Fan-coil units at the heart of effective air conditioning

Following the recent publication of a CIBSE Technical Memorandum, Mike Price reviews why chilled-water fan-coil units are one of the most effective means of providing air conditioning. In recent years, installations of fan-coil units have become one of the most effective means of providing air conditioning in new-build and refurbished commercial premises. The benefits provided by fan coil unit systems include the following. • Significantly smaller ventilation plant and distribution ductwork than an all-air system.
• High cooling capacity.
• Comparatively low cost of capital equipment.
• Individual zone control of space temperature, if suitable controls fitted.
• Flexibility to accept future changes in load and space layouts (office ‘churn’). Fan-coil units are particularly suited to applications with intermittent medium to high sensible cooling loads and where close humidity control is not required, e.g. offices, hotels, restaurants etc. Most of these installations use units mounted in the ceiling void as they do not occupy floor space within the building, although a significant number of projects still specify vertically mounted cased/recessed units. The basic reason for installing a fan-coil system is to provide ‘comfort conditioning’ at the specified environmental conditions within the space.

Fig. 1. Fan-coil units can be installed in a ceiling void, with the ceiling void used as a plenum for the return air from the room to be re-conditioned and to introduce ventilation fresh air from a central air-handling unit.

Whilst not an inherently complicated system, a number of problems can arise with an installation comprising fan-coil units — such as noise, thermal output and draughts. Care therefore needs to be taken in the manufacture, design, selection and installation of the equipment to avoid common pitfalls. The recently launched CIBSE Technical Memorandum, TM43 ‘Fan coil units’* sets out to explain all aspects of this technology and includes guidance and advice on the common problems encountered. As way of explanation, fan-coil units predominantly recirculate return air from the room, but fresh air can also be introduced via the fan-coil unit. Fig. 1 shows how the fan-coil unit and other ventilation services are positioned in a ceiling void. The ceiling void is used as a plenum for the return air from the room to be re-conditioned and to introduce ventilation fresh air from a central air-handling unit (AHU). These two air flows are introduced into the vicinity of the air inlet of a fan-coil unit and then drawn into the unit for conditioning. For vertically mounted, cased or recessed fan-coil units, these normally provide air conditioning at building perimeter locations, such as under windows where cooling loads can be higher, see Fig. 2.

Fig. 2. Vertically mounted, cased or recessed fan-coil units normally provide air conditioning around the building perimeter, such as under windows where cooling loads can be higher; they can also be suitable for compartment enclosures within the building core.

They can also be suitable for compartment enclosures within the building core. Fan-coil units can be evaluated against other air-conditioning systems such as constant-volume all-air central plant, single/dual duct VAV, chilled beams/ceilings, water-loop heat pump or split/multi-split systems. Using the SBEM1 calculation tool, the carbon-dioxide emissions for a fan-coil unit system installed in the office type ‘example building’ compare favourably against most other HVAC systems (Fig. 3). If used with a well designed fresh-air AHU a fan-coil-unit system will readily comply with current Building Regulations, surpassed only by more expensive and less flexible systems such as chilled beams/ceilings. A number of design features can be incorporated into fan-coil-unit systems to reduce building emissions.

Fig. 3. Air conditioning using fan-coil units can generally be expected to give a good result when using the SBEM calculation tool and come in better than the target emission rate for Part L, 2006. Commercial software packages validated for Part L may give more, or less, favourable results to those shown.

Good commercial building design simulation software packages, validated for Part L, should be able to incorporate the following features into the emission calculation.
• Reduction of the specific fan power (SFP) of the fan coil by using 4-pole AC fan motors or further still by using DC electronically commutated DC/EC fans.
• Variable air volume by reducing the fan speed when higher design-day air volumes are not necessary. This can be achieved in speed steps using suitable controls or by variable speed control using DC/EC motor fans.
• Fan coil units to meet the entire cooling load and most of the heating load. The fresh-air AHU operates with a depressed heating set point. Modern AHU fan technology can also be incorporated to reduce the specific fan power of the central plant.
• Fan-coil units with oversize cooling coils can be specified for use with lower-grade (higher-temperature) chilled water. This will promote more efficient operation of the central chillers or enable the use of free cooling systems and ground-source heat pumps, see BSRIA BG8/2004: ‘Free cooling systems’2.
• 2-port water control valves can be used to save energy by reducing the water flow rate of system pumps below their ‘design day’ performance during part-load conditions.
• The fresh-air AHU and its associated energy emissions can be removed and fresh air ducted direct to the fan-coil units. The use of an advanced damper system could provide a fan-coil unit that operates in recirculation and fresh-air modes but still satisfies Part L building air-leakage requirements.
Mike Price is a member of the Hevac fan-coil unit group. References: 1. Simplified Building Energy Model (SBEM): Calculation tool for non-domestic buildings to demonstrate compliance with the National Calculation Methodology (Garston: Building Research Establishment) (2005) (available from 2. Free cooling systems BSRIA BG8/2004. Building Services Research & Information Association, 2004.
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