In the modern day working environment, as we know it, we all take for granted that we work and live in a controlled atmosphere. Whether it is in an office, hospital, leisure centre, or simply shopping in a mall, all these areas have a controlled atmosphere. By a controlled atmosphere, it is meant that the air in a building is regulated for its quality and temperature, ensuring an optimum temperature for human comfort. Along with the controlling of the air comes the problem involved with ensuring the check of air-borne organisms.
The Operation of an Air Conditioning System
The basic principles of an air conditioning system is to draw air in at a particular point, put it through a process, and then to extract it, at another point at a specified temperature.
Regular Air Conditioning Maintenance Requirements and Reporting
Maintaining a healthy air conditioning system goes well beyond the mechanical services work that most contractors cover. Cleaning and exchanging filters is a primary requirement for a clean system and forms the introductory level of regular maintenance. Beyond that, air conditioning ducting needs to be inspected and cleaned free of any build up. Whilst dust and other particles are expelled from the vents and provide a visual indicator of a need for a clean, other build ups including sources of odours can remain hidden within the ducting. Cooling coils are another area that remain “out of sight” until a proper inspection is undertaken. With a routine maintenance schedule in place you can rest assured of the efficiency of your system and in turn savings in running costs.
AMCoil is equipped to provide a comprehensive inspection and report on the condition of your air conditioning duct work, filters and cooling coils system and the services needed to rectify any problems. AMCoil also issues a job completion report which is often a requirement for insurance and landlord reporting.
Sick Building Syndrome
Much research has been done in relation to Sick Building Syndrome, especially in the United States, and the U.K. Results of these researches show that there are two forms of SBS; the first is related to the design of the building, and the second, to the air quality. Although, all the results are not yet conclusive particularly in relation to the design, there is however a definite link, between air quality, and sickness in a building. It is easier to see the relationship in the latter due to the fact that, most people are already aware of the existence of air borne organisms. It is essential to ensure the air is clean as it comes out of the ductwork. Many of the earlier causes of the transfer of diseases is because the air has gone through the system, and picked up a virus/bug, at one point of the building, and has carried it to another.
Microbial contamination of buildings.
Disease can result form microbial contamination of buildings. Most potential pathogens are water-borne microorganisms, and if the system is kept clean, the potential for disease is very low. A methodical and mainly traditional approach to the subject is recommended. This includes attention to fundamental principles. The paper introduces some specific control measures applicable to the various water and air systems commonly found in buildings.
There is increasing public concern about health hazards from air and water systems in buildings. Today’s industrial age has produced many environmental niches, which happen to suit newly discovered water-borne microorganisms such as Legionella. But the principles of control are similar to those, which apply to the other microbes: good hygiene.
Microbial colonisation of air handling systems can lead to Humidifier fever or other allergic reactions.
Effect of water on microbial growth
Contamination of heated water systems such as cooling towers, hot water systems introduce the risk of Legionella growth in the system. Many types of microorganism can flourish and proliferate in dirty water, particularly when it is warm. In clean water, luxuriant growth is not possible but many microbes survive quite happily and a few even manage to reproduce slowly on insignificant nutrients – for example remaining in distilled water.
We now know that the organisms found suspended in water are outnumbered by those present in the accumulations and films, which cover all surfaces. On these surface traces of nutrients are absorbed and aggregate, hence sustaining passing microbes, which settle and flourish there.
Anti-microbial chemicals are usually less effective against surface biofilm organisms, than against the same organism, that is in suspension in the water. It is essential to use suitable bio-dispersants, in conjunction with old fashioned elbow grease, to lift the accumulations, disperse them to give the chemical a chance to do it’s task.
