HVAC
Chilled beam commissioning field guide: active and passive
Active and passive beams, the warm-water dew point margin, the DOAS pairing, and the condensation test that proves the coil stays dry.
Direct answer
A chilled beam is a ceiling-mounted water coil that cools a space by convection with no fan in the room, doing sensible cooling only. Passive beams cool by natural convection; active beams induce room air across the coil with primary ventilation air. The chilled water stays above the room dew point, and a dedicated outdoor air system handles humidity.
Key takeaways
- Chilled beams do sensible cooling only; a DOAS dries the outdoor air below space dew point to carry the latent load.
- Run warm chilled water, commonly 57 to 60°F, kept at least about 2°F above room dew point so the coil never sweats.
- Active beams induce 3 to 5 units of room air per unit of primary air, so deliver primary air at design nozzle static, not just volume.
- Never send chilled water to beams in a wet building: start the DOAS first and pull space dew point to design.
- The condensation test under design humidity must confirm the water reset holds the coil dry and the condensation sensor closes the valve and alarms.
A chilled beam, and where it fits
A chilled beam is a finned water coil mounted at the ceiling that cools the space by moving air across cold water, with no fan running in the room. It does sensible cooling, dropping the air temperature without wringing out moisture. The quiet comes from there being no compressor, no fan, and almost no moving parts in the occupied space. The work happens at the coil, and the air moves itself.
Where it fits is offices, labs, schools, and other spaces with a high sensible load and a modest latent load, where people want quiet and the owner wants low energy and low maintenance. A fan coil unit puts a fan, a filter, and a condensate pan over the ceiling tile, and every one of those is a maintenance call waiting to happen. A VAV box throws a lot of air to carry its cooling. A chilled beam carries most of the cooling in water, which is far denser than air, so the ducts shrink and the fan energy drops.
The catch is that a chilled beam only does half the job by itself. It handles sensible heat. The moisture in the air, the latent load, has to be handled somewhere else, by the primary air. Miss that and you get condensation, which is the failure that defines this whole system.
Active or passive chilled beam: what is the difference?
Passive and active beams both cool with a water coil. The difference is where the air movement comes from. A passive beam has no primary air connection at all. The coil cools the air touching it, that air gets denser and falls, warmer room air rises to replace it, and the loop runs on natural convection alone. Passive beams do cooling only, and they need a completely separate system to bring in ventilation air.
An active beam adds a primary air supply. Ducted ventilation air, already conditioned by the air handler, blows through small nozzles inside the beam. That jet of primary air induces room air up through the coil, several times more room air than the primary air alone. So an active beam does two jobs at once. It delivers the ventilation air, and it drives a lot more air across the coil than convection ever would. More air across the coil means more capacity in the same footprint.
That extra capacity is why active beams are the common choice on real projects. A passive beam is the right tool where you want pure sensible cooling with no ducting to the unit, like over a corridor or a space where the ventilation comes from somewhere else. For most offices and labs, the active beam earns its place because it folds ventilation and cooling into one ceiling unit.
How does an active beam's induction work?
Induction is the engine of the active beam, and the number that describes it is the induction ratio: how much room air the beam pulls across the coil for every unit of primary air pushed through the nozzles. Commonly that ratio runs about 3 to 5 to 1, so a small stream of ducted primary air drags three to five times its own volume of room air through the coil. The manufacturer's data sheet gives the real number for the unit, because it depends on the nozzle design and the primary air pressure.
The primary air is doing three things at once. It carries the outdoor ventilation air the space needs. It carries the dehumidification, the dry air that handles the latent load. And its velocity through the nozzles is what creates the induction that drives the coil. Drop the primary airflow or the nozzle pressure and you lose induction, which means you lose coil capacity and you lose the dehumidified air that keeps the space dry. The primary air is not optional trim. It is what makes the beam work.
This is the part a balancer cannot eyeball. The induction ratio is set by the nozzle size and the supply pressure at the beam, so the primary air has to be delivered at the design static, not just the design volume. A beam starved of nozzle pressure underperforms quietly and never throws a flag.
Why do chilled beams use warm water?
