How to use the fan coil unit sizing calculator
A fan coil unit takes chilled water and cools room air directly, so matching capacity and unit count to the room load is where the design starts. Enter the floor area, the cooling load density, a design margin and the rated capacity of one unit, and you get the total load, the number of fan coils required, the installed margin after rounding, the floor area each unit covers and an estimated supply airflow.
Watch the installed margin. Because unit count rounds up, a 10% design margin routinely becomes 30% or more once the units are installed. Excess capacity leads to short cycling, weak humidity control and wasted first cost, so it is often worth trying a smaller unit size or mixing two capacities.
Several small units beat one large one for temperature distribution, part load response and redundancy when a unit is out for service. The counterweight is more piping branches, more condensate drains, more ceiling access panels and more maintenance labor, so the room shape and occupancy pattern decide.
The load densities in the table are planning values. Real loads swing widely with glazing area and orientation, equipment heat and occupant density, so a full load calculation governs. Catalog ratings also need correction for the actual entering water and air conditions, and a mechanical engineer should confirm the final selection.
Frequently asked questions
Several smaller units usually win. Temperature distribution is more even, part load response is better, and one failure does not shut the space down. The cost is more piping branches, more condensate drains, more access panels and more maintenance labor.
Ten to twenty percent over the calculated load is typical. Rounding up to whole units pushes the installed margin higher, and once it climbs far past fifty percent the system short cycles, humidity control suffers and first cost is wasted.
Actual capacity shifts with entering water temperature, water flow, entering air conditions and fan speed. If site conditions differ from the catalog rating point, apply the correction tables first and confirm the final selection with the manufacturer's performance data.