EFFECTS OF POOR AIR AND WATER QUALITY CONTROL
Diseases carried by water and air
This bacterium is common in natural water systems such as rivers, ponds, and wet soil, but unfortunately it has found a new niche in man-made water systems, which provide suitable conditions for growth. It feeds on the by-products of slime forming microbes and hence any situation, which is warm and contaminated with microbial slimes, could encourage the proliferation of Legionella. It is not difficult to kill in water because it succumbs readily to heat and most disinfectants. However it is considerably protected when entrained in biofilm or within other microorganisms such as protozoa.
Legionella are likely to be detected in most warm, slimy locations, but the higher the numbers, the greater the potential hazard. It is only a potential hazard, as Legionella is an ‘opportunistic’ pathogen – i.e. It can only become established in a susceptible host, usually a male in late middle-age, probably a smoker and probably with a low resistance to infection (for example on medication).
There are no person-to-person transmissions and most investigations have implicated warm showers or cooling towers. Legionella is a form of pneumonia, which may have previously been attributed to other causes. Although there are test methods available for Legionella organisms, it is more practicable to simply aim to suppress the development of all microbial slimes, and keep systems free of fouling.
Legionella in Cooling Towers
It is vital that building owners ensure proper housekeeping of their cooling towers. When the cooling tower circulating water is sprayed through air drawn into the tower it washes out various pollutants such as acidic gases, dirt, soot, fibres etc., which are present in the atmosphere.
If an excess of these solid matters are allowed to accumulate in the collecting basin of the cooling tower or evaporative condenser, it may work its way past strainers and into the rest of the water circulating system and cause partial and even complete clogging of pipes, and condensers. It may support microbial growth in the tower, and the dirtier the system, the more favourable the conditions, for the growth of slime and algae.
Chemical control of slime formation to be effective must be combined with extensive periodic cleaning operations. In addition particles of dirt, and slime can coat the condenser water systems, which not only represent a potential hazard, but also reduce the effectiveness of the heat transfer, which is the purpose of the equipment in the first place.
The efficient operation and thermal performance of a cooling tower depends on tower cleanliness as well as operational maintenance.
Removing deposits can carried out in a number of means, but in the end it manual cleaning of the cooling tower itself (which is the starting point of entry of solid matter, and an indicator of the total cleanliness of the total water system) will be necessary. AS3666 requires this activity to be carried out at least twice per year. This is generally regarded as good practice for reasons of thermal performance apart from benefits obtained in microbial control.
Note: Field studies carried out have highlighted the need for a biocidal residual of some sort (eg water chemical treatment) to be in place in the water system n addition to the periodic cleaning activity. Both measures are necessary and compliment each other.
- Frequent checks, to identify problems, at an early stage, before they become major difficulties.
- Systematic documentation of the condition of the equipment. – An Inspection Report.
- Chemical water treatment, to protect the system submerged under the water, and the parts not under water kept wet with continuous sprays.
- Ensure the cleanliness of the spray equipment.
- Regular cleaning of system.
- The benefit of a clean system is beyond the control of microbial growth. There is also;
- Reduced energy consumption due to more efficient thermal processes.
- Increased equipment life
- Reduced corrosion of metal parts
- Reduced abrasive erosion of tubes, seals, and the like.
- Reduced chemical consumption as cleaned systems respond to water treatment better than fouled ones.
- Better reliability of operation.
Cleaning of Cooling Towers
- The basin and interior surfaces of cooling towers should be hosed down and flushed out to remove airborne grime that has accumulated during the winter. This should be done before the system is filled with water. Ensure a convenient hose connection is available.
- Float valve should be adjusted to close completely when the water level in the basin is at the required distance below the point of overflow. There should be no overflow when the sprays are on unless this is used as the means of bleed off. The discharge of makeup water should always be above the water level in the basin.
- Pump packing should permit only enough water leakage for lubrication and should not allow an excessive amount of water to run to waste.
- Spray nozzles, water distribution tray and strainers should be inspected and cleaned if necessary.
The environment of the equipment is an important factor in determining how fast airborne solid material and microbial matter will accumulate in the system. All of the items on the following list should be checked weekly or at least monthly, but after a time experience will dictate how often most of them will actually need attention.
