Intermittent Operation HVAC for Assembly Spaces

Overview

Intermittent operation characterizes HVAC systems serving assembly spaces with irregular or predictable non-continuous occupancy patterns. These facilities—including worship centers, auditoriums, convention halls, and sports arenas—experience extended unoccupied periods followed by scheduled high-density events. Proper engineering of setback and recovery sequences yields substantial energy savings while ensuring occupant comfort at event start.

Thermal Response Fundamentals

The thermal behavior of intermittently operated spaces depends critically on building construction characteristics and environmental conditions. During unoccupied periods, space temperature drifts toward ambient conditions at a rate governed by heat loss mechanisms.

Heat Loss During Setback

The instantaneous heat loss during setback operation follows:

$$Q_{loss} = UA(T_{space} - T_{ambient}) + \dot{m}{inf}c_p(T{space} - T_{ambient})$$

Where:

• $UA$ = overall building conductance (Btu/hr·°F)
• $T_{space}$ = current space temperature (°F)
• $T_{ambient}$ = outdoor temperature (°F)
• $\dot{m}_{inf}$ = infiltration mass flow rate (lb/hr)
• $c_p$ = specific heat of air (0.24 Btu/lb·°F)

Thermal Mass Effect

Building thermal mass moderates temperature swing rates. The effective thermal capacitance determines cooldown characteristics:

$$C_{eff} = \sum_{i} m_i c_{p,i}$$

Where $m_i$ and $c_{p,i}$ represent mass and specific heat of each building component (structure, furnishings, air volume).

Setback Recovery Calculation

Recovery time—the period required to restore space temperature from setback to occupied setpoint—governs system start time before scheduled events.

Recovery Time Estimation

The approximate recovery time for a space with thermal mass:

$$t_{recovery} = \frac{C_{eff}(T_{occupied} - T_{setback})}{Q_{available} - Q_{loss,avg}}$$

Where:

• $t_{recovery}$ = warm-up time (hours)
• $T_{occupied}$ = occupied setpoint temperature (typically 68-72°F)
• $T_{setback}$ = unoccupied setback temperature (°F)
• $Q_{available}$ = available heating capacity (Btu/hr)
• $Q_{loss,avg}$ = average heat loss during recovery (Btu/hr)

Building Time Constant

The thermal time constant provides a simplified recovery estimate:

$$\tau = \frac{C_{eff}}{UA + \dot{m}_{inf}c_p}$$

Recovery to within 5% of setpoint requires approximately $3\tau$ hours.

Operation Pattern Strategies

gantt
    title Weekly Operation Pattern - Worship Center
    dateFormat HH:mm
    axisFormat %H:%M

    section Sunday
    Setback 55°F    :done, 00:00, 07:00
    Warm-up         :active, 07:00, 09:00
    Occupied 70°F   :crit, 09:00, 13:00
    Setback 55°F    :done, 13:00, 24:00

    section Wednesday
    Setback 55°F    :done, 00:00, 17:00
    Warm-up         :active, 17:00, 18:30
    Occupied 70°F   :crit, 18:30, 21:00
    Setback 55°F    :done, 21:00, 24:00

    section Other Days
    Continuous Setback 55°F :done, 00:00, 24:00

Setback Temperature Selection

Setback temperatures balance energy savings against recovery time and freeze protection:

Energy Impact Analysis

Intermittent operation reduces heating energy consumption through decreased heat loss during unoccupied periods, offset partially by cycling losses during recovery.

Energy Savings Calculation

Net weekly energy savings compared to continuous operation:

$$E_{saved} = \int_{t_{unoccupied}} Q_{loss}(T_{occupied}) , dt - \int_{t_{recovery}} Q_{additional} , dt$$

Where $Q_{additional}$ represents excess energy input during accelerated warm-up.

Cycling Loss Factor

Recovery inefficiency introduces a penalty factor:

$$\eta_{cycling} = 1 - \frac{E_{recovery,excess}}{E_{saved,setback}}$$

Typical cycling efficiencies range from 0.85-0.95 depending on recovery rate and thermal mass.

Comparative Energy Performance

Based on 50,000 sq ft assembly space, climate zone 5, medium thermal mass

System Design Considerations

Heating Capacity Sizing

Systems serving intermittently operated spaces require additional capacity beyond steady-state loads to achieve acceptable recovery times:

$$Q_{design} = Q_{steadystate} \times F_{pickup}$$

Pickup factors typically range from 1.2-1.5 for assembly spaces based on desired recovery time.

Control Sequences

Optimal start algorithms adjust system start time based on:

• Current space temperature measurement
• Outdoor temperature
• Historical recovery performance
• Building thermal response characteristics

ASHRAE Guideline 36 provides sequences for optimal start/stop control in intermittently operated facilities.

flowchart TD
    A[Event Scheduled] --> B{Calculate Current Recovery Time}
    B --> C[Measure T_space, T_outdoor]
    C --> D[Lookup Historical τ]
    D --> E{Recovery Time > Time Until Event?}
    E -->|Yes| F[Start System Now]
    E -->|No| G[Delay Start]
    G --> H[Wait 15 Minutes]
    H --> B
    F --> I[Monitor Recovery Progress]
    I --> J{T_space ≥ T_setpoint - 2°F?}
    J -->|No| I
    J -->|Yes| K[Switch to Occupied Mode]

Equipment Selection

Variable Capacity Systems

Modulating heating equipment provides advantages for intermittent operation:

• High capacity during recovery
• Efficient low-fire operation during occupied periods with high internal gains
• Reduced cycling losses

Thermal Storage

Sensible or latent thermal storage can optimize intermittent operation:

• Pre-charge storage during off-peak periods
• Release stored energy during recovery
• Reduce peak demand charges
• Enable renewable energy time-shifting

Special Event Considerations

Assembly spaces hosting special events require flexible scheduling:

Rapid Response Capability: Systems must recover from deep setback within 2-3 hours for unexpected event additions.

Humidity Control: Extended setback in humid climates necessitates dehumidification during recovery to prevent condensation and maintain comfort.

Ventilation Pre-Purge: Introduce outdoor air 30-60 minutes before occupancy to establish air quality baseline per ASHRAE 62.1.

Code and Standard References

• ASHRAE 90.1: Requires automatic setback controls for spaces unoccupied >12 hours/day
• ASHRAE 62.1: Ventilation rates may be reduced during unoccupied periods
• IMC Section 403.2.3: Thermostatic controls and setback requirements
• ASHRAE Guideline 36: Optimal start/stop control sequences

Verification and Commissioning

Verify intermittent operation performance through:

• Recovery time measurement under various outdoor conditions
• Setback depth achievement during unoccupied periods
• Schedule accuracy and adherence
• Energy consumption comparison to baseline predictions

Proper commissioning of intermittent operation strategies ensures the predicted energy savings materialize in actual operation while maintaining occupant comfort expectations.