Chilled beams run warm chilled water, commonly around 57 to 60°F supply, sometimes stated as wide as 55 to 63°F, against the 39 to 45°F a conventional cooling coil sees. The reason is condensation. The coil sits in the occupied space with no reliable way to drain water, so the water has to stay above the room dew point or the coil sweats. Warm water is the price of a dry coil.
The warm water pays you back at the chiller. A chiller making 58°F water instead of 44°F has a much smaller lift, and a smaller lift means less compressor work for the same cooling. The figure that gets quoted is roughly 15 to 20 percent better chiller efficiency on the beam loads, and that is before you count the hours a waterside economizer or cooling tower can make 58°F water with the compressors off. In a mild or dry climate, free cooling on the beam loop is a real chunk of the year.
So the warm water is not a compromise you tolerate. It is half the reason the system saves energy. The other half is the fan energy you never spend, because the cooling rides in water instead of air.
Do chilled beams condensate?
Yes, a chilled beam will condensate if you let the coil fall below the room dew point, and that is the single failure that defines the system. A beam doing sensible cooling has no condensate pan, so any water that forms drips onto the desk or the floor below. Keep the chilled water above the space dew point and the coil stays dry. Let the dew point climb above the water temperature and you get drips, stains, and a callback.
The margin people design to is small but firm: keep the supply water at least about 2°F above the room dew point, sometimes stated as 1°C. That is not much room, which is why dew point gets watched actively instead of set once. Two defenses do the watching. A space or return dew point sensor resets the chilled water temperature up when humidity rises, and a condensation sensor on the beam supply pipe or the coil trips the water valve closed if moisture is detected. The reset keeps you out of trouble. The condensation sensor is the last line that catches the day something else failed.
Some beams ship with a small drain pan as backup, but a drain pan is an admission that the controls might fail, not a license to run the coil cold. The dew point side of this is its own subject. Surface temperature, dew point, and where condensation forms are covered in the humidification and dehumidification control guide, and the physics there is exactly what governs the beam coil.
What handles the humidity if the beam only does sensible cooling?
A chilled beam does sensible cooling and almost no latent cooling. It drops the dry-bulb temperature. It does not pull meaningful moisture out of the air, because its coil runs too warm to condense. That is by design, since a dry coil is the whole point. But the building still makes moisture, from people, from outdoor air, from processes, and that latent load has to go somewhere.
It goes to the primary air. The sensible load and the latent load are split, decoupled, and handled by two systems sized for what each does well. The beam takes the sensible heat with warm water and no fan energy. The primary air, dried down hard before it reaches the space, takes the latent load and the ventilation. Each system runs at its best operating point instead of one air system trying to do both and overcooling the air just to wring out moisture.
The decoupling is the design idea behind the whole approach, and it only holds if the latent side actually keeps up. If the primary air cannot hold the space dew point down, the beam coil is the surface that pays for it. How the latent side is built, steam versus cooling-coil dehumidification and how dew point is held, is the subject of the humidification and dehumidification control guide.
The DOAS that feeds the primary air
The primary air on a beam system almost always comes from a dedicated outdoor air system, a DOAS. A DOAS conditions 100 percent outdoor air, dries it well below the space dew point, and delivers that dry ventilation air to the beams. It is the latent half of the decoupled pair, and the beam depends on it completely.
A DOAS delivers a small volume of air at a low dew point, the opposite of a conventional system that moves a large volume at a moderate temperature. Because the beam carries the sensible cooling, the DOAS only has to move the ventilation quantity, so the ducts and the fan are small. The ventilation fan energy on a beam-and-DOAS system runs on the order of 0.25 to 0.5 in. w.c. against the 3 to 8 in. w.c. a central all-air system spends, which is most of the fan-energy savings the system claims.
Pair them correctly and they cover each other. The DOAS holds the dew point so the beam coil stays dry, and the beam takes the sensible load so the DOAS never has to overcool. Break the pairing, undersize the DOAS or let it lose dehumidification, and the beam is the first thing that sweats.
The water side and the flow
On the water side a chilled beam is a coil like any other, balanced to a flow and a delta-T, and the same waterside discipline applies. The flow per beam is small, the pressure drop is low, and the delta-T is modest because the water is warm to begin with. You set flow, not pressure, and you confirm it at the beam or the branch the same way you would any hydronic coil. The balancing approach, circuit setters, PICVs, and proportional balancing, is the subject of the hydronic system balancing guide, and chilled beams ride on that work.
