Commissioning: time to raise the bar
Barry Trewhitt and Iskender Gencer discuss the changing face of the commissioning process and the challenges it faces in the renewables era.
Commissioning of air conditioning and ventilation systems has always had its challenges, but in recent years a number of factors have combined to ‘up the ante’.
From a technical viewpoint, today’s system designs can be far more complex. New buildings often incorporate one or more renewable technologies. Integrating these effectively, with one another and with more traditional air-conditioning and ventilation equipment, can be complicated. In addition, new approaches involving natural, displacement, decentralised ventilation or smoke-extract systems can be unfamiliar ground for the commissioning engineer.
Problems during commissioning can be caused by a number of factors. For example, a run of ductwork prior to a VAV controller might lead to unacceptable levels of turbulence, inaccurate pressure readings and problems with noise — even though the system design and the products themselves are completely sound.
It is also possible that during the commissioning process, the system designer’s intentions might be misinterpreted or unclear. Sometimes the information available to the commissioning/controls engineer leaves questions unanswered. This can be particularly difficult if the system is complex or incorporates new-generation energy-saving components that break away from traditional operating principles, air pressures and water temperatures etc.
If problems encountered during commissioning are not solved, what are the likely outcomes? Such problems often surface as a complaint to the manufacturer that ‘the equipment isn’t working’. Another possibility is the system under-performing throughout its lifetime. In the case of air conditioning, the central air-handling might run at higher pressures than intended, with reduced energy efficiency and higher noise levels.
So what are the solutions?
There are, of course, systems that require no commissioning, but the costs could be prohibitive for most projects. The most valuable solutions are therefore often about reducing opportunities for error across the entire process — from installation to handover and beyond. Trox’s approach is to design ease of commissioning into all its products, and to continually build on the technical support offered to every link in the supply chain, including commissioning engineers.
Prefabrication is an obvious solution for ironing out installation problems. Chilled beams prefabricated away from site by the equipment manufacturer will reduce the opportunities for errors on site. More recently we have also seen increased demand from customers for factory fitting of water-flow self-balancing modulating valves and off-site prefabrication of fan-coil controllers and associated accessories.
Detailed O&M guides for our products are also available on request, which include best practice during commissioning (see panel). We also run training days to bring people up to speed with emerging technologies.
BSRIA’s Soft Landings initiative could also be beneficial. Soft Landings means designers and constructors staying involved with buildings beyond practical completion. The aim is to assist the client during the first months of operation and beyond, to help fine-tune and de-bug the systems, and ensure the occupiers understand how to control and best use their buildings.
There is a growing awareness across the HVAC industry that commissioning can be the make or break. Equipment manufacturers can contribute by investing in the development of products that are easier to commission, by providing good-quality technical information, and by ensuring that CPD training is available to keep engineers abreast of emerging technology.
Barry Trewhitt and Iskender Gencer are product managers with Trox UK.
Fan coil best practice — extract from Trox O&M document for the PWS(H) waterside fan coil unit.
Once the system is operational, air volume flow rate measurements can be made at the air leaving discharge diffusers, for ceiling void units. This is normally done at the design fan speed as given in the manufacturer’s quotation.
The following checks should be carried out before measurements are taken:
• Ductwork joints are sealed properly.
• Sections of flexible duct are not in tight bends.
• The installation of the false ceiling is complete.
• The room has adequate return air grilles.
• Central plant fresh air supply system is balanced.
• All fan coil units are switched to their design fan speed (manual or BMS switching).
• Volume control dampers (VCD) in diffusers or ducting are set to fully open.
• Adjustable cones on diffusers are all in the fully up or fully down position.
• Filters and fan are cleaned (NB filters generally need to be cleaned before commissioning).
• Ambient air temperature is between 10°C and 25°C.
If results are outside of the stated tolerance band it is likely that duct static resistances are excessive or the fans in the fan coil unit are under performing.
Duct static resistances should be checked using the static tapping from a pitot static tube inserted into the ducting from the fan coil unit. Great care must be taken to find tappings that are representative of the static pressure that the ductwork system is imposing across the fan coil unit. The static pressure reading taken will be for the air volume flow rate as measured, not the design air volume flow rate required.
If air volume flow rate measurements are different from those required, it will be necessary to refer to the air flow performance curves for each fan coil unit. These give the relationship between air volume flow rate and external static pressure for the particular fan coil unit. From these laboratory derived curves it is then possible to normalise the measured air volume rate to what it would be at the specified external static pressure, i.e. 30 Pa.
(O & M manuals for TROX products are available on request. Telephone 01842 754545 or email trox@troxuk.co.uk)
