Air Handling Unit Cleaning
In most cases the air-handling plant incorporates the outdoor air intake on the return side of the plant so it can be filtered and conditioned (heated or cooled) before distribution to occupied spaces. All air intakes and exhaust outlets should be inspected at least monthly and cleaned where necessary.
Outdoor contaminants including microorganisms may enter a building with the outdoor air. Sometimes wall-mounted air intakes are inadvertently located near sources of pollution, e.g. intakes in close proximity to:
- Cooling Towers (bacteria & trace chemicals)
- Car Park vehicle exhaust points
- Bird & vermin nests (fungal spores)
- Boiler emissions (airborne hydrocarbons or exhaust gases
- Odours from garbage containers
- Toilet exhausts
- Kitchen exhausts
Air filters are usually installed in air-handling plant in order to clean the air by removing particles. Provision of particulate air filters is strongly recommended in the Standard. Air cleaning is aimed at preventing build-up of particulate contaminates in occupied spaces as well as enhancing equipment efficiency.
Once installed, filters require regular maintenance in order to ensure good performance and to obviate the filters themselves becoming a major pollutant source by encouraging mould growth should they become wet. Filters should be inspected monthly and cleaned or replaced, where necessary.
Evaporative Coolers are a viable alternative to refrigerated air-conditioning. The range of air coolers available today is extensive and varies from small, portable domestic units to large, fully ducted systems for commercial and industrial use.
While each unit has individual features, the main components used in all of them are similar and the principle of operation identical. Regular cleaning and draining is considered to be the most appropriate means of microbial control. The Standard requires 3 monthly cleaning as a minimum but some Australian localities (such as very dry & dusty places) require more frequent cleaning as they are severely affected by the build up of dust on pads and filters.
Evaporative air-conditioning equipment should be inspected at least every 3 months while the system is in use and if not in use for period's greater than 1 month, they should be drained.
Cooling Coils incorporated in AHUs will accumulate organic dust with neglect or poor air-filtration. The water running over its surface 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.
Cleaning of coils should take place during installation and is mandatory at the commissioning phase. All coils in the AHU should be inspected monthly and cleaned when necessary.
Trays & Sumps - the cause of many of the outbreaks of respiratory disease such as hypersensitivity pneumonitis, was traced to the presence of stagnant water in drip trays under cooling coils. This stagnant water was found to be a favourable environmental niche for the growth of microbial slimes. These areas should be inspected monthly and cleaned when necessary.
Inadequate drainage not only invites health hazards, it also leads to costly maintenance. In a study carried out by the Building Services Research and Information Association (BSRIA) in the UK, it was found that air-conditioning represented about 234 Mil (Approx AUS$500 Mil) 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.
Condensate drains, tundishes and traps should be checked monthly for effectiveness and drainage lines flushed cleaned.
Ductwork - Access panels are necessary in ductwork to facilitate ongoing inspection where moisture is likely to arise. The need for cleaning inside ductwork depends largely on the standard and location of filters at the air-handling plant. In theory, there should be little need for the cleaning of ductwork if they were thoroughly cleaned at the installation and commissioning stage, filters are changed regularly, there is minima ingress of moisture and the whole system is well-maintained.
However, there is increasing evidence from Australian and overseas sources which points to the need for periodic cleaning of all ductwork. Ductwork should be inspected annually and cleaned when necessary.
The Standard states that all items of equipment must be provided with access for cleaning, draining or periodic replacement. Contamination by fungi or bacteria can develop on wet surfaces. It has been suggested that an acceptable level of airborne viable microorganisms in occupied spaces is around 500 to 1000 colony forming units per cubic metre.
Levels of 3000, 5000 and 10,000 have been associated with hypersensitivity pneumonitis outbreaks and it is generally thought that a level of 1000 is sufficient to warrant investigation and improvement (which is not to say that the air is unsafe or hazardous as such assessment requires the services of medical practitioners and epidemiologists).