- Check water treatment equipment eg liquid level in holding drums.
- Identify unusual noise or vibration from pumps, motors, fans and other components: check starting and stopping of fans.
- Check nozzles and return header. Keep clean to avoid clogging and build-up of organic or biological matter.
- Check tower casing for leaks and corrosion.
- Clean strainers and screens.
- Keep pump packing adjusted.
- Ensure outlet of cooling tower sump is flooded to avoid air being sucked into the system. Adjust water level or install baffles.
- Checks float valve adjustment.
- Inspect bottom of basin, if dirty drain and flush.
- Check louvres, fill, drift eliminators and the like for clogging, scaling, presence of slime or other microbial growths, and any signs of deterioration. Manual cleaning may be necessary if there are deposits attached to surfaces.
- Hose down all loose deposits on eliminators or spray equipment and the fill of cooling towers.
- Check sand filters (if fitted) for water leaks, pre-strainer clogging correct Air operating pressure and correct operation of backwash facility.
- Always comply with appropriate safety measures.
MICROBIAL CONTAMINATION OF AIR HANDLING SYSTEMS
The provision of acceptable indoor quality in a building begins with consideration of the outdoor air quality ie what is commonly described as ‘fresh air’.
There is considerable confusion over the term ‘fresh air’. When asked whether they consider the air in their workplace to be fresh, some occupants base their subjective assessment on their perception of freshness around their faces, usually associated with local skin temperatures being sufficiently low to be pleasant without being perceived as a draft. Others may consider that the air can only be ‘fresh’ if they can see a source of outside air, such as a n open window.
In fact the question can be misinterpreted: fresh air, in this context is outdoor air which alters the contaminant constituency of a room through which it flows, mixing with or displacing room air and contaminants. Of course air contains many contaminants, which may be unwelcome guests indoors if the concentrations are high. Refer AS 1668 Part 2 gives a recommendation for the maximum acceptable concentrations of common contaminants in outdoor air. It is not only the quantity of fresh air that is an important factor in producing an acceptable indoor environment but also the quality of that fresh air.
Microbes in Air
AS 1668 Part 2 requires the outdoor air to be brought to an acceptable level of purity and filtration may be needed.
Contaminants such as vapours, which cannot be removed by economic particulate filtration techniques and which exceed recommended limits, may have settled at sound attenuators, coils, trays, and the internal walls of the ductwork. In areas where moisture can collect, such as fresh air inlets subject to rain penetration, coil or spray trays, and spray eliminators, microorganisms can thrive on a mixture of iron oxides and sludge. This contamination can result in strong odours being emitted into the supply air. In severe cases large concentrations of microorganisms have been carried in the supply air into the workplace producing allergic responses. One such allergic response is known as Humidifier fever, which has been evident in outbreak proportions, although such outbreaks are very rare. Its precise causes are not clear but seem to be related to the build-up of organic matter on humidifiers, operating on predominantly in an environment in which there is considerable cellulose in the air eg paper printing works, textile mills.
“House” dust is a very significant irritant, which can also produce allergic reactions. It compromises a mixture of biological material, such as bacteria, spores, pollen, mites, insect debris, human skin scale, mineral dust, mineral fibres and fabric fibres. Although some of these constituents may enter with the outdoor air, many are generated internally within the occupied space and distributed by local room air currents, for example as people move about.
Substances of a non-microbial type, which may contribute to poor indoor air quality, include a range of chemicals, which may out gas from materials in the building. This aspect is not covered in the AS3666, but it is worth knowing that in a room with soft furnishings, textured wall coverings, carpets and so on, may have thousands of square metres of surface on which airborne chemicals can be absorbed or dust particles trapped. The trapped dust actually increases the area available for absorption. As temperatures and vapour pressures change, some of the absorbed chemicals may be emitted from certain surfaces, thus adding to the odour burden in the room. In some cases this emission occurs when the room is not occupied and contaminants can be purged by introducing fresh air some hours before they arrive.
