Active Chilled Beams and Induction Units
What are they?
Active chilled beams consist of a ceiling-mounted cooling coil (supplied by high temperature chilled water, typically 14°C) with air circulated from an air handler forcing airflow through it. Induction units are an older version of the same thing, and come in ceiling and floor mounted types.
Active chilled beams and induction units are frequently used to service the perimeter of buildings due to the relatively high capacity of the units; in such cases, a VAV system is often used to service the internal zones of the building. Less commonly, active chilled beams or induction units are used for internal zones as well.
Outside air, and commonly heating, is provided by central air handling units (AHUs). Most active chilled beams and some induction units also require humidity control within the occupied space to prevent condensation forming on the coils.
Advantages / Disadvantages
- Medium energy efficiency.
- Reduced ductwork and AHU size.
- Reduces flexibility of internal space.
- Operating requirements of active chilled beams and induction units can make the central AHUs inefficient.
- Need for a high temperature chilled water circuit circulating around the occupied areas of the building.
- Dehumidification operation can be energy intensive
- AHU supply air pressure requirements, particularly for older induction units, can lead to excessive fan energy and high levels of AHU air leakage.
- Need for humidity control means that they must only be used in well sealed buildings
Active chilled beams and induction units can achieve good efficiency but are not as efficient as passive chilled beams and are matched or surpassed by better VAV systems.
Dehumidification control can be a significant weak-spot for the efficiency of these systems, leading to excessive chiller and boiler operation.
Running costs for these systems are average to good depending on the quality of installation and operation.
A building management system (BMS) is critical to the successful operation of the system
Retrofit / improvement opportunities
The major areas for improvement for active chilled beam systems are:
- Recommissioning and maintenance of high temperature chilled water flow balancing and chilled beam valve control.
- Recommissioning of primary airflows to the active chilled beams and induction units
- 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
Active chilled beams are often retrofitted to replace older induction units and in doing so typically improve heating/cooling performance and efficiency. Retrofitting would, however, need to be undertaken when a building is vacant.
Mainly used in office buildings.
Floor plate implications
These are generally similar to other air-based systems in terms of floor plate. As they are typically supplied by central AHUs, there can be some loss of floor space due to the riser ducts. The primary supply ducts for these system are generally smaller than for other AHU systems.
Temperature control / Occupant comfort
Active chilled beams and induction units can provide temperature control when well controlled and commissioned. However, neither technology has particularly good air-diffusion characteristics, which may lead to occupant complaints about draughtiness.
Active chilled beams and induction units have a 25 year lifespan, during which the major challenge is to ensure that both waterside and airside balance are well managed, as either can cause the unit to become ineffective.
Ceiling mounted active chilled beams and induction units are difficult to tell from other air-based systems unless one looks in the ceiling space to find the chilled water pipes. Floor mounted induction units are generally mounted around the perimeter of an office floor and are easy to spot.
Questions to ask
- Are they passive or active chilled beams?
- When were the water and air balance last checked?
- Is there a building management system (BMS)?
- Is the system regularly maintained (at least annually) and seasonally commissioned?
AIRAH Guide DA08 HVAC&R An Introduction