coefficient (Pa·s²/m²)
• $v$ = face velocity (m/s)
• $\alpha$ = dust loading coefficient (m²/g)
• $m_d$ = dust mass per unit area (g/m²)

Thermal Comfort Compliance

WELL Feature T01 requires thermal conditions meeting ASHRAE Standard 55 with enhanced monitoring and control capabilities.

Thermal Comfort Calculation

Predicted Mean Vote (PMV) must remain within:

$$-0.5 \leq PMV \leq +0.5$$

The PMV calculation incorporates six factors:

$$PMV = f(M, W, I_{cl}, t_a, \bar{t_r}, v_{ar}, p_a)$$

Where:

• $M$ = metabolic rate (met)
• $W$ = external work (typically 0 for office work)
• $I_{cl}$ = clothing insulation (clo)
• $t_a$ = air temperature (°C)
• $\bar{t_r}$ = mean radiant temperature (°C)
• $v_{ar}$ = relative air velocity (m/s)
• $p_a$ = water vapor partial pressure (Pa)

graph TD
    A[WELL Thermal Comfort Requirements] --> B[Individual Control]
    A --> C[Operable Windows]
    A --> D[Thermal Zoning]

    B --> E[Thermostat Access]
    B --> F[Personal Comfort Systems]

    C --> G[Natural Ventilation Option]
    C --> H[Mixed-Mode Operation]

    D --> I[≤ 600 ft² per zone]
    D --> J[Occupancy-Based Control]

    E --> K[User Override Capability]
    F --> L[Task Fans/Heaters]

    style A fill:#2c5282,color:#fff
    style B fill:#4299e1,color:#fff
    style C fill:#4299e1,color:#fff
    style D fill:#4299e1,color:#fff

Moisture Management Requirements

WELL Feature W02 mandates humidity control to prevent mold growth and maintain occupant comfort:

Relative humidity bounds:

$$30% \leq RH \leq 60%$$

The moisture removal rate required for dehumidification:

$$\dot{m}w = \rho \cdot \dot{V} \cdot (W{in} - W_{out})$$

Where:

• $\dot{m}_w$ = moisture removal rate (kg/s)
• $\rho$ = air density (kg/m³)
• $\dot{V}$ = volumetric flow rate (m³/s)
• $W_{in}$ = inlet humidity ratio (kg water/kg dry air)
• $W_{out}$ = outlet humidity ratio (kg water/kg dry air)

WELL HVAC System Integration

flowchart LR
    A[Outdoor Air] --> B[MERV 13+ Filtration]
    B --> C[Heat Recovery<br/>ERV/HRV]
    C --> D[Conditioning Coils]
    D --> E[MERV 8+ Final Filter]
    E --> F[VAV Distribution]

    F --> G[Zone 1<br/>T Control]
    F --> H[Zone 2<br/>T Control]
    F --> I[Zone 3<br/>T Control]

    G --> J[Return Air]
    H --> J
    I --> J

    J --> K{Air Quality<br/>Monitoring}
    K -->|CO₂ > 800 ppm| L[Increase OA]
    K -->|PM2.5 > 15 μg/m³| M[Activate Filtration]
    K -->|Within Limits| C

    L --> C
    M --> C

    style B fill:#48bb78,color:#fff
    style E fill:#48bb78,color:#fff
    style K fill:#ed8936,color:#fff

Air Quality Monitoring Requirements

WELL Feature A07 requires continuous monitoring of:

The sensor placement density requirement:

$$n_{sensors} = \frac{A_{floor}}{f_{coverage}} \geq n_{min}$$

Where:

• $n_{sensors}$ = number of sensors required
• $A_{floor}$ = floor area (ft²)
• $f_{coverage}$ = coverage factor (7,500 ft²/sensor for PM, 2,500 ft²/sensor for CO₂)
• $n_{min}$ = minimum sensors per floor (typically 1)

Energy Recovery Requirements

WELL Feature A04 mandates energy recovery ventilation for climate zones with significant heating or cooling loads:

Enthalpy recovery effectiveness:

$$\varepsilon_h = \frac{h_{supply} - h_{outdoor}}{h_{exhaust} - h_{outdoor}}$$

Where:

• $\varepsilon_h$ = enthalpy effectiveness (fraction)
• $h_{supply}$ = supply air enthalpy after recovery (kJ/kg)
• $h_{outdoor}$ = outdoor air enthalpy (kJ/kg)
• $h_{exhaust}$ = exhaust air enthalpy (kJ/kg)

WELL requires minimum effectiveness of 60% sensible and 50% latent for Energy Recovery Ventilators (ERV).

Sound and Vibration Control

WELL Feature S02 establishes maximum background noise levels for HVAC systems:

The sound power attenuation through ductwork:

$$L_{out} = L_{in} - \alpha \cdot L - 10 \log_{10}\left(\frac{A_{out}}{A_{in}}\right)$$

Where:

• $L_{out}$ = outlet sound pressure level (dB)
• $L_{in}$ = inlet sound pressure level (dB)
• $\alpha$ = attenuation coefficient (dB/m)
• $L$ = duct length (m)
• $A_{out}, A_{in}$ = outlet and inlet duct areas (m²)

Implementation Strategies

Achieving WELL certification requires integrated design approaches:

1. Oversized filtration sections to accommodate MERV 13+ filters while maintaining acceptable pressure drops below 0.8 in. w.g. at design flow
2. Dedicated outdoor air systems (DOAS) to decouple ventilation from thermal loads, enabling superior humidity control
3. Demand-controlled ventilation with CO₂ sensors maintaining levels below 800 ppm above outdoor concentrations
4. Individual thermal comfort control providing user adjustment within ±3°F of setpoint
5. Low-velocity distribution limiting terminal velocities to 500 FPM to minimize noise generation

The total system static pressure increase from enhanced filtration:

$$\Delta P_{total} = \Delta P_{MERV8} + \Delta P_{MERV13-MERV8}$$

Typically adds 0.3-0.5 in. w.g., requiring fan selection with adequate pressure capability and motor efficiency to maintain WELL Feature A09 demand ventilation requirements.

WELL Building Standard transforms HVAC design from code-minimum compliance to health-optimized performance, demanding sophisticated engineering analysis, continuous monitoring, and integrated building systems that prioritize occupant well-being alongside energy efficiency.