Michael Wild, Manager TROX Service Division, explains the need to upgrade VAV units and the consequential saving in carbon emissions.

Variable air volume (VAV) was one of the most popular systems dating back to the mid-1970’s.  At its peak the UK market was around 80,000 VAV boxes.  In recent years its popularity has reduced in favour of air water systems, particularly multi service chilled beams.  There are still applications for VAV with variable cooling loads such as meeting rooms and specialist process air conditioning market, for example, pharmaceuticals and laboratories.

It is estimated that there are over 1 million VAV boxes still operating mainly in the comfort air conditioning applications within the office sector.  Some of the original VAV boxes had what was known as a “mechanical damper”.  It incorporated springs and weights so that a range of pre-set air volumes could be controlled irrespective of downstream duct pressure changes.  These dampers required quite a high static pressure to get the actions of the springs and damper mechanism operational and typically 200 Pascals (Pa) was required.  Today’s VAV boxes and VAV valves require very much lower pressures to operate them with 20 Pa being typical, which would show a significant energy saving over the original units.

This saving  200 - 20 = 180 Pa related to a   building of,  say,  40,000 m²   with   an  airflow   of 15l/sec/m² would achieve a reduction of 308,000 kwh per annum, or in terms of CO₂ emissions a staggering 132,440 kgs.  This in itself should be the motivation to the client to survey and refurbish his VAV system.

The accuracy with which these original VAV boxes can control the air flow depends very much on the life of the springs and other mechanisms.  Generally speaking VAV units which are in continuous operation would have a useful working life of 15-20 years.  During the life of the unit regular maintenance and monitoring should take place to ensure that they operate effectively.  Even with this maintenance there is a risk that operational difficulties will be found within the VAV unit which could lead to much wider variations in supply air volumes – that is the box drifts away from its normal tolerances.  This leads to much wider fluctuations in temperature because the room at times could be over heated or over cooled.  Again there are significant energy issues when this happens since the VAV box is not effectively controlling the environment, and would further add to the CO₂   emissions mentioned above.

Another important part of the VAV box is the sensor placed in the duct to measure airflow.  It has been found that sensors mounted in the duct in the airstream can with time be affected by build up of dust.   This again can have a detrimental effect on the energy efficiency of the VAV box.  Modern sensors are like a grid sampling air and passing this through sensitive electronic parts which are now mounted externally as part of the controller as opposed to being in the ducting.

With most of these older style units running inefficiently there is an enormous potential for energy saving if new units were installed.  Obviously replacing these units can be a costly business because it means that everything in the ceiling and above needs to be “stripped out”.  However if the refurbishment could be simplified reducing costs and disruption many VAV buildings may be encouraged to go down the “refurbishment route”.  There is also a strong sustainability argument that means we should be extending the VAV system life cycle, particularly if there is a way to improve the efficiency of the VAV unit by an effective but simple upgrade.

There is now an initial and growing demand for a complete refurbishment of these old VAV boxes and a single point responsibility seems to be preferred.  At TROX we offer site surveys and validation of VAV units, together with a comprehensive installation and maintenance package.  Refurbishing VAV boxes can be a relatively simple task when using a modern VAV valve – (see product details of the TROX TVR).  These valves have a low lift off pressure, 20 Pa, and incorporate a controller which is capable of communicating to the BMS if required.  The actual refurbishment is purely a matter of disabling the old damper or removing the old damper section.  The new valve is then fixed to the inlet of the VAV box and the controller wired into the existing or modified circuit.  All the static parts of the VAV box are retained subject to inspection.  It may be necessary to replace attenuator sections if damaged.  Where boxes can be upgraded in this way it is usually possible to retain the ducting thus minimising the refurbishment costs and potential disruption. 

The VAV valve is suitable for all makes of VAV boxes and therefore offers the ideal solution to VAV refurbishment.

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