Two-pipe and four-pipe both show up. A two-pipe beam is cooling only, or it changes over seasonally between chilled and heating water for the whole zone. A four-pipe beam has a separate heating coil or a second circuit so it can cool and heat without a changeover, which is what you want where some zones need heat while others need cooling on the same day.
The waterside number that bites on beams is dew point, not delta-T. A coil balanced perfectly to flow will still drip if the supply water sits below the room dew point, so the waterside reset that holds water temperature above dew point matters more here than the last fraction of a degree of delta-T.
Layout, throw, and draft
Beam placement is an air-distribution problem, not just a capacity problem. An active beam discharges along its length, and that discharge has a throw and a pattern that has to wash the space without dumping cold air on people. Get the layout wrong and you trade quiet, even comfort for a draft complaint at every desk under a nozzle.
The throw is set by the primary air and the nozzle pattern, and it has to reach far enough to mix the space but not so hard that it lands as a cold jet in the occupied zone. Beams discharging toward each other across an aisle can collide and dump the combined air straight down, which is the classic draft complaint. Beams too far apart leave a warm dead spot between them. The manufacturer's throw data at the design primary airflow is what you lay out to, and the spacing follows the throw, not a tidy reflected-ceiling grid.
Perimeter zones need their own thought. A beam near a glass wall is fighting a downdraft of cold air off the glass in winter and solar gain in summer, so perimeter beams often carry heating or sit where their throw can break up the glass downdraft. Lay the beams out for the air pattern first and let the ceiling grid follow.
The controls and the dew point interlock
The control on a chilled beam is simpler than a fan coil because there is no fan to stage, but it carries one job a fan coil does not: protecting against condensation. The room thermostat modulates the chilled water valve to hold space temperature, opening the valve for more cooling and closing it as the space satisfies. On a four-pipe beam the same loop switches to the heating valve when the space calls for heat.
Layered on top is the dew point and condensation protection, and it overrides comfort every time. A space dew point sensor feeds a chilled water temperature reset, so when humidity rises the plant raises the supply water to stay above dew point, trading a little cooling capacity for a dry coil. A condensation sensor at the beam closes the water valve outright if it detects moisture, regardless of what the thermostat wants. Comfort loses to condensation protection, always, because a warm office is a complaint and a dripping beam is damage.
The primary air and the DOAS sit in the building management system alongside the beams, and the sequence ties them together. The BMS has to know not to send cold water to the beams until the DOAS has the space dew point under control, which is the startup interlock that keeps a humid building from sweating its ceilings on day one.
Commissioning a chilled beam system
Commissioning a beam system is four checks that have to pass together: the water is balanced, the primary air is right, the space stays dry, and the space stays comfortable. Skip the order and you can chase a comfort problem that is really an airflow problem, or a sweating beam that is really a controls problem.
Start with the water. Balance the flow to each beam or branch and confirm the delta-T, the same waterside test and balance covered in the hydronic balancing guide, because a beam starved of flow makes no capacity and a beam with runaway flow steals it from the next one. Then the primary air. Set the primary airflow and, on active beams, confirm the static pressure at the nozzles, because the induction ratio and the dehumidified-air delivery both depend on nozzle pressure, not just volume. Verify the DOAS is actually drying the air to its design leaving dew point.
Then the test that matters most, the condensation test, under design humidity. And finally the comfort check: walk the space under load and confirm no drafts, no dead spots, and that the beams hold setpoint. AABC and NEBB procedures cover the test-and-balance side. The controls and dew point verification follow the sequence of operations and the manufacturer's startup. The agent's job is to make the system fail loudly on the bench before it fails quietly over an occupant's desk.
The condensation test
The condensation test is the heart of beam commissioning, because it proves the one failure the system is built to avoid will not happen. You raise the space to its design humidity, or as close as you can drive it, then watch whether the coil stays dry and whether the protection trips when it should.
Two things get verified. First, that under the design space dew point the chilled water reset holds the supply water above that dew point and the coil runs dry, no beading on the pipe, no drips. Second, that the condensation sensor actually works. Push the conditions until the sensor should trip, or test the sensor directly, and confirm it closes the water valve and the BMS sees the alarm. A condensation sensor nobody ever proved is a condensation sensor that fails silent the first humid weekend.
