Packaged Constant Air Volume (EnergyPlus HVACTemplate)

The Packaged Constant Air Volume (CAV) system in EnergyPlus HVACTemplate is a robust climate control solution. It maintains a consistent airflow regardless of heating or cooling demands, making it suitable for various building applications. This packaged unit ensures reliable and steady air volume, providing stable indoor conditions for optimal comfort and functionality.

The Packaged Constant Air Volume comprises one Plant HVAC component and one Space HVAC component (per selected Space).

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

• Space HVAC component - Constant 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 Constant Air Volume System (Air Cooled) Space HVAC component.

Constant Air Volume (CAV) (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 CAV systems, these results determine the constant airflow delivered to each zone. Since airflow remains constant regardless of load, zones are served with fixed volume rather than modulating flow. The zone sizing establishes the continuous airflow rate and the reheat capacity needed to maintain comfort across varying load conditions throughout the year.

Air Terminal Single Duct Constant Volume Reheat

The air terminal unit is the zone-level equipment containing fixed airflow controls and a reheat coil. It delivers constant airflow to the zone whilst modulating reheat to maintain temperature setpoints. Key design parameters include the Design Maximum Air Flow Rate in L/s (constant airflow delivered continuously during system operation) and the Maximum Air Temperature in °C (limits discharge air temperature to prevent overheating).

The reheat coil provides heating when zone loads are low. Design parameters include the Maximum Reheat Capacity in kW, which may be autosized based on zone heating loads at the fixed supply airflow.

During operation, airflow remains constant whenever the system runs. When zone temperature drops below setpoint, the reheat coil activates to warm the constant supply airstream. This creates energy penalties through simultaneous cooling and reheating, which is inherent to constant volume systems and why supply air temperature control is critical.

Coil Heating Water

The hot water heating coil provides heating through hydronic heat transfer from a central plant. This coil type is 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 Design Coil Load in kW (maximum heat delivery to the air stream), Design Size Design Water Flow Rate in L/s (hot water flow rate required to meet the load), and U-Factor Times Area Value in W/K (overall heat transfer coefficient).

For central heating coils, capacity is determined by system heating load. For terminal reheat coils, capacity is based on zone heating load at constant airflow. The coil is sized assuming entering hot water temperature, leaving hot water temperature, and air conditions.

During operation, the heating coil modulates water flow through a control valve to maintain setpoint temperature. The valve position varies from fully closed at no load to fully open at design load, controlling heat transfer to the air stream.

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 (constant system airflow that runs continuously when the system operates), 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 CAV system delivers constant airflow to all zones. The supply fan runs at fixed speed to maintain constant volume. Supply air temperature is controlled through modulating the cooling and heating coil water flows. Zone temperature control is achieved through reheat coils rather than airflow modulation, resulting in higher energy use than variable volume systems.

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, which is the constant system flow rate).

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

During operation, the controller maintains ventilation continuously at a fixed rate since system airflow is constant. 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 whilst total airflow remains constant.

Coil Cooling Water

The chilled water cooling coil provides cooling through hydronic heat transfer from a central chiller plant. Design parameters from system sizing include the Design Size Design Coil Load in kW (total heat removal including both sensible and latent cooling), Design Size Design Water Flow Rate in L/s (chilled water flow rate required to meet the load), Design Size Design Air Flow Rate in L/s (volumetric airflow across the coil), and U-Factor Times Area Value in W/K (overall heat transfer coefficient).

The coil is sized assuming entering chilled water temperature (typically 6.7°C), leaving chilled water temperature (typically 12.2°C), and design air conditions. The sizing establishes the heat transfer surface area required to meet the cooling load.

During operation, the cooling coil modulates chilled water flow through a control valve to maintain supply air temperature setpoint. The valve position varies to match cooling loads, controlling heat transfer from the air stream to the chilled water. The coil operates continuously whenever the system runs and cooling is required.

Fan Constant Volume

The fan system model simulates central supply fan performance with constant speed operation. During system sizing, it establishes the Design Maximum Air Flow Rate in L/s (constant airflow capacity delivered continuously during system operation) and Design Electric Power Consumption in kW (constant power draw during operation 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 operates at constant speed whenever the system runs, delivering fixed airflow regardless of zone loads. Unlike variable volume fans, power consumption remains essentially constant during all operating hours. The fan cycles on and off with the system rather than modulating speed, resulting in higher annual energy consumption compared to variable volume alternatives.

