Question 7 of 8: VAV systems and indoor air quality

Question 7: VAV systems and indoor air quality (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — variable air volume systems (10 points)

A variable air volume system holds the supply air temperature roughly constant and varies the quantity of air delivered to each zone in response to that zone's thermostat, using a throttling terminal box in the branch duct. It stands in contrast to the constant-volume reheat system, which delivers a fixed quantity of cold air everywhere and burns reheat energy to spoil it back up to the required temperature, and to dual-duct systems, which mix hot and cold streams at the terminal.

The principal advantage is energy. Because fan power varies roughly as the cube of the volume delivered, a zone operating at half flow draws about an eighth of the fan power, and the savings persist through the whole shoulder season when almost every zone is at part load. The system also eliminates simultaneous heating and cooling: no reheat energy is spent cancelling cooling that has already been paid for. It exploits diversity honestly, since the central plant and the main ducts can be sized for the coincident peak of all zones rather than for the sum of their individual peaks, which typically reduces installed capacity by 10 to 30 per cent and shrinks both the shaft space and the capital cost. Zone control is genuinely independent, one terminal box per zone, and the boxes are inexpensive, purely mechanical devices with no piping, no condensate and little maintenance. Retrofit and future rearrangement of partitions are relatively easy because only the terminals move.

The disadvantages all follow from the same fact: at low load the air quantity is small. Ventilation is the first casualty. If the outdoor-air fraction at the air handler is fixed, then throttling the supply throttles the fresh air with it, and a lightly loaded zone can fall below the outdoor air its occupants require; ASHRAE Standard 62.1 addresses this through the multiple-zone recirculating system ventilation-rate procedure, and modern practice adds outdoor-air measurement and control, or a dedicated outdoor air system in parallel. Second, air distribution suffers: at low flow the supply jet loses throw and its Coanda attachment to the ceiling, so cold air dumps into the occupied zone and stratification and draught complaints follow. This is why terminals are given a minimum flow setting, typically 30 to 50 per cent, and why the choice of diffuser matters more than in a constant-volume design. Third, humidity control at part load is poor, because the coil runs at reduced load with a higher sensible heat ratio and dehumidifies less just when the latent load from occupants has not fallen. Fourth, a simple cooling-only VAV system cannot heat, so perimeter zones need reheat coils, baseboard heating or fan-powered boxes, which returns part of the reheat penalty the system was meant to avoid. Finally, the system is control-intensive: static pressure control, variable-speed drives, terminal calibration and proper commis-sioning are essential, and a poorly commissioned VAV installation is a common source of comfort complaints and of energy performance far below prediction.

VAV is therefore recommended where the building is large, has many zones with genuinely different and non-coincident load profiles, and is dominated by sensible cooling: office towers, institutional and government buildings, large retail, university and hospital administrative areas, and interior zones of any deep-plan building. It is a poor choice where the latent load is high or variable relative to the sensible load, as in restaurants, natatoria and laboratories; where a constant air change rate is mandated, as in operating theatres, isolation rooms and many laboratory and industrial exhaust applications; and in small buildings with one or two zones, where the control complexity is not repaid. In Canadian practice, perimeter zones with large winter heating loads are usually handled by VAV with reheat or by a separate perimeter heating system, and the National Energy Code for Buildings effectively requires variable-speed fan control on systems above a modest size.

Part (b) — indoor air quality (10 points)

Indoor air quality is determined by the balance between the rate at which contaminants are generated indoors or admitted from outside and the rate at which they are removed by ventilation, filtration or source control. The influencing factors fall into four groups. Occupants and their activities generate carbon dioxide, water vapour, bioeffluents and particulate, and their number and density set the ventilation requirement. Building materials and furnishings emit volatile organic compounds and formaldehyde, most strongly when new; adhesives, paints, sealants, carpets and pressed-wood products are the usual sources. Equipment and processes contribute: printers and photocopiers emit ozone and ultrafine particles, cooking produces grease and combustion products, and unvented or badly vented combustion appliances and attached garages are the classic route for carbon monoxide. The building and its services contribute their own: moisture accumulation leading to mould, radon entering through the foundation, which is a significant concern across much of Canada, and the HVAC system itself, whose wet cooling coils, drain pans, humidifiers and dirty filters become amplification sites for microbial growth. Outdoor air quality is the fourth factor, since traffic, industry and nearby exhaust stacks are drawn in through the intake if it is badly located.

The measures follow the hierarchy of control. Source control is always first and cheapest: specify low-emitting materials, prohibit smoking, separate and directly exhaust printing rooms, kitchens, chemical stores and parking garages at negative pressure, seal the foundation against soil gas, and locate outdoor air intakes well away from loading docks, cooling towers and exhaust discharges with the separation distances given in ASHRAE Standard 62.1. Ventilation is the second line: supply outdoor air at the rates of Standard 62.1, using either the ventilation rate procedure or the indoor air quality procedure, and ensure it actually reaches the breathing zone — the zone air distribution effectiveness factor recognises that ceiling supply and ceiling return with warm air is far less effective than displacement ventilation from the floor. Demand-controlled ventilation using carbon dioxide sensors is appropriate in densely and intermittently occupied spaces such as the cafeteria of Problem 5. Filtration is the third: MERV 8 as a minimum, MERV 13 or better where outdoor particulate matters, with activated carbon where gaseous contaminants are the concern. Humidity must be kept between roughly 30 and 60 per cent relative humidity, low enough to suppress mould and dust mites and high enough to limit respiratory irritation and static; in a Canadian winter the upper limit is also set by condensation risk on windows and in walls. Finally, the system must be maintained and commissioned: filters changed, drain pans and coils cleaned, outdoor-air dampers verified, and the building flushed out before occupancy.

The governing documents are ANSI/ASHRAE Standard 62.1 for commercial and institutional ventilation and 62.2 for dwellings; ANSI/ASHRAE Standard 55 for thermal comfort; and in Canada the National Building Code Part 6, the National Energy Code for Buildings, CSA Z204 and the Health Canada residential indoor air quality guidelines, together with provincial occupational health and safety regulations and the ACGIH threshold limit values where an occupational exposure is involved. Thermal comfort is inseparable from perceived air quality: occupants reliably report air as stuffy when it is merely too warm. The ASHRAE comfort chart plots operative temperature against humidity ratio and shows the winter and summer comfort envelopes for sedentary activity in typical clothing — broadly 20 to 23.5 °C in winter and 23 to 26 °C in summer, with humidity between about 0.004 and 0.012 kg/kg. Its basis is Fanger's predicted mean vote model, which combines the six governing variables — air temperature, mean radiant temperature, air speed, humidity, metabolic rate and clothing insulation — into a single index, with the envelope drawn where the predicted percentage dissatisfied falls below ten per cent. Local discomfort from draught, radiant asymmetry, vertical temperature difference and cold floors must be checked separately, and in a Canadian winter the radiant asymmetry from a large cold glazing area is often the real complaint behind a request for more heat.