The battle for indoor air quality
Overheads cannot be controlled at the expense of indoor air quality, and good indoor air quality is more than just ventilation. JIM KILCOYNE weighs up the various issues.Most people spend up to 90% of their time indoors. In a move to become more energy efficient, buildings are becoming more airtight, thereby reducing energy leakage. In fact the air tightness of buildings is one of the key issues in Part L of the Building Regulations. Humidity control no longer warrants the same level of importance as temperature control. The squeeze to control overheads often has a negative impact on maintenance and operating budgets. At the same time, there is rising awareness and concern in relation to personal health and fitness, being driven by regulation and increasingly by litigation. The battle lines between needs of occupants, Government regulations and the dictates of the financial director have been laid down. A mediator is required, and an indoor-air-quality programme could just prove to be the right choice. Poor indoor air quality affects a building in several ways. Occupants can feel unwell, leading to loss of productivity and rising absenteeism. This, in turn, can result in increased costs, which may include greater healthcare costs for the company. The building itself can be affected; damp and visible mould can drive tenants away, raise operation and maintenance costs and even reduce its investment potential. The three major contaminants to be found in every building are particulates (dust, pollen), gaseous matter (volatile organic chemicals, odours) and microbiological substances (moulds, fungi). Some enter the building from outside, while others are generated indoors via furnishings and work-related processes. The element that encourages their growth is moisture.
Moisture management Microbial growth, such as mould, mildew and bacteria, can emit spores that produce odours, allergens and, in some cases, toxins. We are all aware of the discomfort that bad odours can inflict upon us, but they are rarely lethal. Long-term exposure to allergens and toxins, however, can lead to serious health concerns, including bodily irritation, allergic reactions, and respiratory illness, such as asthma and lung disease. The building also suffers, as excessive moisture can stain wall surfaces, damage paint, corrode metal surfaces, ruin building insulation, and generally accelerate the deterioration of a building’s furnishings and structural materials. Controlling indoor relative humidity to an acceptable level minimises microbial growth potential and results in consistent thermal comfort in the occupied space. Maintaining an internal relative humidity below 60% significantly reduces the potential for microbial growth in buildings. It is also the easiest constituent to manage.
Constant volume versus VAV It is important to understand that the ability of HVAC systems ability to control indoor relative humidity varies. HVAC systems are typically sized for the sensible and latent loads at design conditions. However, with some forms of control, the system may not have sufficient latent (dehumidification) capacity when partially loaded. For example, a constant-volume air-handling unit controlled by space temperature only may maintain temperature and relative humidity very well on a summer day. On a cool, rainy day, as the thermostat modulates the cooling to maintain the desired temperature, the ability to dehumidify is significantly reduced or lost completely. Temperature control is maintained, but humidity control is not. To ‘actively’ control indoor humidity, the measurement of both the bulb temperature and relative humidity in the space and air supply are essential, as well as a means of controlling humidification/dehumidification. These are often overlooked because of initial and on-going operating costs, which may prove to be shortsighted. VAV systems do a good job of controlling indoor RH, so they could offer a better solution because of the generally lower supply-air temperature (normally 13°C) at all load conditions, thereby indirectly controlling space relative humidity. The temptation to reset the supply-air temperature upward at low-load conditions, such as winter should be avoided. This may look like a short-term energy-saving approach, but often results in the loss of space humidity control.
Controlling pressure Building pressure also plays a role in controlling indoor moisture, as differing climates may have different methods. In predominantly cool and mixed climates, by maintaining positive building pressure relative to outside, the infiltration of moist outside air is minimised, while the ‘exfiltration’ of dry (low dewpoint) indoor air into wall cavities is maximised. Operating with positive building pressure encourages dry indoor air to flow through the wall structure straight through to the outside. To maintain positive building pressure, make-up airflow must always exceed the airflow needed to replace exhausted air. In climates where buildings require heat for a significant proportion of the year (such as ours!), the building could be operated with neutral or slightly negative building pressure to avoid forcing moist indoor air into the walls.
Distribution and filtration Proper distribution of supply air in a building is also important, so that all spaces, occupied as well as unoccupied, such as mechanical equipment and storage rooms, are properly pressurised. Air distribution systems should be balanced at start-up and periodically re-balanced during the life of the building — especially at fit-out stage to ensure proper distribution and, thus, proper pressurisation. Finally, the selection and maintenance of filters is paramount. The graph compares filter capability against typical particulate contaminants found in buildings. Many particulates could easily bypass ordinary filters, so consideration should always be given to fitting high-efficiency filters where possible. A good portion of particulates will then be prevented from entering the building at the intake ducts. Good indoor air quality is more than just ventilation. Consideration should be made to measurement (temperature, humidity, carbon dioxide, air quality), control (humidity, fresh air) and filtration — along with considering initial costs, running costs and energy management.
Jim Kilcoyne is controls manager for Trane Northern Europe, a member of the Building Controls Industry Association, PO Box 872, Chippenham, Wilts SN15 5WA.
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