For rooms in which heavy intermittent tobacco smoking occurs, such as in hotel bars, clubs, it has been found that the odour strength during occupied times could be greater than the source strength of smoking. This is due primarily to the ongoing odour source strength of the desorbed tobacco smoke.
Dust and microorganisms can accumulate inside air-handling system. If dust penetrates the filters then it will settle out on all surfaces, such as heating and cooling coils.
Other Indoor Contaminants
There is a wide range of contaminants which may be either generated in the workplace or enter from outside. In the context of the modern office building, hospital, club, shopping centre, etc contaminants of particular concern include, volatile organic compounds, ozone, airborne particles. These substances may contribute to discomfort and in some cases perhaps symptoms associated with sick building syndrome. Some authorities on the subject have speculated on the possibility of a “cocktail’ effect, resulting from a synergistic combination of exposure to a number of substances at sub-threshold concentrations. An example of this might be when an irritant, such as formaldehyde, combines with airborne dust and low room humidity to produce eye irritation, particularly among contact lens wearers.
A wide range solvents are released from new floor coverings, treated timber, decorative finishes, furniture and equipment mostly from adhesives used in their manufacturer or installation. These emissions fall to zero once the drying process is complete – between six months an three years after construction, depending on the volatility rates. Solvents are also emitted during certain cleaning operations such as dry cleaning of carpets and other fabrics.
There are many potential sources of odour in the modern building. Apart from the occupancy-dependant body odours and tobacco smoke, there are odours released from the solvents used in the manufacture and cleaning of furnishings and building fabric, odours stimulated by airborne micro-organisms such as mould spores, odours which are absorbed on to internal surfaces and released some time later, smelly external outdoor pollutants not removed by the filtration system and odours released from the internal surfaces of inadequately cleaned air-handling systems.
If smoking is banned and odours form non-human sources are minimal then the fresh air supply has only to dilute body odours of the occupants and reduce carbon dioxide to an acceptable concentration.
Clean Air Design
Although the provision of an appropriate quantity of outdoor air is the ammunition in the battle to provide an acceptable indoor air quality, the accuracy of the aim has to be ensured by a correctly designed air-distribution system. The correct quantity of uncontaminated outdoor air has to leave the ductwork system in an appropriate way to meet the needs of the occupants in every part of the building.
A clean-air approach to design and maintenance is required. This is the objective of the companion Australian Standards AS 1668 Part 2 and AS3666.
Microbial Control of Air Handling Systems
Air handling systems in buildings are designed for the environmental needs of occupants and to satisfy regulatory requirements.
Inadequacies in these systems can encourage building related illness. Various coping strategies such as filtration, cleaning a proper air distribution serve to assure occupants that their comfort and health needs are satisfied.
The role of air filters is highlighted. Due to increasing expectations for the working environment to be clean, good standards for air filtration are becoming increasingly important.
These issues are discussed in this paper. Theories about the Sick Building Syndrome phenomenon are also discussed.
Microbial control of air handling systems at the planning/design/construction stage include attention to:
- Location of air intakes;
- Accessibility and drainage
- Air filtration
- Ventilation and air distribution
- Other contributing factors are:
- Pollutants emitted inside the building
- Contamination from outside sources and
- Biological contamination
Sick Building Syndrome
The term ‘sick building’ has been coined in recent years. A building is characterised as ‘sick’ when its occupants complain of health and comfort problems that can be related to working or being in the building. There are two types of problems:
Sick building syndrome
– not yet fully understood by scientists
– not covered in AS3666
Building related illness
– covered in the standard
These terms generally apply to problems related to indoor air pollution and are not to be confused with complaints related to discomfort causes such as inadequate temperature or humidity control, noise, glare, etc.
