Fan Coil Units: How They Work and Who Maintains Them

What a fan coil unit is, and the two things the name can mean

The 2010 Operations and Maintenance Best Practices guide, prepared by Pacific Northwest National Laboratory, defines a fan coil unit as a small terminal air handling unit with a fan and coil(s). A coil is tubing wrapped in thin metal fins, with cold or hot fluid running through it, and the fan moves room air across the fins. Trane and Daikin manuals also list a filter and a condensate drain pan among the parts to check. Condensate is the water that forms on a cold coil as it takes moisture out of the air.

In residential HVAC, Carrier says, the terms air handling unit and fan coil unit are often used interchangeably. Carrier's page at its fan coils web address is titled Air Handler Units for Residential HVAC. Trane's residential glossary describes a fan coil unit as an indoor coil, a fan and an outdoor condensing unit, with refrigerant in the coil. The Trane and Daikin manuals cited here describe fan coils connected to chilled water pipes instead. Our reading: the name covers both the refrigerant type and the chilled-water type, so the first thing to learn about your unit is which one it is. Central air conditioning system types covers split systems and air handlers, and variable-speed air handlers covers the blower in that indoor unit, so this page stays on the fan coil.

Chilled water or refrigerant: what feeds the coil

A 2000 Florida Solar Energy Center report for the Florida Department of Education carries a notice that its views are the authors' own, not the Center's. Its authors describe fan coil systems as fan units, one for each zone, connected by water loops and pumps to a central plant. Trane's 2003 manual for a chilled-water fan coil lists two ways to control it: fan speed, and cycling the water flow to the coil. The same manual says the wall thermostat controls the water line stop valve. Our reading: in a water-fed unit, the room thermostat works a valve, and the cold itself is made somewhere else in the building. Pump and hydronic services covers the pumps and piping that move chilled water, so this page covers the unit end only.

Carrier says an air handler is the indoor unit that circulates air, while the outdoor condenser cools the refrigerant, and the two work together as a split system. The Florida report also describes a split direct-expansion system, where the cooling coil and the condenser are separate. Direct expansion, or DX, means the refrigerant evaporates inside the coil itself. The report adds that fan coil units can be chosen to match given condensing units. For who may handle refrigerant, see EPA Section 608 rules.

Trane says its vertical high rise fan coil unit is used in high-rise hotels, condominiums, dormitories, military barracks and residential buildings. We have not measured how many Tampa Bay condominiums use each kind of feed, so ask your association which one your building has.

Two-pipe or four-pipe, and what the Florida report says

A Vanderbilt University residence hall guide says a building's two-pipe or four-pipe label describes the water distribution system that serves its climate equipment. In a two-pipe system, it says, each unit has one supply line and one return line, and the entire building is in either heating mode or cooling mode. A four-pipe system has both hot and chilled water lines, so it can supply heat to one room while cooling another.

The Florida report says two-pipe units can heat or cool through the same pair of pipes, but not at the same time, with a seasonal change between the two modes. Its authors add that two-pipe units are not recommended for Florida's climate, while a four-pipe unit can provide heating and cooling simultaneously. Our reading: that report is from 2000 and was written for school buildings, so it is a reason to ask which system your building has, not a verdict on your building.

Moisture control: what the Florida report says fan coils can and cannot do

The same report says conventional air conditioning systems mainly control temperature, not humidity. It says that removing moisture with fan coils depends critically on not oversizing the equipment for the load. It also says fan coil units can supply minimum outside air for ventilation, but are not particularly appropriate in humid conditions.

For water-fed units, the report describes a modulating fan coil, which throttles the chilled water flow to match the cooling need. It says even small cuts in the chilled water flow can severely reduce the moisture-removal potential. Our reading: that is a 2000 statement from a report on schools, so treat it as a sign that valve control matters, not as a measure of your unit. Our reading: sizing and valve control are set when a building is designed or changed, so they are questions for the association or its engineer. For a home with a ducted system, humid house with the AC on goes deeper on moisture removal.

