Chilled beams: active and passive systems

Chilled beams cool spaces by circulating chilled water through a finned coil mounted at or in the ceiling. They move much less air than all-air systems because water carries most of the cooling, and water is far more effective than air at transporting heat. That can mean smaller ducts and air handlers, lower fan energy, quiet operation and reduced floor-to-floor height.

Chilled beams have been widely used in Europe and are increasingly common in Canadian offices, laboratories, healthcare facilities and higher education buildings. This guide explains the difference between active and passive beams, their limits, and the design details that matter most.

Passive chilled beams

A passive chilled beam is a coil inside a casing, mounted at or above the ceiling, with no ducted air connection. Warm room air rises to the beam, is cooled as it passes over the coil, becomes denser and falls back into the room. Cooling occurs by natural convection.

Characteristics:

• No fan or primary air connection, so they are silent and have no moving parts.
• Cooling capacity is modest, limited by natural convection.
• Require a separate system for ventilation, dehumidification and heating.
• Need open ceiling area or perforated ceiling around the beam so air can circulate.

Passive beams suit spaces with moderate sensible loads and a separate source of ventilation, such as a DOAS with displacement or ceiling outlets.

Active chilled beams

An active chilled beam is connected to a primary air supply, usually from a dedicated outdoor air system. Primary air is discharged through nozzles inside the beam, creating a high-velocity jet that induces room air through the coil. The mixed air, primary plus cooled room air, is discharged into the space through slots along the beam.

Characteristics:

• Higher cooling capacity than passive beams because induction forces airflow across the coil.
• Primary air provides ventilation and latent cooling.
• Can include a heating coil (four-pipe) or use the same coil for heating and cooling with changeover (two-pipe).
• Discharge pattern is similar to a linear slot diffuser, with Coanda attachment to the ceiling.
• Require primary air at sufficient pressure to drive the nozzles, typically in the range of about 50 to 250 Pa (0.2 to 1.0 in. w.c.) at the beam, depending on the product.

Comparison

Condensation control: the critical issue

Chilled beams are sensible-only devices. They are not designed to drain condensate, so the coil surface must stay above the room dew point. If the space dew point rises above the chilled water supply temperature, water condenses on the coil and drips into the space.

The design approach has three parts:

1. Elevated chilled water temperature: beams typically use chilled water supplied at roughly 14 to 17 C (57 to 63 F), well above the 6 to 7 C (43 to 45 F) used for conventional coils.
2. Primary air dehumidification: the DOAS must deliver air dry enough to absorb the latent load of the space, keeping the space dew point below the beam water temperature.
3. Controls safeguards: dew point sensors in the space or condensation sensors on the beam piping, with logic to reset chilled water temperature upward or close beam valves if dew point approaches the supply water temperature.

Building envelope matters too. Operable windows, poor air sealing or frequent door openings can let humid outdoor air in during summer, so many designers interlock window contacts with beam valves. See condensation and mould prevention for the broader topic.

Chilled water plant implications

The elevated chilled water temperature opens opportunities:

• Chillers serving beams can run at higher evaporator temperatures, improving efficiency.
• A dedicated medium-temperature chilled water loop, or a blending arrangement off a primary loop, can serve beams while low-temperature water serves the DOAS coil.
• Waterside economizers can provide free cooling for many more hours per year in Canadian climates, since outdoor conditions often allow cooling tower or dry cooler water at 14 C or warmer for much of the year.
• Heat recovery chillers can supply both the beam loop and heating loops.

See chilled water system design fundamentals for loop design.

Where chilled beams fit and where they do not

Good candidates:

• Offices with moderate sensible loads and controlled envelope.
• Laboratories where air change requirements are set by safety but sensible loads are high, letting beams handle cooling without increasing airflow. This depends on the lab's minimum air change rate.
• Patient rooms and clinics, subject to CSA Z317.2 and infection control review.
• Classrooms and university buildings.

Poor candidates:

• Spaces with high latent loads, such as kitchens, pools, gyms or densely packed assembly areas.
• Spaces with high or frequent outdoor air infiltration.
• Spaces where the primary airflow required for ventilation is much greater than what the beams need, eliminating the benefit.

Design checklist

1. Calculate sensible and latent loads separately for each zone.
2. Size the DOAS supply dew point to handle the latent load.
3. Select beams for sensible capacity at the chosen water temperature and primary airflow.
4. Check primary airflow against ventilation requirements in ASHRAE 62.1 or local code.
5. Coordinate beam layout with lights, sprinklers and ceiling systems.
6. Design hydronic piping with balancing and isolation per beam or group; plan for water balancing.
7. Specify dew point monitoring and condensation prevention controls.
8. Plan access for coil cleaning and nozzle inspection.

Frequently asked questions

Do chilled beams need maintenance?

Very little compared with fan coils. There are no fans, filters or drain pans. Coils should be vacuumed or cleaned periodically, and active beam nozzles inspected.

Can chilled beams provide heating?

Active beams can heat with warm water, typically at low supply temperatures that pair well with condensing boilers and heat pumps. Overhead heating has limits at cold perimeters, so heavily glazed spaces may still need perimeter heat.

Key takeaways

• Passive beams cool by natural convection; active beams use primary air to induce room air through the coil.
• Chilled beams handle sensible cooling only; ventilation and dehumidification come from primary air.
• Space dew point must stay below the chilled water temperature, typically 14 to 17 C.
• Higher water temperatures improve chiller efficiency and extend free cooling hours in Canada.
• Beams suit offices, labs and healthcare but not high-latent or leaky spaces.