Central Plant and Air Handling System
The Central Plant and Air Handling System collaborate as a unified function, efficiently managing heating, cooling, and ventilation for optimal building energy performance.
It is composed of:
Condenser Water Loop
Hot Water Loop
Chilled Water Loop
Air Handling Unit(s)
Air Terminal(s)
The 'Select HVAC Template' form provides options, to allow the user to configure the system with:
Heating via either Hot Water or Electric
A single AHU connected to multiple Air Terminals or multiple AHUs connected to multiple Air Terminals
Choice of Spaces for Air Terminals
Central Plant and Air Handling System 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 central plant systems with 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.
Zone Meters
Zone meters are virtual sub-meters that track energy flows within each zone throughout the annual simulation, providing monthly summaries. They track two categories: electricity meters record electrical demand in kWh for fans and terminal units (representing utility costs), whilst supplied energy meters track the thermal energy in kWh delivered for heating and cooling (the useful output).
Each meter records total monthly energy in kWh, peak instantaneous power in kW, and the timestamp when peak occurs. You can use this data to compare zones and identify consumption patterns, assess efficiency by comparing electrical input to thermal output, and validate that load diversity and part-load performance match expectations.
Plant Meters
Plant meters track the central equipment, which for central plant systems means the chillers, boilers, cooling towers, and pumps serving all air handling units. The system separates tracking by equipment type: Chiller Electricity shows summer peaks occurring in mid-to-late afternoon, Boiler Gas or Electricity shows winter peaks in early morning hours, Cooling Tower Electricity tracks condenser heat rejection, and Pump Electricity shows 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 plant equipment sizing against rated capacity. The meters also expose operating inefficiencies such as excessive pump energy or simultaneous heating and cooling.
Plant Staging
Plant staging determines how multiple pieces of plant equipment (chillers, boilers, cooling towers, pumps) are brought online or taken offline in response to changing building loads. Key parameters include the Equipment Loading Scheme (sequential, uniform, optimal, or uniform PLR), Minimum Part Load Ratio (typically 0.10-0.20 for chillers and boilers), and Staging Setpoints (load thresholds that trigger equipment to stage on or off).
Sequential loading operates equipment in a defined order, bringing each unit to full capacity before starting the next. Uniform loading distributes load equally across all available equipment. Optimal loading selects equipment combinations that maximise plant efficiency based on performance curves.
During operation, the staging logic continuously evaluates building load against available capacity, stages equipment on when load exceeds capacity plus a buffer margin, and stages equipment off when load drops below minimum efficient operation. Proper staging minimises equipment cycling whilst maintaining efficient part-load performance.
Plant Part Load Ratio
Plant part load ratio (PLR) represents the instantaneous load on plant equipment divided by its available capacity, ranging from 0.0 (no load) to 1.0 (full load). This metric is critical because equipment efficiency varies significantly with PLR. Chillers typically achieve peak efficiency at 40-80% PLR, whilst efficiency degrades below 20% PLR. Boilers maintain relatively constant efficiency across a wider PLR range.
The PLR affects performance through capacity and efficiency modifier curves that adjust rated performance based on operating conditions. System PLR is calculated as total building load divided by total operating equipment capacity.
During simulation, PLR is tracked continuously for each piece of equipment. Monthly average PLR reveals how well equipment sizing matches building loads. Frequent operation below 30% PLR indicates potential oversizing, whilst frequent operation above 90% PLR suggests undersizing or need for additional capacity.
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 over-cooling, 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 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 minimum 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 (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 VAV 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 may reset based on outdoor air temperature or zone loads 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.
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 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.
Plant Loop
The plant loop represents the complete hydronic distribution system connecting the central plant equipment (chillers, boilers, cooling towers) 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.
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.
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.
Cooling Tower Variable Speed
The variable speed cooling tower rejects heat from the chiller condenser water loop to the atmosphere through evaporative cooling. Design parameters include the Design Size Design Water Flow Rate in L/s (condenser water flow from the chiller), Design Size Design Air Flow Rate in L/s (fan airflow capacity), Design Size Design U-Factor Times Area Value in W/K (overall heat transfer effectiveness), and Design Size Free Convection Capacity in kW (heat rejection without fan operation).
The tower is sized based on design wet-bulb temperature, approach temperature (typically 3-6°C), and range (typically 5-8°C). Tower capacity must match or exceed chiller heat rejection at design conditions.
During operation, the tower fan modulates speed to maintain condenser water supply temperature setpoint. At low loads, the tower may operate in free convection mode without fan power. Fan speed increases with cooling load and ambient wet-bulb temperature, with power following a cubic relationship with fan speed.
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.
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