6.6 Mechanical ventilation and air conditioning (MVAC)
Mechanical ventilation is a primary energy intensive process and air conditioning is even more so. When considering the installation of mechanical ventilation (not including refrigeration) and air conditioning (including heating and cooling elements) (MVAC), attention should therefore be given to the following:
form and fabric of the building
energy efficiency of the equipment, and
control of the equipment.
For new buildings the zone by zone approach explained in the guidance to Standard 6.1 allows designers the flexibility to assign cooling only to those zones of an otherwise heated only building, where heating and cooling is required, therefore reducing the overall energy use and carbon dioxide emissions of the building.
Designers may wish to design beyond the current guidance and consider the risks of higher internal temperatures occurring more often due to solar gains. CIBSE Technical Memorandum 37 (TM37) ‘Design for Improved Solar Shading Control'(http://www.cibse.org/) gives guidance on this issue.
The designer should consider natural ventilation strategies appropriate for the building geometry (which could include a combination of brise soleil, natural ventilation controls and daylight controls). Particular attention should be paid to limiting solar gains by ensuring that areas of the external building fabric which are susceptible to solar gain have appropriate areas of solar shading. A ventilation strategy that incorporates night cooling and the thermal mass of a building should also be considered.
Free cooling should be optimised in order to minimise the need for mechanical ventilation and air conditioning. When the external air temperature is higher than the space temperature the system design should allow the provision of a minimum level of fresh air. Enthalpy control should also be considered to improve free cooling.
Should natural ventilation fail to achieve the required occupied period temperature, the designer could consider mixed-mode ventilation. A mixed-mode building integrates the use of air conditioning when and where it is necessary, with use of natural ventilation whenever it is feasible or desirable, to maximise occupant comfort whilst reducing energy use (compared to 'year round' use of air conditioning).
Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirement of this standard in so far as is reasonably practicable, and in no case worse than before the conversion (regulation 12, schedule 6).
The form and fabric of the building should mitigate solar gains and result in minimum installed capacity of mechanical ventilation and cooling equipment. When considering the proportions of glazing in the building, the designer should also consider the provision of daylight controls and adequate levels of daylight – refer to BS 8206-2: 2008 'Lighting for buildings. Code of Practice for daylighting' for guidance on daylighting. For example, CIBSE suggest that for office type spaces, the number of occupied hours above 28ºC should not exceed 1% of the annual occupied period. CIBSE Technical Memorandum 37 (TM37) ‘Design for Improved Solar Shading Control' (http://www.cibse.org/) provides a method for assessing the risks of excessive temperatures occurring in the building.
Guidance on the efficiency of mechanical ventilation and air conditioning systems is given in the Non-domestic Building Services Compliance Guide for Scotland http://www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks/ndbscg.
The document replicates guidance published in support of building standards elsewhere in the UK and supports standardisation of the specification and expected performance of fixed building services throughout the UK. The guidance applies to new systems and replacement, in whole or in part, of existing systems. It also addresses improvement work to existing systems as a consequence of replacing components.
Clause 6.6.3 provides information on situations not addressed in that document.
Where work to an existing building is subject to a building warrant and includes the provision of new fixed building services or alters or extends the capacity of existing fixed building services, the opportunity should be taken to review and improve the performance of fixed building systems.
Guidance on the extent to which improvement should be made is given in annex 6.D – ‘Improvement to the energy performance of existing building services when carrying out building work’.