Do this with an infrared thermometer or a surface probe on the supply pipe and the coil, and a reliable space dew point reading, not the building sensor you have not yet calibrated. The test is not theoretical. On a hot, humid commissioning day you can often create the condition for real by holding the DOAS back briefly under controlled watch. Never do that with the building occupied, or with anything below the beam you would not want dripped on.
Startup sequence and the wet building
Do not send chilled water to the beams in a wet building. This is the rule that gets broken most, usually during construction when the building is still full of moisture from concrete, paint, and an open envelope, and someone wants to prove the beams cool. Cold coil plus humid air equals a sweating beam, and now the first thing the beams ever did was drip.
The sequence is DOAS first. Start the dedicated outdoor air system and let it pull the space dew point down to design before any chilled water reaches the beams. On a humid day the DOAS runs and dries the building, the space dew point comes down, and only then does the chilled water go to the beams. If the DOAS cannot get the dew point low enough, you do not start the beams. You find out why, either more dehumidification at the DOAS or warmer water from the plant, before cold water ever touches a coil in that air.
This applies every startup, not just the first. After a long shutdown, a power loss, or a weekend in summer, the building can be wet again, so the BMS has to re-establish dew point control before it re-enables the beams. Re-energize the beams into a wet building and you have recreated the condensation failure on purpose.
Cleaning and maintenance
The maintenance case is one of the strongest things about beams, and it comes from what is not there. No fan, no motor, no condensate pan, and on most beams no filter in the unit. There is nothing in the occupied-space unit to fail, clog, or grow mold, which is exactly the list of things that generate fan-coil service calls.
What a beam coil does collect is dust, slowly, because room air passes over the fins by induction. Without a filter at the unit, that dust lands on the coil and on the induction nozzles over years. A fouled coil loses capacity and dirty nozzles shift the induction pattern, so the maintenance is periodic coil cleaning, commonly a vacuum and a brush or a light wash per the manufacturer, on a multi-year interval rather than the quarterly filter change a fan coil demands. The DOAS carries the filtration for the system, so its filters are the ones on the regular schedule.
Because the units are simple and passive in the space, the access design matters more than the maintenance frequency. Make sure the beams can be reached and the coils cleaned without tearing the ceiling apart, because a beam nobody can reach is a beam nobody cleans.
The heating chilled beam
An active beam can heat as well as cool, and on a four-pipe layout that is how perimeter zones get their heat without a separate system. A heating coil, or a second circuit in the same beam, takes warm water, and the same induction that drives cooling drives the heating air across the coil. The thermostat and the control valve switch between the chilled and heating circuits as the space calls.
Heating off a beam has a limit worth knowing. The beam discharges its air near the ceiling, and warm air wants to stay near the ceiling. Drive too much heat through a ceiling beam and the warm air stratifies up high while the floor stays cold, so beams carry a modest heating load well but are a poor match for a space with a big heating demand or a lot of glass downdraft. That is why perimeter heating is sometimes left to a separate element under the glass, with the beam handling the cooling and ventilation.
A two-pipe changeover beam heats too, but the whole zone switches between chilled and heating water seasonally, so it cannot cool one room while heating the next. Where simultaneous heating and cooling is needed, four-pipe is the answer, at the cost of the extra piping.
Energy and comfort
The energy story is two numbers. Fan energy drops because the cooling rides in water, so the DOAS only moves the ventilation air, on the order of 0.25 to 0.5 in. w.c. against 3 to 8 for an all-air system. Chiller energy drops because the warm water cuts the lift, roughly 15 to 20 percent on the beam loads, with more from waterside free cooling in the right climate. Together that is a genuinely low-energy way to cool a sensible-heavy space.
Comfort is the other reason owners pick beams. The cooling is part radiant off the cool coil and surfaces and part gentle convection, without a fan blasting air, so the space is quiet and the air motion is low. People who work under beams tend to stop noticing the HVAC, which is the goal. The quiet is real. No fan in the ceiling means a low background noise level, an advantage in offices, libraries, classrooms, and exam rooms.
The comfort only holds if the throw was laid out right, which loops back to layout. A beam dumping a cold jet on a desk is not quiet comfort. It is a draft complaint, and it is the most common comfort failure on an otherwise good system.
