Fan Coil Units

Four-pipe fan coil unit schematic diagram. (Image reproduced with permission from source: Kolderup Consulting)

A fan-coil unit is typically a piece of equipment located in the room, above the ceiling, or in a mechanical space using forced convection and consisting of a return air opening, filter, fan, heating and cooling coil or coils, and supply air opening. Most often, they use hot and cold water from a primary system (chiller and boiler) serving the coils, but sometimes the term “fan-coil unit” is also applied when the cooling for the unit is based on a refrigerant coil. For modeling, the term is used almost exclusively for units with hydronic (water) coils, and that will be the focus of this page. They come in two-pipe changeover systems that change from heating to cooling based on centralized control and four-pipe systems that can heat or cool depending on the needs of the space. When fan-coil units provide ventilation air, it is usually through a direct outdoor opening or short duct, so it is common for fan-coil units to be located adjacent to exterior walls. Usually, the outdoor air is not controlled, so no type of economizer operation is possible. A system similar to a fan-coil unit with an economizer is typically called a unit ventilator.

Like other in-room terminal systems, fan-coil units have the following advantages:

• Reduce or eliminate ducting
• Individual room control
• Easy replacement when a failure occurs
• Uses centrally managed heating and cooling plant

Some of the disadvantages of fan-coil units include:

• Usually limited to perimeter spaces
• Fan noise
• Managing the removal of condensate
• Maintenance within occupied spaces

The 2020 ASHRAE Handbook for Systems and Equipment in chapter 5 - “In-Room Terminal System,” provides additional advantages and disadvantages as well as more detailed information.

How It Works

Fan-coil units themselves are very basic HVAC devices. They are located in the room or sometimes other locations and are connected to a central heating and cooling system by pipes of water, and a fan draws air from the room, through a filter, over the heating and cooling coil or coils, and back out to the room. A thermostat in the room regulates when the unit runs, and if outdoor air is provided, it is often through a small inlet with the fan-coil unit located adjacent to an exterior wall. If no outdoor air is provided by the unit, a separate dedicated outdoor air system (DOAS) may be used. Most units draw the air through the coils, but some put the fan before the coils and blow air through them.

Fan-coil units can be packaged in vertical or horizontal configurations based on the direction of the airflow through the unit. Condensation occurs when the cooling coil temperature is below the dewpoint temperature of the air (a measure of the humidity), and then the condensed water needs to be removed from the unit, often by piping. Fan-coil units with a vertical configuration will often have the necessary piping in a riser adjacent to the unit that serves many floors where the units are directly above one another. Horizontal units may be visible near the ceiling or hidden by the ceiling between floors, depending on the application.

The piping system to the fan-coil unit has two general configurations:

• Four-pipe systems where the heating supply and return piping and the cooling supply and return piping are plumbed to each unit, and the heating and cooling coils are separate. This allows each room thermostat to allow water through the coil that is needed and means that heating or cooling can be performed at the same time by fan-coil units in adjacent rooms. Since this requires more plumbing and two coils, it is the more expensive option compared to two-pipe systems.
• Two-pipe systems only have a supply and return pipe to each unit connected to a single coil. This means that either the entire building or at least large segments of the building are only supplied with either hot water or chilled water, and the fan-coil units can only supply that. While this is less expensive than a four-pipe system, it creates problems since each room has its own loads, and some may need cooling while others need heating. This is especially true for building elevations facing different directions but can also occur on the same face of the building. Often two-pipe fan-coil units will have supplementary electric heating available within each unit that is energized when the central plant is only providing chilled water, but heating is called for by the thermostat. Optimizing the controls for when to switch between heating and cooling in a building that uses two-pipe fan-coil units is often complicated and seldom optimal, and due to this, two-pipe systems are usually less efficient than four-pipe systems. Occupant complaints during shoulder months of being too warm tend to drive two-pipe systems to favor cooling for more and more of the year, which then results in the use of even more supplemental electric heat use over time.

An older and very inefficient configuration using three pipes, a cooling supply, a heating supply, and a common return. The common return mixes cooler and warmer water from units providing cooling and heating, respectively, and increases the energy needed by the chiller and boiler without providing many benefits.

Information Needed for the Model

For each fan-coil unit, the information needed for modeling includes the following:

• system configuration (two-pipe or four-pipe)
• fan control method
• sensible and latent cooling capacity
• heating capacity
• supplementary backup heat capacity (typically for two-pipe systems)
• air flow rate
• outdoor air flow rate
• maximum and minimum hot water flow rate
• maximum and minimum cold water flow rate
• supply air temperature for heating
• supply air temperature for cooling
• output control method

For many modeling programs, two-pipe systems are not explicitly modeled, and instead, a four-pipe system is modeled with schedules indicating when heating and cooling are available with no overlapping hours.

To achieve control on output (see Common Control Options below), it is common for simulation programs to need fan information for:

• airflow at each speed for multi-speed fans
• airflow at the minimum and the maximum speed for variable speed fans
• fan efficiency
• air pressure rise across the fan
• motor efficiency

The primary system consisting of chillers, boilers, cooling towers, pumps, and controls is also required for a hydronic fan-coil unit to be described.

