Packaged Air Handling Unit
A packaged air handling unit (PAHU) is a self-contained HVAC system designed to provide both heating and cooling to a building or specific zones. It typically includes components such as supply and return fans, heating and cooling coils, filters, dampers and integrated controls. The heating coil can use hot water, electric resistance or gas, while the cooling coil may use chilled water or a direct expansion (DX) refrigeration system. Filters clean the air before distribution, and dampers manage airflow and ventilation rates. The controls coordinate system operation based on the building's heating, cooling and ventilation requirements.
It is composed of:
Air Handling Units
Air Terminal
The 'Select HVAC Template' form provides options, to allow the user to configure the system with:
A single AHU connected to multiple Air Terminals or multiple AHUs connected to multiple Air Terminals
Choice of Spaces for Air Terminal
Packaged Air Handling Unit Metered Outputs
Zone Sizing
Zone sizing determines peak heating and cooling loads for each zone by simulating extreme design days, 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 varies depending on orientation and internal gains. A sizing factor, typically between 1.15 and 1.25, is applied as a safety margin.
For packaged air handling units, these results reveal load diversity across zones. Since different zones peak 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.
Plant Meters
Plant meters track the central equipment, which for packaged air handling units means the DX cooling coil, DX heating coil, and supply fan serving all zones. The system separates tracking by equipment type: DX Cooling Electricity shows summer peaks occurring in mid-to-late afternoon, DX Heating Electricity shows winter peaks in early morning hours, and Supply Fan Electricity tracks air circulation energy throughout the year.
These meters provide monthly energy totals and peak demands, revealing the coefficient of performance under real conditions, total building HVAC electrical demand, and validation of packaged unit sizing against rated capacity. The meters also expose operating inefficiencies such as excessive runtime or simultaneous heating and cooling.
Air Terminal Single Duct VAV No Reheat
The air terminal unit is the zone-level equipment containing a damper assembly and controls without a reheat coil. It modulates airflow to maintain zone temperature setpoints through variable air volume control alone. 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, typically 20-30% of maximum).
Without reheat capability, zone heating is provided by reducing cooling airflow to minimum and allowing the warmer supply air temperature (during heating mode) to satisfy the heating load. This eliminates the energy penalty of simultaneous heating and cooling.
During operation, the damper modulates from full open during peak cooling to minimum during low loads. The system relies on supply air temperature reset and proper minimum flows to maintain comfort without reheat. This approach is most suitable for interior zones with consistent cooling loads.
Air Loop HVAC
The complete air loop system integrates all components: outdoor air system, supply fan, DX cooling coil, DX 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 among zones), 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 system continuously adjusts to meet zone demands. The supply fan modulates airflow based on zone damper positions to maintain constant static pressure. Supply air temperature resets based on outdoor air temperature or zone loads to maximise efficiency. This coordinated control efficiently serves diverse zone loads without reheat.
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 whilst maximising free cooling opportunities.
Heat Exchanger Air To Air Sensible And Latent
The air-to-air heat exchanger is a central air handling unit component with integrated bypass dampers used for exhaust or relief air heat recovery. It transfers both sensible energy (temperature) and latent energy (moisture) between supply and exhaust air streams. Key design parameters include the Design Size Nominal Supply Air Flow Rate in L/s (rated airflow capacity).
Performance is defined entirely through effectiveness specifications with no geometric inputs required. The heat exchanger recovers waste energy from exhaust air to precondition incoming outdoor air, reducing heating and cooling loads. Bypass dampers modulate to enable economizer operation, suspending heat exchange when outdoor conditions favor free cooling.
During operation, multiple frost control strategies prevent ice formation during cold weather, including preheat coils and exhaust-only mode. This approach is most suitable for buildings with high ventilation requirements where energy recovery justifies the added equipment complexity.
Fan System Model
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.
Coil Cooling DX Single Speed
The single-speed DX cooling coil provides mechanical cooling using direct expansion refrigeration technology with on/off cycling control. Design parameters include the Design Size Rated Air Flow Rate in L/s (volumetric airflow across the coil at rated conditions), Design Size Gross Rated Total Cooling Capacity in kW (total heat removal including both sensible and latent cooling at rated conditions), and Design Size Gross Rated Sensible Heat Ratio or SHR (fraction of total cooling devoted to sensible versus latent cooling, typically around 0.75, meaning 75% sensible and 25% latent).
Rated conditions are standardised: air entering the cooling coil at 26.7°C dry-bulb and 19.4°C wet-bulb, with outdoor air entering the condenser at 35°C dry-bulb. The coil capacity and efficiency vary with actual operating conditions according to performance curves.
During operation, the single-speed DX coil cycles on and off to meet the cooling load. When operating, it runs at full capacity. The cycling ratio represents the fraction of each time step that the coil operates. At part-load conditions, cycling losses reduce efficiency slightly compared to full-load operation.
Coil Heating DX Single Speed
The single-speed DX heating coil provides heating using heat pump technology with on/off cycling control, extracting heat from outdoor air. Design parameters include the Design Size Rated Air Flow Rate in L/s (airflow across the coil during heating) and Design Size Gross Rated Heating Capacity in kW (total heat delivery capacity at rated conditions).
Rated conditions assume outdoor air at 8.3°C dry-bulb and 6.1°C wet-bulb. Performance decreases as outdoor temperature drops, with significant capacity degradation below 0°C and potential for defrost cycling below -5°C depending on outdoor humidity conditions.
During operation, the coil cycles on and off to meet heating loads. When operating, it runs at full capacity. The cycling ratio represents operating time fraction. Defrost cycles activate periodically in cold, humid conditions to maintain performance, temporarily reducing heating output whilst melting frost from the outdoor coil.
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