Chilled beams in offices, labs, and data centers
Chilled beams make sense in spaces with a high sensible load, a modest latent load, a ceiling to hang them from, and an owner who values quiet and low maintenance. Offices are the bread and butter: steady sensible load from people and equipment, ventilation that a DOAS handles cleanly, and tenants who want a quiet space. Schools and universities use them in classrooms and labs for the same reasons, plus the low noise.
Laboratories are a strong fit because the sensible load is high and the air-change requirement is often driven by ventilation and fume exhaust anyway, so decoupling the sensible cooling from the air matches how a lab already runs. The beams carry the cooling, and the lab air system carries the air changes and the exhaust makeup.
Data centers are the careful case. The dry, sensible-heavy load looks ideal for beams, and high-temperature chilled water aligns with the warm supply the ASHRAE TC 9.9 thermal guidelines now allow. But the heat density in a modern white space is often too high for ceiling beams, and the room dew point has to be controlled hard so the coil never sweats over live equipment, which raises the stakes on the condensation protection. Where they fit a data hall, beams are a quiet, efficient sensible-cooling option. Where the density is high, close-coupled cooling usually wins. Either way, the chilled water and humidity discipline is the same as the hydronic balancing and humidity-control guides describe.
What to document
The beam record has to answer two questions later: did each beam get the flow and air it was designed for, and how much margin is there between the water and the dew point. Capture both per beam or per zone, because the dew point margin is the number that predicts whether the system will ever sweat.
For each beam or zone record the type, the chilled water supply temperature and flow, the delta-T, the primary airflow, the nozzle static on active beams, the design space dew point, and the margin between the supply water and that dew point. Record the condensation sensor test result and the dew point reset setpoint, because those are the protections, and a protection nobody documented is a protection nobody can audit.
| Beam or zone | Type | CHW supply temp / flow | Primary air (CFM / nozzle static) | Dew point margin |
|---|---|---|---|---|
| Open office A1 | Active, 4-pipe | 58°F / 0.6 gpm | 55 CFM / 0.6 in. w.c. | Water 2.5°F above space dew point |
| Lab L2 | Active, 4-pipe | 59°F / 0.9 gpm | 80 CFM / 0.7 in. w.c. | Water 3°F above space dew point |
| Corridor C1 | Passive, 2-pipe | 60°F / 0.4 gpm | None (DOAS to space) | Water 2°F above space dew point |
Common mistakes
- Running the chilled water below the room dew point, which sweats the coil and drips on whatever is below.
- Starting the beams in a wet building before the DOAS has pulled the space dew point down.
- No DOAS, or an undersized one, so nothing holds the latent load and the beam coil pays for it.
- No condensation sensor, or one that was never tested, so the last line of defense fails silent.
- Setting primary air to volume only and ignoring nozzle static, so the induction ratio and dehumidified-air delivery fall short.
- Water flow never balanced, so some beams starve and others run away.
- Laying beams out to the ceiling grid instead of the throw, creating drafts where jets collide and dead spots where they do not reach.
- Treating the beam as if it dehumidifies, and sizing the latent load to it instead of to the DOAS.
Field checklist
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Standards and references
The framework for beams lives across a few bodies, and the manufacturer's data sits at the center, because it sets the water temperatures, flows, nozzle pressures, and induction the unit is rated for. ASHRAE provides the application and design guidance for active and passive beams, including the dew point and decoupled-load approach, and ASHRAE Standard 200 is the test method for rating the equipment. REHVA's chilled beam guidance covers the same design ground on the European side and is widely referenced.
AHRI Standard 1240 (I-P), with 1241 in SI units, is the performance rating standard for active chilled beams. It certifies water flow rate, water pressure drop, water coil capacity, primary air flow rate, induced air flow rate, and sound power, which is the data set you balance and commission against. It does not cover passive beams, which rely on the manufacturer's own data. Test and balance follows AABC or NEBB procedures for the waterside flow and the primary airflow.
The exact edition, the rated conditions, and the setpoints all trace back to the project specification and the manufacturer's submittal. Confirm the water temperature, dew point margin, and primary air requirements against the actual equipment and the spec before you commission, not against a rule of thumb, because the dew point margin in particular is unforgiving.
