Fan coil units in commercial buildings

A fan coil unit is a small air handler serving one room or zone. It contains a fan, a coil supplied with chilled water, hot water or both, a filter and a drain pan, and it recirculates room air through the coil to heat or cool the space. Fan coils are common in hotels, condominiums and apartments, offices, hospitals and schools, especially where a central plant already supplies chilled and hot water.

Fan coils give each room independent temperature control without the large ducts of an all-air system. They also bring maintenance obligations: hundreds of small fans, filters, drain pans and valves spread throughout a building. This guide covers the main types, piping arrangements, ventilation strategies and the practical issues owners should plan for.

Unit configurations

Horizontal ducted units allow more flexible air distribution, while vertical exposed units are easier to service but take floor space.

Two-pipe and four-pipe systems

Two-pipe

A two-pipe system has one supply and one return pipe to each unit. The whole system, or each zone, is switched between chilled water in summer and hot water in winter. Two-pipe systems are cheaper to install, but they cannot provide heating in one room and cooling in another at the same time.

Changeover is the main weakness. In Canadian shoulder seasons, sunny south-facing rooms may need cooling while north rooms need heat. Switching the whole loop takes time, since a large mass of water must change temperature, and frequent changeover wastes energy. Some buildings add electric heat to the units to cover shoulder seasons, which raises operating cost.

Four-pipe

A four-pipe system has separate chilled water and hot water circuits, with either separate coils or a single coil with a changeover valve. Each unit can heat or cool independently at any time. Four-pipe costs more to install but gives much better comfort control, which is why it is common in hotels, offices and healthcare.

Pipe and valve considerations

• Use two-way control valves with variable speed pumping for energy efficiency; three-way valves bypass flow and waste pump energy.
• Pressure-independent control valves simplify balancing in large systems. See control valve selection and authority.
• Provide isolation valves, strainers and unions at each unit for maintenance.
• Insulate chilled water piping and valves fully with a vapour barrier to prevent condensation.

Providing ventilation

Fan coils recirculate room air and do not, on their own, provide adequate outdoor air. Common approaches include:

• Dedicated outdoor air system: a central DOAS delivers conditioned outdoor air directly to each room or to the fan coil inlet. This is now the most common approach. See dedicated outdoor air systems.
• Corridor supply: outdoor air is supplied to corridors and transferred under doors. This older approach, common in apartments, is unreliable and is not considered adequate ventilation under current practice.
• Through-wall outdoor air: individual units draw outdoor air through a wall louver. This is difficult to control and creates freeze risk in Canadian winters.

Delivering DOAS air directly to the room, rather than into the fan coil return, makes ventilation independent of whether the fan coil fan is running. If DOAS air is ducted into the fan coil, the fan must run whenever the space is occupied, which adds fan energy.

Fan coil fan motors

Older fan coils commonly used permanent split capacitor (PSC) motors with three fixed speeds. Electronically commutated (EC) motors are now standard on most new units. EC motors are significantly more efficient, especially at reduced speed, and allow variable speed control. Running fans at low speed most of the time reduces both energy and noise.

Upgrading existing units to EC motors or modern fan coils is a common energy measure in hotels and apartment buildings.

Noise

Fan coils sit inside or right next to occupied rooms, so noise matters. Considerations include:

• Select units at a speed that meets the room noise criterion at design airflow, not at maximum fan speed.
• Avoid excessive external static pressure on ducted units, which forces higher fan speed.
• Use flexible pipe connections and vibration isolation for ceiling units over sensitive spaces.
• Locate horizontal units above corridors or washrooms where possible rather than over beds or desks.

Condensate management

Fan coils serving cooling loads remove moisture, producing condensate. Problems with drain pans are a frequent source of water damage and microbial growth.

1. Slope drain pans to drain, made of corrosion-resistant material such as stainless steel or plastic.
2. Trap and vent condensate lines correctly for the unit's static pressure.
3. Slope drain lines continuously, typically at least 1% (1/8 in. per ft), and size for the connected load.
4. Provide secondary drain pans or water sensors with shutoff for units above finished ceilings, where codes or owners require.
5. Clean pans and lines as part of preventive maintenance.

Maintenance planning

In large buildings with hundreds of units, access matters as much as frequency. Horizontal units need access panels large enough to remove the filter and fan assembly, and condominium owners need clear guidance on filter changes.

Controls

Fan coils are typically controlled by room thermostats, either standalone or networked to a BAS. Networked controls allow:

• Occupied, unoccupied and setback schedules.
• Integration with hotel room booking or occupancy sensors.
• Alarms for high condensate levels or failed fans.
• Trending of valve positions to identify problems.

Frequently asked questions

Are fan coils more efficient than VAV?

It depends. Fan coils with EC motors and a DOAS can have low fan energy because ventilation air is moved separately and fan coil fans run at low speed. VAV systems benefit from airside economizers. An energy model comparing systems for the specific building is the best basis for a decision.

Can existing two-pipe systems be improved?

Yes, through better changeover controls, zoning by exposure, adding small heating coils, or converting to four-pipe during a major renovation. A high-efficiency HVAC review can identify the best option.

Key takeaways

• Fan coils provide room-by-room heating and cooling using central chilled and hot water.
• Four-pipe systems allow simultaneous heating and cooling; two-pipe systems struggle in Canadian shoulder seasons.
• Ventilation should come from a DOAS, ideally delivered directly to each room.
• EC motors reduce energy and noise and are a common retrofit measure.
• Condensate management and access for maintenance are critical to long-term performance.