There is popular worry about ‘sick building syndrome’, which is creating a somewhat exaggerated public suspicion of air-conditioning. Although there is much scientific inquiry current to try to understand it, there is not yet a comprehensive definition of the term ‘sick building syndrome’ and it has been viewed with some scepticism. The cause is not known, the symptoms are non-specific, there is nothing abnormal to measure and laboratory tests cannot be carried out to reproduce the symptoms: therefore a definition in the usual scientific terms is not possible.
One definition of ‘sick building syndrome’ is
“A description of a building in which complaints of ill health is more prevalent than might otherwise be expected”
A building is said to manifest SBS when a substantial proportion (eg over 20%) of building occupants complain of symptoms associated with acute discomfort.
- Eye, nose or throat irritation
- Dry cough
- Dry or itchy skin
- Dizziness or nausea
- Difficulty in concentrating
- High sensitivity to odours
- And complainants report relief upon leaving the building
- SBS is generally associated with air-conditioned buildings but it has also been evident in non air-conditioned buildings.
Building Related Illness
AS3666 does concern itself with ‘building related illness’ which by definition is less obscure and is better understood than SBS because the term describes clinically defined illness’s diagnostically attributable to exposure to microbial contaminants causing:
- Allergic reactions (eg mould or fungal spores leading to respiratory illnesses
- Such as hypersensitivity pneumonitis, occupational asthma or Humidifier fever
- Bacterial (eg Legionnaires disease)
- Viral (eg influenza, chicken pox, measles epidemics resulting from the droplet nuclei produced when sneezing)
- Fungal (eg direct airborne infection of immunocompromised individuals and the elderly from fungal species such as Aspergillus fumigatus, which may result from bird droppings)
- Other rare allergic reactions attributable to chemical out gassing from building products or processes are not within the terms of reference of AS3666. Outbreaks of respiratory illness have also been reported as a result of exposure to waste products of dust mites, from pollen and animal related allergens arising from dander etc. The cause of work related illness in buildings might lie within the building itself as well as the services.
Principles of control
- In the face of the widely differing medical symptoms involved, it is virtually impossible to produce an exact standard, if followed to the letter will alleviate these all health problems. Especially as there are other factors involved, and all individuals have differing reactions to the built environment. AS3666 is the beginning. It addresses critical areas for which there is clear evidence that health hazards can result if the principles in the Standard are not followed.
- Preventative measures described in the Standard are consistent with principles established by authorities such as WHO, ASHRAE, BSRIA, and others.
- The principle include:
- Prevention of moisture incursion into occupied spaces and air conditioning components; humid or damp conditions should also be prevented.
- Removal of any stagnant water or slimes that do collect in mechanical systems.
- Use of steam rather than atomised water as the moisture source in humidifiers (note that ultrasonic humidifying devices can produce respirable aerosols and should be avoided.
- Design of air-conditioning systems to exclude the use of water sprays; if these are necessary for thermodynamic reasons, particular maintenance attention is needed (note: such equipment is rarely used in comfort air-conditioning applications in Australia due to the benign climatic range)
- Use of good quality air filters
- Appropriate maintenance programs for all air-conditioning plant as well as building surfaces.
- Adequate ventilation air – Ref: AS 1668 Part 2
- Reasonable separation between outside air intakes and exhausts.
While planning strategies are worthwhile passive control measures, and a whole chapter in itself, active measures to control microbes such as Legionella can also be taken. It is better to implement such control measures to eliminate microbiological problems rather than to rely on a favourable aerodynamic environment to minimise the hazard of exposure to airborne contamination.
These active measures include primary actions as the design features of cooling towers; maintainability, accessibility, trainability,, cleanability, safety, drift elimination, which addresses the potential source of Legionella, protozoa, and other microbes.