It is especially important when evaluating the difference between two-pipe systems and other systems, including four-pipe fan-coil systems, that a true diversity of space conditions are modeled in the building. Modeling many zones all having the same thermostat settings, internal loads, and schedules will not show the impact of diversity on the central system and reveal zones where the loads are not being met because they are cooling when most other zones are heating and visa-versa. Instead, varying the thermostat setpoints, internal loads, and schedules over a reasonable range may reveal where two-pipe systems do not provide the same level of service as other systems.

Common Measures

Some common energy-efficiency measures to consider are shown below:

• High-efficiency fans with ECM or DC fan motor
• Variable-speed fans
• Use of a four-pipe system instead of a two-pipe system
• Optimizing fan speed and water flow control
• Resetting the chilled water supply temperature
• Resetting the hot water supply temperature
• Optimization of changeover temperature in two-pipe systems.

Many measures for the overall system that uses fan-coil units will be on the primary equipment, such as improving the full or part load efficiency of the chiller and boiler or improving the performance of the cooling tower or pumps.

It is not uncommon for systems using fan-coil units to be a baseline or original design configuration, and other options, such as VRF, may be considered efficiency measures.

Common Control Options

Output control (i.e., how much heating or cooling is provided) for a fan-coil unit can either be by modulating the water or airflow or a combination and is generally controlled by a thermostat or a building automation system. On/off, multiple-speed, or variable-speed fans are very common. And constant or variable flow of hot or chilled water options are common. For multiple-speed fans, it is common for the occupant to control the fan speed by a dial on the unit and it makes modeling this type of control more difficult because the occupant behavior must be taken into consideration. Water flow can be controlled by the room air thermostat as well. In a real building, on/off fan control using a return air thermostat located in the unit can suffer from not sensing the room air temperature well and may be best modeled with a wide deadband. Humidity issues, especially during shoulder seasons, can occur in the space when constant fan speed systems are controlled by modifying the chilled water flow.

Common Applications

Fan-coil units, like other in-room terminal systems, are commonly applied to apartments, classrooms, offices, small retail, hotels, and motels. Since they are commonly applied to individually controlled perimeter spaces, buildings with many individual rooms and little core area are good applications. They are typically not used to condition large spaces that might occur in a big box or large showroom retail, warehouse, or performance spaces. They also are not typically used within healthcare facilities, laboratories, or cooking facilities since they lack outdoor air controls that high ventilation spaces typically need. Since four-pipe fan-coil units allow individual room control, they are good fits for applications with many zones of different space types, perhaps with different thermostat setpoints, and are adjacent to one another as well as applications with a large portion of the year having some zones heating and others cooling.

Model Output Checks

The following output checks should be considered when modeling fan-coil systems:

• Examine the sensible cooling output, latent cooling output, heating output, run time, and fan speed for a variety of conditions to verify the controls are operating correctly.
• For two-pipe systems, verify that heating and cooling are not occurring simultaneously based on the hydronic system and also check if supplementary heating is occurring when the system is in cooling mode.
• Check those supply temperatures for chilled and hot water and for the supply air coming from the unit to be consistent with inputs.

Related Energy Code Requirements

Fan-coil units are mentioned only a few times in ASHRAE Standard 90.1-2022:

• Section 6.4.3.10 describes when Direct Digital Controls (DDC) are required
• Section 6.5.3.6 on Fractional Horsepower Fan Motors provides an exception for the efficiency and adjustment except for fan-coils that are only used during heating.
• Energy Cost Budget (now Section 12 but historically Section 11) as budget System 5 and 7.
• Performance Rating Method (Appendix G) as a qualifier related to new requirements for alterations as well as as a substitute for Systems 1 and 2 when purchased chilled water and purchased heat are used.

Both AHRI and ASHRAE provide standards that define the way to test and report the ratings for fan-coil units. For 1500 cfm or below, either ASHRAE Standard 79 or AHRI Standard 440 can apply. For larger units below 3000 cfm, AHRI 840 applies.These rating procedures are primarily for heating and capacity, airflow, power, and water pressure drop.

Similar or Related Systems

Various system configurations that have hydronic in-room terminals without being connected to a central air system and therefore are similar to fan-coils include:

• Unit ventilators - very similar to fan-coils but usually provide outdoor air that is controlled and is usually for larger spaces.
• Passive chilled beams - another type of in-room hydronic terminal unit relying on radiant and natural convective heat transfer and typically located in ceilings. Passive chilled beams are similar to active chilled beams but active beams are connected to a central air handler with the primary air flowing through a nozzle that induces room air to be drawn to the heating and cooling coil before being delivered to the room.
• Radiant panel or floor - a hydronic in-room terminal, usually without fans usually, for radiant heating

Similar to fan-coils because they often have individual room terminals but use refrigerants instead of heated and chilled water, include variable refrigerant flow and water-loop heat pumps .

Additional Resources

• 2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 5 - “In-Room Terminal System.”

• 2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 20 - “Room Air Distribution Equipment”

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References

ASHRAE Standard 79-2015 -- Method of Test for Fan-Coil Units (ANSI Approved).

AHRI Standard 440 (IP)/441(SI) - 2019 Standard for Performance Rating of Fan Coil Units.

AHRI Standard 840 (IP)/841(SI) - 2015 Standard for Performance Rating of Unit Ventilators.