Pump Variable Speed

The variable speed pump circulates water through the hydronic loop, modulating flow to match system demands. During sizing, it establishes the Design Maximum Flow Rate in L/s (maximum water flow capacity based on coil requirements) and Design Power Consumption in kW (power at maximum flow based on head pressure, efficiency, and motor efficiency).

Key design parameters include Rated Flow Rate in L/s, Rated Pump Head in Pa (typically 150,000-300,000 Pa depending on system size), and Motor Efficiency (typically 0.85-0.90 for premium motors).

During operation, the pump modulates speed to maintain differential pressure or flow setpoint. Pump power follows an approximately cubic relationship with flow, so reducing flow to 50% reduces power to roughly 12.5% of maximum. This part-load efficiency makes variable speed pumps ideal for systems with varying loads.

Plant Loop

The plant loop represents the complete hydronic distribution system connecting the central plant equipment (chillers, boilers) to the demand side equipment (coils). Design parameters include the Maximum Loop Flow Rate in L/s (total flow capacity), Loop Design Temperature Difference in °C (temperature difference between supply and return, typically 5.6°C for chilled water and 11°C for hot water), and Load Distribution Scheme (determines how multiple chillers or boilers share loads).

The plant loop includes supply and demand sides connected through a common pipe. The supply side contains the central plant equipment and primary pump, whilst the demand side contains the coils and secondary pump for decoupled systems.

During operation, the plant loop maintains setpoint temperatures by modulating equipment operation. Flow varies based on demand, with pumps maintaining pressure whilst equipment cycles or modulates to maintain supply temperature. The loop continuously balances supply and demand whilst minimising energy consumption.

Boiler Hot Water

The hot water boiler generates thermal energy for the heating plant loop. Design parameters include the Design Size Nominal Capacity in kW (maximum heat output to the water), Design Size Design Water Flow Rate in L/s (water flow rate through the boiler at design conditions), and Nominal Thermal Efficiency (ratio of useful heat output to fuel input, typically 0.80-0.85 for conventional boilers and 0.90-0.95 for condensing boilers).

The boiler is sized to meet the building heating load accounting for distribution losses and pick-up factors. Sizing considers the design hot water supply temperature (typically 82°C) and return temperature (typically 71°C).

During operation, the boiler modulates firing rate to maintain hot water supply temperature setpoint. Modern boilers can modulate down to 10-20% of capacity, improving part-load efficiency. The boiler cycles on and off or modulates continuously depending on load and equipment capabilities.

Pump Constant Speed

The constant speed pump circulates water through the hydronic loop at fixed flow rate. During sizing, it establishes the Design Flow Rate in L/s (constant water flow delivered continuously during pump operation) and Design Power Consumption in kW (constant power draw based on head pressure, efficiency, and motor efficiency).

Key design parameters include Rated Flow Rate in L/s, Rated Pump Head in Pa (typically 150,000-300,000 Pa), and Motor Efficiency (typically 0.85-0.90).

During operation, the pump runs at constant speed whenever enabled, delivering fixed flow regardless of system loads. Power consumption remains constant during all operating hours. The pump cycles on and off with system demand rather than modulating, resulting in simpler control but higher energy consumption compared to variable speed alternatives where three-way valves maintain constant flow.

Chiller Electric EIR

The electric chiller generates cooling for the chilled water plant loop using vapour compression refrigeration. Design parameters include the Design Size Reference Capacity in kW (cooling capacity at reference conditions), Design Size Reference Chilled Water Flow Rate in L/s (evaporator water flow at design conditions), and Reference COP (coefficient of performance at rated conditions, typically 5.0-6.5 for modern water-cooled chillers and 2.8-3.5 for air-cooled chillers).

The chiller is sized using the EIR (Energy Input Ratio) method, where EIR equals 1/COP. Reference conditions assume 6.7°C leaving chilled water temperature and specific condenser entering conditions (29.4°C for water-cooled, 35°C for air-cooled).

During operation, the chiller modulates capacity to maintain chilled water supply temperature setpoint. Performance varies with part-load ratio and operating conditions according to performance curves. The chiller cycles or modulates based on cooling demand, with efficiency typically degrading at very low part-load conditions below 20-30% capacity.