Packaged Variable Air Volume (EnergyPlus HVACTemplate)

The Packaged Variable Air Volume (VAV) system in EnergyPlus HVACTemplate is a comprehensive climate control solution. It efficiently adjusts air volume based on heating or cooling requirements. This packaged unit incorporates a variable air volume system, enhancing energy efficiency by dynamically adapting airflow. Ideal for maintaining precise and comfortable indoor conditions in diverse building settings.

The VAV comprises one Plant HVAC component and one Space HVAC component (per selected Space).

• Plant HVAC component - Variable Air Volume (System Air Cooled)

• Space HVAC component - Variable Air Volume (System Air Cooled)

The 'Select HVAC Template' form allows for multiple Spaces to be served. Clicking into the 'Spaces' field shows a list of available Spaces. Each selected Space will be served with one Variable Air Volume System (Air Cooled) Space HVAC component.

Variable Air Volume (VAV) (EnergyPlus HVACTemplate) Metered Outputs

Zone Sizing

Zone sizing establishes peak heating and cooling loads for each zone through design day simulations, analysing the hottest day for cooling and coldest day for heating rather than full annual periods. The outputs include design load in kW and required supply airflow in L/s for each zone. Peak occurrence timing differs by zone depending on orientation, internal gains, and solar exposure. A sizing factor, typically ranging from 1.15 to 1.25, is applied as a safety margin to account for uncertainties.

For VAV systems, these results are critical because they reveal load diversity across zones. Since different zones reach peak loads at different times, the central air handling unit can be sized using coincidence factors rather than the sum of all individual zone peaks, which optimises equipment capacity and reduces first costs. The zone sizing also establishes minimum airflow requirements that prevent overcooling and ensure adequate ventilation during part-load conditions.

Air Terminal Single Duct VAV Reheat

The air terminal unit is the zone-level equipment containing a damper assembly, reheat coil, and controls. It modulates airflow to maintain zone temperature setpoints. Key design parameters include the Design Maximum Air Flow Rate in L/s (peak zone cooling airflow) and the Minimum Air Flow Rate or Fraction (lowest allowable airflow for ventilation and to prevent overcooling, typically 20-30% of maximum).

The reheat coil provides heating when zone loads are low. Design parameters include the Design Maximum Reheat Air Temperature in °C and Maximum Reheat Capacity in kW, which may be autosized based on zone heating loads.

During operation, the damper modulates from full open during peak cooling to minimum during low loads. When heating is required at minimum airflow, the reheat coil activates, creating energy penalties through simultaneous cooling and reheating.

Coil Heating Electric

The electric heating coil provides heating through resistance elements, used either as the central heating coil in the air handling unit or as a reheat coil in terminal units. Design parameters include the Design Size Nominal Heating Capacity in kW (maximum heat delivery) and Design Size Nominal Electric Input in kW, which equals the capacity since electric resistance heating is essentially 100% efficient.

For central heating coils, capacity is determined by system heating load during design days. For terminal reheat coils, capacity is based on zone heating load at minimum airflow conditions.

During operation, the heating coil modulates its output to maintain setpoint temperature. Electric heating elements stage or modulate to match the load, maintaining supply air temperature for central coils or tempering cold supply air for reheat coils.

Air Loop HVAC

The complete air loop system integrates all components: outdoor air system, supply fan, cooling coil, heating coil, and distribution to zones. Design parameters include the Design Supply Air Flow Rate in L/s (maximum central system airflow accounting for diversity), Design Supply Air Temperature in °C (typically 12.8°C for cooling mode), and Design Supply Air Temperature for Heating in °C (typically 35-40°C for heating mode).

The air loop includes branch definitions specifying component sequence, node connections linking components, and sizing information establishing design conditions.

During operation, the VAV system continuously adjusts to meet zone demands. The supply fan modulates airflow to maintain constant static pressure. Supply air temperature is typically held constant during cooling but may reset to reduce reheat. This coordinated control efficiently serves diverse zone loads.

Controller Outdoor Air

The outdoor air controller manages ventilation air and economiser operation, determining the mixture of outdoor air and return air delivered to the coils. Key design parameters include the Minimum Outdoor Air Flow Rate in L/s (ventilation requirement based on occupancy and space type) and Maximum Outdoor Air Flow Rate in L/s (equals total system airflow during full economiser operation).

The controller includes economiser logic using outdoor air for free cooling when conditions are favourable, with control based on dry-bulb temperature, enthalpy, or dewpoint.

During operation, the controller maintains minimum ventilation continuously. When cooling is required and outdoor conditions are favourable, the economiser modulates outdoor air up to 100%. Outdoor air, return air, and relief air dampers coordinate to maintain proper mixture.

Coil Cooling DX Two Speed

The two-speed DX cooling coil provides mechanical cooling with two discrete operating speeds, allowing better part-load performance than single-speed equipment. Design parameters include the Design Size High Speed Rated Air Flow Rate in L/s and Low Speed Rated Air Flow Rate in L/s (volumetric airflow at each speed), Design Size High Speed Gross Rated Total Cooling Capacity in kW (total heat removal including sensible and latent cooling), and Design Size High Speed Rated Sensible Heat Ratio or SHR (fraction devoted to sensible versus latent cooling, typically around 0.75).

Low speed typically operates at 50-67% of high speed capacity. Rated conditions are standardised at 26.7°C dry-bulb and 19.4°C wet-bulb entering air, with 35°C dry-bulb outdoor conditions.

During operation, the coil stages based on demand. At low loads, it runs at low speed for better dehumidification. Each speed cycles to match loads between zero and full capacity at that speed.

Fan Variable Volume

The fan system model simulates central supply fan performance with variable speed capability. During system sizing, it establishes the Design Maximum Air Flow Rate in L/s (total airflow capacity accounting for diversity) and Design Electric Power Consumption in kW (power at maximum airflow based on pressure rise, total efficiency, and motor efficiency).

Key design parameters include Fan Total Efficiency (typically 0.55-0.65), Pressure Rise in Pascals (typically 500-1000 Pa), and Motor Efficiency (typically 0.85-0.93 for premium efficiency motors).

During annual simulation, the fan modulates airflow to match system loads. Fan power follows an approximately cubic relationship with airflow, so reducing airflow to 50% of design reduces power to roughly 12.5% of maximum. The fan includes a Minimum Flow Fraction (typically 0.25-0.30) preventing excessively low speeds where efficiency degrades.