Heating, Ventilation & Air Conditioning

The characteristics of centralised HVAC plant used in the base-build will directly influence the design, installation and operation of HVAC plant specified for the fit-out. Typically, a base-build may provide centralised heating plant, and potentially centralised chillers and air-handling plant. 

The base-build plant can provide heating, cooling and fresh air in a variety of different ways such as hot/chilled water, hot/cooled air or even through piped refrigerant. These system characteristics will largely dictate the types of suitable fit-out plant, but within such constraints there are opportunities to influence overall efficiency, effectiveness and sustainability of the completed systems.

The occupier should discuss with the owner whether there are opportunities to upgrade base-build systems if plant is inefficient or coming to the end of it's life e.g. boilers or chillers.

The base-build plant will be specified and sized using assumptions about uses, occupancy density and operational patterns. These assumptions should be reviewed against the requirements of the incoming occupier, with differences raised and addressed in the plant design and configuration.

Thermal comfort is an important element of occupier satisfaction with internal temperatures being one of the biggest items of complaint by buildings users. Undertaking a thermal model can ensure conditions during occupancy are optimised and risk of overheating is minimised. 

CIBSE AM11 provides a thermal comfort modelling standard that can support the selection of an appropriate service strategy using CIBSE Guide A.

Ska Rating requires projects valued under £500K, if unable to undertake the above modelling, to: 

• provide an overlay of the furniture and mechanical plans; 
• provide written evidence in the form of meeting notes to demonstrate discussion has taken place with the client regarding occupant comfort; and 
• provide a list of the solutions and actions to be taken following the client review. 

The issues that must be discussed as a minimum are: locations of cold/hot spots (from HVAC equipment locations and downdrafts), radiant temperatures and overheating near windows and atria.
 

If the building configuration allows it, natural ventilation strategies should be used, as the reduction in operational costs and maintenance can be significant compared to mechanical systems. Natural ventilation solutions range from providing manually openable windows, through to fully automated systems with BMS-controlled louvres and windows. The type of systems suitable for a specific project will be influenced by the base-building, intended activities in the space as well as local outside air-quality. Openable windows can suit extensively occupied spaces, whereas automated systems are better suited to areas with transient users, such as circulation spaces. 

When adopting natural ventilation strategies airflow paths should be given full consideration during the design process to ensure they are not obstructed by walls or furniture. Undertaking ventilation modelling using the proposed layout can be used to verify that a natural ventilation strategy will provide a comfortable working environment. 

If natural ventilation alone is not suitable to provide a comfortable environment throughout the year, then a mixed-mode ventilation system can be considered. In such systems, natural ventilation is used for the majority of the time, with mechanical ventilation and/or cooling employed only when the natural system is unable to maintain a comfortable environment.

If openable windows are provided then consider providing interlock controls on the HVAC systems. These prevent the heating and cooling systems running when the windows are open, avoiding wasted energy and associated costs. 

Where possible, the distance between the heating/cooling source and the end users should be minimised to cut losses from energy distribution. Pipework and ducts should be properly insulated, and the number of bends kept to a minimum to reduce the loads on pumps and fans.

Where mechanical ventilation and air conditioning equipment will be installed within the tenant demise the selection of efficient equipment will reduce operational costs. The occupier should consider relative pros and cons of different options with their designers. An options appraisal should take a whole lifecycle costing approach, including: aesthetics, operational efficiency, response time, adaptability, installation, maintenance and operational costs.

A useful source of efficient technology is the Energy Technology List. Examples could include the use of:

• Efficient motors and fans.
• Variable speed drives that regulate the electrical supply based on the required demand of motors.
• Air-to-air heat exchangers 
• Chilled beam and ceiling systems that uses chilled water circulated through a coil cooling the air within a space either passively or actively via a fan. Chilled beams are typically one of the most energy efficient options for cooling space.
• Efficient fan coil units (FCUs). FCUs are one of the most common HVAC options utilised due to their flexibility. Using direct current versions can help reduce operational costs.
• Night time purging / free cooling, where a natural ventilation strategy is used at night to purge excess heat and cool the building. This is typically most effective where a building has high levels of exposed thermal mass and outdoor air-quality is high.
• Internal window blinds that reduce the amount of 'solar gain' entering a space, gutting glare and reducing the requirement for cooling. 

Wet central heating systems

When specifying wet central heating systems consider using local temperature sensors and regulating controls, such as TRV (thermostatic radiator valves) or individual room thermostats, to support efficient use and help reduce overheating risk. Such local controls can be particularly beneficial where the base build is not conducive to enabling defined heating and cooling zones.

Mechanical ventilation & air conditioning systems

An effective fit-out HVAC strategy must consider the different layout zones created by different uses, orientations, fenestration and occupancy patterns. Heating, cooling and ventilation needs of the different zones must be determined, and the plant, controls and operational configuration designed accordingly.

The level of control vs automation needs to be carefully considered and how such systems will ingrate with the existing base-building controls and any Building Management System (BMS). 

The HVAC solution will have a direct influence on the internal environmental conditions experienced by building users. This includes aspects such as:

• Air Temperature (°C)
• Air Velocity (m/s)
• Relative Humidity (%)
• Carbon Dioxide Emissions (CO_² / ppm)
• Volatile Organic Compounds (VOC) (μg/m³)
• Noise (dB)

The design of a HVAC system can support good indoor environmental quality by taking into consideration the following aspects.

Appropriate specification

When developing the HVAC design, consideration of indoor environmental quality should feed into the levels and quality of ventilation specified, the thermal comfort and cooling strategy and the level of controls and monitoring installed.

Aspects beyond the HVAC design that will also affect overall indoor environmental conditions will include the resources allocated to Commissioning, maintenance procedures put in place Into Occupancy, as well as, the presence of Materials that may emit pollutants.

Review the existing air intake & distribution infrastructure

For buildings with a centralised air distribution system, the fit-out layout must reflect and respect the air distribution infrastructure, e.g. locations of ducts, intake/extract points and filter locations etc. Failure to achieve the required co-ordination can result in poor internal environmental conditions, as well as complex and costly maintenance e.g. from restricted access to change filters.

Internal air quality will also be affected by the air intake positions and filtration systems and it is important such a factor is reviewed to be reassured over meeting any internal air quality strategy. Sometimes aesthetic consideration places outdoor air intakes in areas of the building that may be exposed to heavy traffic, industrial discharges, or other sources of unwanted pollutants. Such situations could lead to premature loading of filters, increased maintenance and cleaning costs and unexpected wear and tear on the equipment. Corrective action may involve upgrading to more effective filters or, in extreme cases, relocating the outdoor air intake.

Implement a construction dust management programme

Construction dust can clog up ventilation units which will affect commissioning and system performance if not identified early. It is important that the construction team co-ordinate with property management team to clearly understand capabilities of the base-building and any requirements during construction phase.

The owner and occupier should require the implementation of a construction management program that ensures key ventilation components are protected from contamination during construction. Ensure that construction filters placed in ductwork and mechanical equipment are routinely inspected and replaced as needed.

In addition, carbon filters should ideally not be installed until all construction work, including dry wall and painting, has been completed.