Secondary actions relate to the path of transmission of Legionella bacteria in the air. Not much can be done about the open air itself but design of ducted systems within buildings should take into account the potential for airborne Legionella and protozoa to be carried through plant and ductwork. Of course there are other paths of transmission such as building entrances and open windows. It is nonetheless incumbent upon air-conditioning designers to consider plant items such as:
- And the role they play in regard to Legionella control.
These Plant items are themselves potential sources of general microbial growths and are included in the standard. Paths of transmission such as fans and ductwork are also included;
- Evaporative coolers
- Cooling coils
- Terminal unit
All air conditioning air handling systems should incorporate them for various functional reasons. Little detail is given in AS3666 on filter selection; the designer has freedom of choice on filter performance appropriate to the application.
Filters come in a wide range of qualities, dust holding capacities, and dust arrestance efficiencies, i.e. the ability to remove microscopically sized particles from the air stream.
The Australian Standards AS1132, 1323, 1324 set standards for air filters for commercial buildings. There are several parts to these standards, in particular AS1132 Part 5; Air Filters – Air Conditioning and General Ventilation Methods of Test – Part 5 – Arrestance Efficiency and Dust Holding – Test Dusts Nos 1,2 & 3
Test dust No.2 comprises particles predominantly in the 3-10 micron range. Most bacteria are in the size range of 1-5 microns; Legionella is stated to average 3 microns long and one-micron diameter. Now bacteria nearly always agglomerate either at other microbes such as protozoa or to dust particles.
They therefore travel in dirt/droplet particles that may be somewhat larger than the microbe itself. It follows that a filter that has high dust arrestance efficiency to AS1132 test dust No. 2 (a weight collection test). Will be a very effective ‘secondary’ trapping device against Legionella. Such filters are readily available and efficiencies of around 95% or greater are common. While these filters are highly effective against particles of the size of bacteria, they are much poorer against particles of the size of viruses (0.02-0.3 microns).On the other hand, it is known that viruses which produce the well known viral infections (smallpox, chickenpox, measles, mumps, influenza, common cold) are principally attached to droplet nuclei, which are produced during sneezing. The median size of these droplets is around 4 microns. The expelled virus rapidly loses infectivity in the air. It is close proximity for long periods, which is the most common route of infection. Air filters may yet have a role to play because some studies have shown the droplet nuclei can remain airborne for some time before drying (or attaching to surfaces) and the larger size of such droplet nuclei compared with the virus itself, more readily permits removal at the filter in the same way that bacteria may be readily removed.A reasonable efficiency against test dust No.1 (particle size 0.3-1.0 micron size predominantly), is necessary to keep the interior of a building clean; it is the very fine dust particles which are the main cause of staining surfaces. Test dust No 3, comprises the ‘rocks’ in terms of size of test dust; particles size of 10-30 microns.
Irrespective of the filter chosen it is important that it be well constructed with good edge sealing so that there is no leakage around the filter. This point is made in AS3666 as a Note. It is followed by a statement about moisture. For virtually all bacteria (including Legionella), moulds, fungus, and other microorganisms, moisture is an essential growth component. They are all aquatics life forms. Moisture provides a solvent for the microbe’s food, it prevents desiccation and it carries away waste products form the microbial cell itself.
Outbreaks of respiratory disease associated with fungal spores have occurred overseas. They typically result from contamination of wet filters at air intakes. In one outbreak, a badly fitted air filter allowed fungal spores, which were generated at an adjacent building site to enter the ventilation system and the building itself.
Generally due to the mild climatic conditions around most Australian cities, humidifiers are necessary only at special purpose applications (eg computer rooms to preclude static electricity effects)
This situation compares with many northern hemisphere cities which experience conditions of extremely low moisture content in the outside air in winter and humidification (after preheating incoming air) is essential. Hence the greater propensity for health problems related to air conditioning equipment.