Units, terms, and conversions
Chilled beam work crosses water-side and air-side units, plus the dew point math that ties them together, so the same idea can read differently across a submittal, a TAB report, and a sequence of operations.
Water flow is gallons per minute (gpm) in I-P and liters per second (L/s) in SI. Air pressure is inches of water column (in. w.c. or in. wg) or pascals (Pa). Temperature is °F or °C, and dew point carries the same units as the air temperature it is measured against. Cooling capacity is in Btu/h or watts, and the induction ratio is dimensionless, a ratio of induced room air to primary air.
- Active chilled beam
- A beam fed with ducted primary air whose nozzles induce room air across the coil, delivering ventilation and added sensible cooling
- Passive chilled beam
- A cooling-only beam with no primary air, cooling the space by natural convection alone
- Induction ratio
- The volume of room air induced across the coil per unit of primary air, commonly about 3 to 5 to 1
- Primary air
- The ducted, dehumidified ventilation air supplied to an active beam, which also drives the induction
- DOAS
- Dedicated outdoor air system, the unit that dries the outdoor ventilation air and carries the latent load
- Sensible vs latent
- Sensible cooling lowers air temperature; latent cooling removes moisture. Beams do sensible; the DOAS does latent
- Dew point margin
- The gap between the chilled water supply temperature and the room dew point, commonly held to at least about 2°F
- Changeover
- A two-pipe arrangement that switches the whole zone between chilled and heating water seasonally; four-pipe avoids it
FAQ
What is a chilled beam?
A chilled beam is a ceiling water coil that cools a space by convection with no fan in the room, handling sensible heat only. Unlike a fan coil, it has no fan, no filter, and no condensate pan over the occupied space, which is why it runs quiet and needs little maintenance.
Active or passive chilled beam: what is the difference?
A passive beam cools by natural convection with no primary air and does cooling only, needing a separate ventilation system. An active beam blows primary ventilation air through nozzles that induce room air across the coil, delivering ventilation and more cooling capacity from the same unit.
Why do chilled beams use warm water?
Chilled beams run warm water, commonly 57 to 60°F, because the coil sits in the space with no drain, so the water must stay above the room dew point to keep the coil dry. The warm water also cuts the chiller lift, giving roughly 15 to 20 percent better chiller efficiency on those loads.
Do chilled beams condensate?
A chilled beam condensates only if the coil falls below the room dew point. Keep the chilled water at least about 2°F above the space dew point and the coil stays dry. A dew point reset and a condensation sensor protect it, and on a sensible beam there is no condensate pan to catch drips.
Do chilled beams need a condensate drain pan?
Most chilled beams doing sensible cooling have no condensate pan, because the coil is kept above dew point so it never sweats. Some models add a small drain pan as backup, but it is a safety net, not a license to run cold water. Dew point control is the real protection.
What is the induction ratio of an active chilled beam?
The induction ratio is how much room air an active beam pulls across its coil per unit of primary air, commonly about 3 to 5 to 1. Higher induction means more cooling capacity, but it depends on nozzle design and supply pressure, so use the manufacturer's rated value, not a generic number.
Do chilled beams need a DOAS?
Chilled beams need a separate system for ventilation and humidity, almost always a dedicated outdoor air system. The beam does sensible cooling only; the DOAS dries the outdoor air below the space dew point and carries the latent load. Without it, nothing holds the dew point and the beam coil sweats.
Can chilled beams provide heating?
Yes. On a four-pipe layout a beam heats with warm water through a heating coil or second circuit, switching from cooling as the space calls. Heating off a ceiling beam stratifies, so it suits a modest heating load. Spaces with heavy glass downdraft often add separate perimeter heat.
Why can't you start chilled beams in a wet building?
Cold water in a humid building sweats the coil, so you run the DOAS first to pull the space dew point down before sending chilled water to the beams. This applies every startup, including after a summer shutdown, because a wet building plus a cold coil drips on whatever is below.
How are active chilled beams rated?
Active chilled beams are rated under AHRI Standard 1240 (I-P), or 1241 in SI, which certifies water flow, water pressure drop, coil capacity, primary airflow, induced airflow, and sound power. Passive beams are not covered and rely on the manufacturer's own data. Commission against the rated values.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.