Where the water goes, and who answers for the unit

EPA's 2013 moisture guidance sets a design goal of keeping normal condensate from overflowing drain pans and preventing condensation on cold pipes, valves, ducts and air handler cabinets. Trane's 2003 manual says condensate protection for the chilled water valves and piping must be provided by the installer. It says a drain pan extension should sit under the valves, or else the valves and piping should be thoroughly insulated. Daikin's manual says the installing contractor must supply service valves and insulation for water piping in line with local codes.

Both manuals say the drain pan is pitched on purpose: Daikin says not to level the pan, and Trane says to level the unit by its casing. Our reading: pan slope is set at installation, so it is a question for the installer, not something to adjust yourself. EPA says a drain trap can dry out when a system is not in use, which can let sewer gases or other gases be drawn into the unit and spread through the building. Our reading: that matters most for a unit left unused while the owner is away. Air conditioner leaking water covers the drain path link by link.

Florida's condominium law says maintenance of the common elements is the association's responsibility, except for maintenance of limited common elements that the declaration assigns to the unit owner. It defines common elements as the parts of the condominium property not included in the units. Limited common elements are those reserved for the use of certain units, as the declaration specifies. Chapter 718 does not use the term fan coil. Our reading: the statute leaves it to the declaration to say whether a given unit, its drain line and its valves belong to the owner or the association. The statute lists the recorded declaration among the association's official records, and says any association member may inspect those records at all reasonable times. This page is not legal advice, and a Florida condominium attorney can read a declaration.

What the manuals say to check, and how often

Daikin's 2018 manual for a ceiling-concealed fan coil says a dirty filter reduces air flow and unit performance. It says to check the filter monthly and replace or clean it as required. Its monthly list also includes inspecting and cleaning the condensate drain pan and checking the drain piping for proper flow. Trane's 2003 manual also lists a monthly check of the air filter and of the drain pan. Its annual list includes draining and treating the whole system to control pipe scaling.

A 2013 Trane manual for another fan coil line says untreated or improperly treated water may result in scaling, erosion, corrosion, algae or slime. It recommends a qualified water treatment specialist to decide what treatment, if any, is required. EPA's guidance also says to inspect coils, pans and condensate lines regularly for cleanliness and to make sure the drain is working. Our reading: these lists are written for maintenance staff, so ask who in your building does each item, and keep the answer in writing. AC filter replacement covers filter sizes, ratings and change intervals.

Does a fan coil unit fit your home or building?

A fan coil fits when the building can supply what its coil needs, and when its size, space, noise, drain and controls suit the room. Trane's 2003 manual says servicing needs access through removable ceiling panels, and Daikin's says the unit needs enough space for installation and maintenance. The Florida report says proper sizing, never oversized, is critical to how well fan coil systems remove moisture. AC tonnage and sizing in Florida explains how sizing is judged for cooling equipment.

The report also says zone-located fan coil units tend to have greater operating sound levels than conventional air systems, and that this should be carefully evaluated. It notes that when these units need service, maintenance personnel may have to disrupt class or work time. Our reading: in a condo, that means service may need a visit to the unit, so ask how access is arranged. AC noises decoded covers what different sounds can mean. HVAC blower motor repair covers blower motors in ducted air handlers, a similar part. HVAC permits in Florida covers when a permit applies if a unit is replaced.

Questions to put to the association and any contractor

Put these questions to the association, its engineer or any contractor, and get the answers in writing.

1. What feeds this unit: chilled water from a central plant, or refrigerant from an outdoor condensing unit?
2. Is the building two-pipe or four-pipe, and does it change over from cooling to heating by season? The Florida report says two-pipe units are not recommended for Florida's climate.
3. Under our declaration, who maintains the unit, its filter, its drain line and its valves? Ask for the page of the declaration or the maintenance chart that says so. The statute also lists plans, permits and warranties provided by the developer among the association's official records.
4. Who checks the filter and drain pan, and what does the written schedule say? Two manuals list a monthly check of both.
5. Does anyone check the drain trap on units that sit empty for months?
6. Was the unit sized for the load and never oversized, and how is the water flow to the coil controlled? The Florida report ties moisture removal to both sizing and the water flow.

A 2013 Trane manual says service work on the refrigeration and electrical systems should be done only by qualified and experienced personnel. The state licensing agency says a license check starts from the license number, or from the name of the contractor or the business.