Fan Coil Unit System (Electric Heating)
A Fan Coil Unit System (Electric Heating) is an HVAC setup using fan coils for air distribution and electric heating elements to regulate indoor temperature.
It is composed of:
Condenser Water Loop
Chilled Water Loop
(Optional) Air Handling Unit
Fan Coil Unit(s)
The 'Select HVAC Template' form provides options, to allow the user to configure the system with:
Zone Reheat Type (Electric or Hot Water)
Optional connection to a central Air Handling Unit (for outside air supply)
A single AHU connected to multiple Fan Coil Units or multiple AHUs connected to multiple Fan Coil Units
Choice of Spaces for Air Terminals
Fan Coil Unit System (Electric Heating) 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, electric heating coil capacity, and fan airflow needed to meet zone demands throughout the year.
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 fan coil systems with electric heating means the chillers, cooling towers, and chilled water pumps serving all zones. The system separates tracking by equipment type: Chiller Electricity shows summer peaks occurring in mid-to-late afternoon, Cooling Tower Electricity tracks condenser heat rejection, Pump Electricity shows circulation energy throughout the cooling season, and Fan Coil Electric Heating shows winter peaks in early morning hours at the zone level.
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 poor chiller part-load performance.
Plant Staging
Plant staging determines how multiple pieces of plant equipment (chillers, 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 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. Cooling towers maintain relatively stable performance across varying PLR ranges.
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.
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 for chilled water and electric heating. 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 Cold Water Flow Rate in L/s (chilled water flow for cooling coil), and Maximum Heating Capacity in kW (electric heating capacity).
The configuration allows simultaneous availability of cooling and electric heating, with independent control of the cooling coil through a water valve and the electric heating coil through staged or modulating elements.
During operation, the unit cycles the fan between speeds based on zone thermostat demand, modulates chilled water flow through the cooling coil valve, and stages or modulates electric heating elements. The controls prevent simultaneous cooling and heating 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 Electric
The electric heating coil provides heating through resistance elements within the fan coil unit. Design parameters include the Design Size Nominal Heating Capacity in kW (maximum heat delivery to the air stream) and Design Size Nominal Electric Input in kW, which equals the capacity since electric resistance heating is essentially 100% efficient.
The coil capacity is based on zone heating load at the available airflow rates. The coil is sized to maintain zone setpoint temperature when supply air is at its coldest and airflow is operating.
During operation, the heating coil modulates its output to maintain zone temperature setpoint. Electric heating elements stage or modulate to match the load, tempering the air stream whilst the fan operates at the selected speed. The simplicity and zone-level control of electric heating eliminates the need for a hot water distribution system.
Air Loop HVAC
The air loop represents the dedicated outdoor air system (DOAS) when provided, delivering ventilation air to the fan coil units. Design parameters include the Design Supply Air Flow Rate in L/s (total ventilation airflow), Design Supply Air Temperature in °C (typically neutral or slightly conditioned), and availability schedules defining when the system operates.
The air loop includes outdoor air intake, optional filtration, optional heating and cooling for tempering, supply fan, and distribution to zones where it connects to the fan coil units.
During operation, the DOAS delivers constant or variable ventilation air to supplement the recirculated air handled by the fan coil units. This decouples ventilation from zone temperature control, allowing fan coil units to operate more efficiently whilst ensuring adequate outdoor air supply. The DOAS may include energy recovery to precondition ventilation air.
Controller Outdoor Air
The outdoor air controller manages ventilation air delivery when a dedicated outdoor air system is provided. 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 DOAS airflow capacity).
The controller may include demand-controlled ventilation logic that adjusts outdoor airflow based on actual occupancy when CO₂ sensors are present, and may include energy recovery control to maximise heat transfer between exhaust and supply air streams.
During operation, the controller maintains ventilation airflow according to the specified method—per person, per area, or based on air changes per hour. When demand-controlled ventilation is active, the controller modulates outdoor air based on measured CO₂ levels. The controller coordinates with energy recovery devices to optimise ventilation energy performance.
Plant Loop
The plant loop represents the complete chilled water distribution system connecting the central plant equipment (chillers, cooling towers) to the demand side equipment (fan coil unit cooling 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 Load Distribution Scheme (determines how multiple chillers 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 chilled water supply temperature setpoint 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 chilled 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.
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