Setback Recovery in Assembly Spaces

Overview

Setback recovery in assembly spaces presents unique challenges due to high thermal mass, large volume-to-occupancy ratios, and intermittent use patterns. Proper recovery strategy design ensures occupant comfort at event start while minimizing energy consumption during unoccupied periods.

Recovery Time Calculation

The fundamental recovery time equation accounts for building thermal capacitance and available heating capacity:

$$t_{recovery} = \frac{C_{building} \cdot (T_{setpoint} - T_{setback})}{Q_{available} - Q_{losses}}$$

Where:

• $t_{recovery}$ = recovery time (hours)
• $C_{building}$ = effective building thermal capacitance (Btu/°F)
• $T_{setpoint}$ = occupied setpoint temperature (°F)
• $T_{setback}$ = setback temperature (°F)
• $Q_{available}$ = available heating capacity (Btu/hr)
• $Q_{losses}$ = envelope heat losses during recovery (Btu/hr)

Effective Thermal Capacitance

For assembly spaces, calculate effective thermal capacitance including structural mass:

$$C_{building} = \sum (m_i \cdot c_{p,i} \cdot f_i)$$

Where:

• $m_i$ = mass of component i (lb)
• $c_{p,i}$ = specific heat of material i (Btu/lb·°F)
• $f_i$ = engagement factor (0.3-0.8 for typical recovery periods)

Material Engagement Factors:

Recovery Curve Characteristics

graph TD
    A[Recovery Initiation] --> B{Outdoor Temperature}
    B -->|Below 32°F| C[Full Heating + Preheat AHUs]
    B -->|32-50°F| D[Staged Heating]
    B -->|Above 50°F| E[Minimum Heating]
    C --> F[Monitor Temperature Rise Rate]
    D --> F
    E --> F
    F --> G{Temperature Rise Rate}
    G -->|Below Target| H[Increase Heating Output]
    G -->|At Target| I[Maintain Current Output]
    G -->|Above Target| J[Reduce Heating Output]
    H --> K[Approach Setpoint]
    I --> K
    J --> K
    K --> L[Occupied Mode]

Temperature Rise Profile

graph LR
    subgraph "Recovery Phase Analysis"
    A[0-30 min:<br/>Rapid Rise<br/>3-5°F/hr] --> B[30-90 min:<br/>Moderate Rise<br/>2-3°F/hr]
    B --> C[90-120 min:<br/>Slow Rise<br/>1-2°F/hr]
    C --> D[Setpoint<br/>Achieved]
    end

Recovery Time Targets

ASHRAE 90.1 does not mandate specific recovery times but requires optimal start controls for buildings >10,000 ft². Typical assembly space targets:

Optimal Start Control Strategy

Optimal start algorithms adjust equipment start time based on current conditions and learned building response.

Basic Optimal Start Algorithm

$$t_{start} = t_{occupancy} - \left(K_1 \cdot \Delta T + K_2 \cdot (T_{outdoor} - T_{ref}) + K_3\right)$$

Where:

• $t_{start}$ = equipment start time
• $t_{occupancy}$ = scheduled occupancy time
• $\Delta T$ = $(T_{setpoint} - T_{current})$
• $K_1$ = temperature difference coefficient (typically 8-15 min/°F)
• $K_2$ = outdoor temperature coefficient (typically 1-3 min/°F)
• $K_3$ = base time offset (typically 15-30 min)
• $T_{ref}$ = reference outdoor temperature (typically 40-50°F)

Adaptive Learning

Modern controls employ adaptive algorithms that refine coefficients based on actual performance:

$$K_{1,new} = K_{1,old} + \alpha \cdot \frac{(t_{actual} - t_{predicted})}{\Delta T}$$

Where $\alpha$ is the learning rate (typically 0.1-0.3).

Thermal Mass Effects

High thermal mass in assembly spaces significantly impacts recovery characteristics.

