When specifying an air-conditioning system, careful selection of components to meet the particular needs of the client is critical. Keith Carter of Mott Green and Wall describes the key decisions when specifying a system
1. Performance requirements
The first step is to identify the client's requirements, which could be:
  • Performance standards such as comfort cooling only or close environmental control
  • Occupier comfort
  • Control strategies
  • Resilience requirements
  • Environmental concerns
  • Operating costs and maintenance
  • Integration of services structure and architecture.

2. Selecting the options
Next, identify options which will meet the client's requirements and the estimated cooling load. The main system options available are:

  • Conditioned space
  • Variable air volume systems
  • Fan coil units
  • All air systems with re-heat
  • Displacement ventilation, potentially with static cooling
  • Dual duct systems
  • Induction systems
  • Main compound options.
Air supply and water
  • Options such as 100% fresh air or partial recycling
  • Primary heat source: LTHW coils, direct gas firing
  • Heat exchange requirements
  • Humidification requirements.
Cooling medium
  • Chilled water or refrigerant gases.
Cooling generation
  • Heat pumps, absorption chillers, centrifugal chillers, reciprocating chillers, screw chillers.
Heat rejection
  • Cooling towers, air-cooled condensers, dry-air coolers.
Alternative systems have different whole-life costs and environmental implications, which should be considered alongside initial cost and programme criteria.

3. Project constraints
The choice of system will also be affected by constraints imposed by the design, utilities infrastructure and statutory requirements, such as:

  • Size and location of plant space
  • Restrictions on the use of roof space
  • Health and safety issues
  • Capacity, location and suitability of riser space
  • Capacity and suitability of the utilities infrastructure.

4. Components of the specification
The specification document will define and confirm the requirements of the project. Discrete sections of the specification are typically written as separate documents by each building services consultant. Use of the "common arrangement" format provided by the National Engineering Specification helps to facilitate the production of an integrated document.

The specification document should include the following sections:

  • References to the contract conditions
  • Scope of works
  • Standards of materials and components
  • Schedules of equipment
  • Commissioning requirements
  • Testing requirements
  • Level of record documentation
  • Proving and witnessing of system performance
  • Off-site testing of packaged plant or components.

5. Standards
The proposals specified should comply with all current codes of practice, Building Regulations (including retrospective issues that may need to be assessed), and health and safety guidelines.

Generally, the specification will refer to the following classes of documents:

  • British standards
  • European standards
  • Heating and Ventilation Contractors Association codes of practice
  • Electrical Contractors Association codes of practice
  • Chartered Institution of Building Services Engineers guidelines
  • Local authority and fire officer requirements
  • Water and waste bylaws.

6. Continuing review of the design documentation
The consultant's standard specification should be regularly reviewed to ensure that it reflects best practice and allows for the most effective selection of materials and on-site installation methods. Good practice in reviewing and updating specification documentation will ensure that an appropriate balance between innovation and the tried and tested is utilised to deliver the best possible solution.

Air-conditioning and environmental control