All humidifiers should be designed for ease of cleaning as this is one of the few applications in which water (or steam) is deliberately admitted into air handling plant. Irrespective of the type of humidifier, in the ductwork, appropriate drainage lines are necessary, because condensate can be produced under some conditions such as start-up or malfunction. Any source of stagnant water, even with a poor supply of nutrients present will in time become contaminated with microorganism.
Water stagnation inside ducts and indeed inside any part of the building should be prevented for reasons of hygiene.
In many arid and temperate parts of Australia, evaporative cooling is a viable alternative to refrigerated air-conditioning. The range of evaporative air coolers available today is extensive and varies from small, portable domestic units to large fully ducted systems for commercial and industrial installations. While each unit has individual features the main components used in all of them are similar, and the principle of operation is identical.
AS3666 highlights the need for the design to ensure there is adequate access space. There is also a requirement for coolers not in use to be kept dry. This is to obviate the accumulation of stagnant water in the base of the cooler. Various proprietary techniques are available to carry out this end-of-cycle water dumping operation.
Evaporative coolers have yet to be implicated in outbreaks of Legionella.
Contributing factors may be:
- Low water quantity stored
- Relatively rapid evaporation of the water into the air stream
- Water is cooled (Legionella is unlikely to multiply below 20°C)
However there is no room for complacency, as evaporative coolers cannot be expected to be 100% effective at evaporating water. Some water carry-over in the form of aerosols can be anticipated. These aerosols are unlikely to dry as the air stream is nearly saturated and distances involved between cooler and occupied space are generally short.
The use of biocides at evaporative coolers is to be avoided. For good air hygiene, evaporative coolers should simply be well maintained and regularly cleaned.
The requirement in AS3666 for evaporative coolers is an example of the general emphasis of the Standard towards ‘what’ is required rather than ‘how’ to achieve it. In this case, the need for cleanability is stated without prescribing specific manufacturing requirements such as rounded corners to trays and sumps.
Cooling coils may at first appear to be innocuous devices in the air handling plant. Any water on them is generally by way of condensate extracted from the air itself and presumed therefore to be microbe free. Further, it is unlikely to stagnate on the coil. There is a need however to ensure the coil surfaces are kept clean (necessary for thermal reasons anyway) and that moisture does not collect on the coil surfaces. With neglect, dust may accumulate and the water running over the surfaces may collect microbial matter before being blown directly into the air path if the air velocity over all or part of the coil is excessive. Studies have shown that aerosols can be produced in this way, although a direct link with health aspects is unproven.
Some coils are continuously sprayed with water for thermodynamic reasons and therefore have a greater need for easy cleaning and drainage facilities.
If components such as drip trays and piping (which are outside the heat exchange coil but still within the air handling plant to improve thermal efficiency) are likely to be lowered to a temperature which is below the dew point of the air passing through the plant then some treatment to prevent condensation may be necessary.
Depending on the psychometric condition of air entering the cooling coil and the coil temperature, a considerable quantity of condensate is often produced at such coils and provision must be made for drainage. Ensure the drain is graded and incorporates a trap having a water seal height greater than the air pressure in the duct (a factor of safety on the height of the water seal height is a sound precaution in case of pressure fluctuations at fan start up or shutdown which may dislodge the water seal). At Note 1 of Clause 2.9.2 the Standard seeks to encourage the use of clear plastic in the drain lines so they can be readily inspected routinely.
In many of the outbreaks of respiratory diseases such as hypersensitivity pneumonitis investigated, the cause was traced to the presence of stagnant water in trays under cooling coils. This stagnant water was found to be a favourable environmental niche for the growth of microbial slimes.
Inadequate drains and drainage is not only inviting health hazards but also leads to costly maintenance. In a study carried out by the BSRIA in the UK it was found that air-conditioning represented about 234 million pounds of annual maintenance cost and that 46% of all maintenance time was related either to cleaning or replacing filters or to attending to condensate problems. Blocked drains are a constant source of problems, particularly that of a propensity for production of slime and difficulty in removing it completely once it grows in the conditions prevailing in the drain line.