Thermal Mass Benefits:

• Reduces temperature swing during unoccupied periods
• Provides thermal energy storage during recovery
• Dampens outdoor temperature fluctuations

Thermal Mass Challenges:

• Extends recovery time requirements
• Increases equipment sizing factor
• Complicates control algorithm tuning

Mass-to-Volume Ratio Impact

$$R_{mv} = \frac{\sum (m_i \cdot c_{p,i})}{V \cdot \rho_{air} \cdot c_{p,air}}$$

Where:

• $R_{mv}$ = mass-to-volume ratio (dimensionless)
• $V$ = conditioned space volume (ft³)
• $\rho_{air}$ = air density (0.075 lb/ft³)
• $c_{p,air}$ = air specific heat (0.24 Btu/lb·°F)

Typical Assembly Space Values:

Recovery Enhancement Strategies

Outdoor Air Reset During Recovery

Minimize outdoor air to design minimum (ASHRAE 62.1 ventilation rates apply only during occupancy):

$$Q_{OA,recovery} = \max(0.15 \text{ cfm/ft}^2, 500 \text{ cfm})$$

This maintains slight building pressurization while minimizing heating load.

Economizer Lockout

Disable economizer operation during recovery to maximize heating coil capacity. Enable only when:

$$T_{space} \geq (T_{setpoint} - 2°F) \text{ AND } t \geq (t_{occupancy} - 30 \text{ min})$$

Staged Equipment Operation

Sequence heating equipment to optimize efficiency:

1. Phase 1 (0-30% recovery): Activate all heating stages, 100% return air
2. Phase 2 (30-70% recovery): Modulate to maintain target rise rate
3. Phase 3 (70-100% recovery): Reduce to occupied mode, introduce ventilation air

Equipment Sizing for Recovery

Total heating capacity must accommodate both envelope losses and thermal mass recovery:

$$Q_{total} = Q_{design} + Q_{recovery}$$

Where:

$$Q_{recovery} = \frac{C_{building} \cdot \Delta T_{setback}}{t_{recovery,target}}$$

For assembly spaces with 2-3 hour recovery targets, this typically results in 1.4-2.0× design heating load capacity.

Economizer Contribution

When outdoor conditions permit, economizer operation can reduce mechanical cooling during shoulder seasons but provides no heating benefit. Do not credit economizer capacity toward recovery heating requirements.

Control Sequence Optimization

Recommended Recovery Sequence:

1. Pre-start (t - 5 min): Enable fans, verify operation
2. Initial recovery (t + 0 min): Full heating, minimum OA, economizer locked out
3. Mid recovery (50% complete): Begin modulating heating output
4. Late recovery (80% complete): Increase OA to design minimum
5. Occupied transition (90% complete): Enable economizer, full occupied mode
6. Occupied mode (100%): Normal control sequences active

Design Recommendations

Per ASHRAE 90.1-2019 Section 6.4.3.3.3: Buildings >10,000 ft² must employ automatic time clock control with optimum start capabilities. For assembly spaces specifically:

• Implement adaptive optimal start with minimum 30-day learning period
• Size heating equipment for 2-3 hour recovery from 10-15°F setback
• Monitor and trend recovery performance monthly
• Adjust setback depth based on unoccupied duration and outdoor temperature
• Consider separate setback strategies for conditioned vs. unconditioned periods

Verification Testing: Commission optimal start by testing at various outdoor temperatures and setback conditions to verify occupancy targets are met within ±2°F at scheduled time.

Performance Metrics

Track these key performance indicators to optimize recovery strategy:

• Percentage of occupancy periods meeting temperature target (target: >95%)
• Average recovery energy per degree-hour (Btu/°F·hr)
• Optimal start accuracy (target: ±15 minutes)
• Equipment runtime during recovery vs. occupied periods
• Seasonal recovery time trends

Reference: ASHRAE Handbook—HVAC Applications, Chapter 4 (Places of Assembly); ASHRAE Standard 90.1-2019; ASHRAE Guideline 36-2021