Fan Coil Unit - Air Cooled (EnergyPlus HVACTemplate)

A Fan Coil Unit (FCU) - Air Cooled, as defined in EnergyPlus HVACTemplate, is a climate control system commonly used in buildings. t consists of a coil through which chilled water circulates, absorbing heat from the air. The fans then distribute the cooled air throughout the space, ensuring effective and energy-efficient cooling. This configuration is particularly suitable for applications where air cooling is preferred over other methods.

A FCU comprises one Plant HVAC component and one Space HVAC component (per selected Space).

• Plant HVAC component - Fan Coil Unit System (Air Cooled)

• Space HVAC component - Fan Coil Unit (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 have one Fan Coil Unit (Air Cooled) Space HVAC component.

Fan Coil Unit - Air Cooled (EnergyPlus HVACTemplate) 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 fan coil systems, these results determine the capacity and airflow for each individual unit. Since fan coil units operate independently in each zone, the sizing establishes the cooling coil capacity, heating coil capacity, and fan airflow needed to meet zone demands throughout the year.

Fan System Model

The fan system model simulates fan performance in fan coil units, calculating airflow delivery and power consumption. During zone sizing, it establishes the Design Maximum Air Flow Rate in L/s (the capacity needed for peak cooling load) and the Design Electric Power Consumption in kW (power required at maximum airflow, which scales with zone size).

Fan coil units typically offer multiple speed settings—low, medium, and high—allowing occupants or controls to adjust airflow based on comfort needs and load conditions. Each speed has corresponding airflow and power consumption characteristics.

During simulation, fans modulate to match actual loads, consuming less power at part-load conditions than at design maximum. When operating at low speed, the fan delivers reduced airflow (typically 50-70% of maximum) and reduced power (typically 30-50% of maximum), improving comfort and efficiency during part-load operation.

Zone HVAC Four Pipe Fan Coil

The four pipe fan coil unit is a zone-level terminal unit containing a fan, cooling coil, heating coil, and controls, with separate piping circuits for chilled water and hot water. Key design parameters include the Design Maximum Air Flow Rate in L/s (airflow at high speed), Design Maximum Outdoor Air Flow Rate in L/s (ventilation airflow when provided), Maximum Hot Water Flow Rate in L/s (hot water flow for heating coil), and Maximum Cold Water Flow Rate in L/s (chilled water flow for cooling coil).

The four pipe configuration allows simultaneous availability of heating and cooling, with independent control of each coil through separate water valves. This enables rapid mode changes and better humidity control compared to two pipe systems.

During operation, the unit cycles the fan between speeds based on zone thermostat demand, modulates hot water flow through the heating coil valve, and modulates chilled water flow through the cooling coil valve. The controls prevent simultaneous heating and cooling operation whilst allowing quick transitions between modes.

Coil Cooling Water

The chilled water cooling coil provides cooling through hydronic heat transfer from a central chiller plant. Design parameters from zone 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 zone temperature setpoint. The valve position varies to match cooling loads, controlling heat transfer from the air stream to the chilled water whilst the fan operates at the selected speed.

Coil Heating Water

The hot water heating coil provides heating through hydronic heat transfer from a central plant. 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).

The coil is sized assuming entering hot water temperature (typically 82°C), leaving hot water temperature (typically 71°C), and design air conditions. For fan coil applications, the heating coil capacity must meet zone heating loads at the available airflow rates.

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

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.