The potential for, and cost of, blocked drains should not be underestimated.
The subject of ductwork inspection and cleaning is a contentious issue and opinions vary greatly. AS3666 is a beginning. Further developments, it could be expected, will be driven by evidence of need rather than by opinion. The general principle for hygiene in buildings is to recognise that floor carpets, walls, drapes and ceilings are surfaces, which collect dust. Occupants bring into the building a dust load on their person and shoes. They shed invisible particles of skin. Paper dust is produced in the building and there are other sources of dusts as well. The surfaces of the building require cleaning. So also may the internal surfaces of ductwork.
Much depends on the standard and location of filters at the air handling plant. Filters are not vacuum cleaning devices. They handle only airborne dust and not dust which precipitates onto other surfaces. Well designed filters in the plant provide continuous removal of most of the dust which would otherwise settle further down the ductwork system or continue, airborne, back into the occupied space.
Critical points in the system for installation of access opening are:
- Between outside air intake and filters
– to be able to attend to extraneous moisture or other matter which enters the system
- The return air duct between the space served and the filter if abnormal airborne particulate matter is likely to be generated in the occupied space
– lint from laundries
– dander from animal holding rooms.
AS3666 primarily addresses the most likely sources of health-related problems, viz plant items where moisture is likely to be produced. Access panels shall be provided at these points. It would be prudent for them to be provided throughout the ductwork system although cleaning, or even inspection, of the whole system (particularly the supply air side) may rarely be needed. Obviously the cleaning of existing duct systems would be very costly and may, in most cases be unwarranted. There is not yet clear evidence for the need for such cleaning, but there is concern. Cognisance has been taken in the Standard to ensure that requirements are realistic and achievable under reasonable commercial conditions. But be alerted: the jury is still considering its verdict on this issue and there is an increasing body of literature from overseas sources which points to the need for periodic inspection/cleaning of ductwork. Of course geographic conditions may differ: mention has already been made, for example of the fact that humidifiers are rarely incorporated in air handling plants in Australia, at least for air-conditioning applications.
In theory there should be little need for the cleaning of ductwork provided:
- The ducts are inspected and cleaned thoroughly at the installation or commissioning stage such that dust does not accumulate;
- Dampers, vanes, insulation and other items remain properly in place under conditions of operation;
- Air filters are installed correctly and are properly maintained;
- No further extraneous matter enters the system;
- Ductwork bracing is in accordance with sound specifications such as Nat spec to ensure that ‘pounding’ of the ducting does not occur a fan start-up and shutdown as this upsets any dust which may have settled in the ductwork, making it airborne.
Systems incorporating distributed equipment items (terminal units) shall have access panels provided. Access panels need obviously to be accessible, a feature not always recognised at the design stage and often forgotten during construction or renovation work.
There is now no requirement in the Standard for the cleaning of fans (unlike the requirements of the public review draft) as it is recognised that fans in air systems are rarely wetted and some fans are virtually impossible to clean, or even internally inspect because the scrolls and rotors are tightly configured and not accessible. Again, the prudent building owner should ensure large fans, which do have a large surface area and do usually incorporate access panels, are kept in clean condition.
The public review draft also included flexible ducting in its listing of plant to be cleaned (or replaced). This item has now been removed from the requirements of the final Standard but note that there are limitations on flexible ducting imposed by AS 1668 Part 1.
There is a requirement for access at terminal units for inspection and cleaning. Many proprietary devices of this type incorporate a fan, filter and cooling/heating coil. It has been found in some cases that the insulation on the casing of such units has eroded or collapsed to the extent that it came in contact with the cooling coil (or its condensate tray) and ‘wicking’ of the porous material took place. This production of unchecked stagnant water has led to health related problems. Similar ‘wicking’ can occur, incidentally at building surfaces such as carpet or ceilings and can lead to production of mould.