💧 Chilled Water Flow
💨 Supply Airflow
Sizing Breakdown
Full Calculation Table
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How to Use the Fan Coil Calculator
Choose 4-pipe if the fan coil needs to provide simultaneous cooling and heating capability, or 2-pipe if the system operates in one mode seasonally.
Enter the zone sensible cooling load and winter heating load. These typically come from a zone-level load calculation for the specific room or suite the fan coil serves.
Enter chilled water and hot water supply and return temperatures matching your central plant design.
Click Calculate Fan Coil Sizing to see coil capacity, chilled and hot water flow rates, and required supply airflow.
Understanding Fan Coil Unit Sizing
Fan coil units serve perimeter zones directly, using a small fan to draw room air across a chilled water or hot water coil, then deliver conditioned air back to the same zone. They are common in hotels, office building perimeter zones, and hospital patient rooms because each unit responds independently to its own zone's thermostat, unlike a central AHU serving many zones through one duct system. Manufacturer capacity ratings for these units follow AHRI's room fan-coil certification program, which is worth checking against any capacity this calculator produces before finalizing a unit selection.
2-Pipe vs. 4-Pipe: The Core Design Decision
A 2-pipe system connects each fan coil to a single set of supply and return piping that the building switches seasonally between chilled water and hot water. This is simpler and less expensive to install, but it means the entire building operates in one mode at a time, which struggles during shoulder seasons when some zones need heating while others need cooling on the same day. A 4-pipe system runs separate chilled water and hot water piping to every fan coil, letting each unit provide whichever mode its own zone needs at any moment, independent of what other zones are doing, the trade-off the ASHRAE Handbook, HVAC Systems and Equipment volume lays out in its fan coil systems chapter.
Why Hotels and Mixed-Orientation Buildings Often Choose 4-Pipe
Hotel guestrooms on different building faces see very different solar exposure throughout the day, and guests set their own thermostats independent of neighboring rooms. A south-facing room overheating from solar gain while a north-facing room needs heating is a common scenario that only a 4-pipe system handles gracefully. The hotel HVAC calculator covers this application in more building-specific detail, including diversity between guestroom and public space loads.
Coil Capacity and Water Flow Rate
Chilled water flow rate follows the same underlying relationship as any hydronic system: flow rate in litres per second equals the coil load in kilowatts divided by 4.186 times the water temperature difference. Fan coils typically share the same chilled water supply and return temperatures as the rest of the building's hydronic system, since they usually connect to the same central plant covered by the chiller plant calculator, and that branch piping can be checked against the hydronic pipe sizing calculator once flow rate is known.
Supply Airflow and Room Comfort
Fan coil supply airflow uses a smaller temperature difference than central AHUs, commonly 8 to 10 degrees Celsius, since the unit sits directly in or near the zone it serves without long duct runs to account for. This tighter temperature difference helps avoid noticeable temperature stratification or drafts in a relatively small, directly-served space like a hotel room or private office, and fan noise at the selected airflow is worth checking against the NC curve calculator since a unit that overshoots the room's target noise criteria undermines the comfort a fan coil is supposed to deliver.
Fan Coils in Multi-Building-Type Applications
Fan coils appear across many commercial building types beyond hotels, including hospital patient rooms where the hospital HVAC calculator addresses additional pressurization and infection control considerations, and general office perimeter zones covered by the office HVAC calculator. This calculator handles the fan coil unit itself; pair it with the building-type-specific tool for broader system context.
Frequently Asked Questions
A 2-pipe fan coil has a single coil connected to either chilled water or hot water piping, switched seasonally, meaning the whole system operates in either heating mode or cooling mode at one time. A 4-pipe fan coil has separate chilled water and hot water coils, or a single coil with independent valves for each, allowing simultaneous heating and cooling across different zones or even within the same unit at different times of day.
Buildings with a mix of orientations, occupancy schedules, and internal gains often need heating in some zones while cooling is needed in others on the same day, particularly during spring and fall shoulder seasons. A 2-pipe system locked into one mode cannot serve this simultaneous demand, while a 4-pipe system lets each fan coil respond independently to its own zone's actual need. See the hotel HVAC calculator for guestroom-specific sizing considerations.
Chilled water flow rate in litres per second equals the cooling coil load in kilowatts divided by 4.186 times the chilled water temperature difference in degrees Celsius. Most fan coil applications use a similar temperature difference to the central chiller plant design, commonly around 5.5 to 6.5 degrees Celsius. See the chiller plant calculator for the central plant serving these units.
Fan coil supply airflow follows the same relationship as any sensible cooling airflow calculation, dividing the cooling load by the supply air temperature difference. Fan coils commonly use a slightly smaller temperature difference than central AHUs, often 8 to 10 degrees Celsius, since they typically serve a single zone directly without long duct runs.
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Zone load inputs for this tool