Passive Chilled Beams
What are they?
Passive chilled beams are essentially a cooling coil (supplied with high temperature chilled water, typically 14°C) attached to or suspended from the ceiling. Chilled water is circulated through the coil, thereby cooling the air in the room. The cooling happens by natural convective movement of air (hence “passive”) through the coil and to the occupied space.
Outside air, and commonly heating, is provided by central air handling units (AHUs) and chillers. Chilled beams also require humidity control within the occupied space to prevent condensation forming on the coils.
Advantages / Disadvantages
- High energy efficiency and low running costs.
- Quiet operation.
- Low maintenance costs.
- Reduced ductwork and AHU size.
- Reduces flexibility of internal space.
- Need for a high temperature chilled water circuit circulating around the occupied areas of the building.
- High installation cost
- Less common system type can lead to design, operation and maintenance issues
- Dehumidification operation can be energy intensive
- Generally small overall plant means that systems maintain a low level of energy use even when operating poorly, but at the expense of occupant comfort.
- Need for humidity control means that they must only be used in well sealed buildings
- Limited ceiling space requirements
Passive chilled beams typically have low energy consumption and running costs, although they are matched by some of the better VAV systems. Passive chilled beams work best when there is an exposed ceiling soffit, but can also be used in conjunction with a suspended ceiling.
Dehumidification control can be a significant weak-spot for the efficiency of these systems, leading to excessive chiller and boiler operation. Superior performance will be achieved in drier climates.
Chilled beams / ceilings can be expected to have low running costs. There will be less demand placed on chillers which will, therefore, consume less energy.
A building management system (BMS) is critical to the successful operation of the system
Retrofit / improvement opportunities
The major areas for improvement for passive chilled beam systems are:
- Recommissioning and maintenance of high temperature chilled water flow balancing and chilled beam valve control.
- Testing and flushing to remove air build up from chilled beams
- Optimisation of dehumidification controls
- Upgrade of controls for boilers, chillers and associated pumps
- Upgrade of chiller plant. Chillers have 15-20 year lifespan, although they generally have been superseded in terms of efficiency well before the end of their operational life. Replacement of chillers is best undertaken in winter, but care is required as passive chilled beams still need chilled water through winter.
Control improvements can be implemented with the tenants in-situ
Chilled beams can be retrofitted into existing buildings as long as there is adequate ceiling space and the building is well sealed. Retrofitting would, however, need to be undertaken when a building is vacant.
Mainly used in office buildings.
Floor plate implications
Chilled beams require more work to relocate than air-based systems. Open ceilings and perforated metal ceilings may not be aesthetically pleasing to some occupants.
Temperature control / Occupant comfort
Passive chilled beam systems can be slow to respond if the demand for cooling increases rapidly and can be less effective at dealing with peak temperatures than more conventional systems.
Passive chilled beams are unusually quiet, to the extent that white noise generators are often added to reduce the distance across which occupant conversations carry.
Passive chilled beams and ceilings have very low maintenance requirements and a 30+ year life span. Passive chilled beams can be badly affected by poor waterside balancing and by the accumulation of air in poorly installed beams.
Passive chilled beams are generally directly visible in the occupied space or are hidden above a perforated metal grid ceiling. The beams look like the wire and tube radiators that used to be on the back of old fridges.
Questions to ask
- Are they passive or active chilled beams?
- When was the water balance last checked for the chilled beams?
- Is there a building management system (BMS)?
- Is the system regularly maintained (at least annually) and seasonally commissioned?
- Is the system providing good comfort control?
AIRAH Guide DA08 HVAC&R An Introduction