Duct cleaning benchmark

29th of June 2011
Duct cleaning benchmark
Duct cleaning benchmark

A new European standard creates a benchmark for the cleanliness of heating, ventilation and air conditioning (HVAC) systems and ductwork. Hartley Milner takes a closer look.

Input from the cleaning and hygiene sector has helped shape a new standard for safeguarding the well-being of people in buildings across the EU. The document from CEN, the European Committee for Standardisation, creates a benchmark for the cleanliness of existing and new heating, ventilation and air-conditioning  (HVAC) systems and ductwork.

As well as being important for comfort and health, regular cleaning of ventilation systems helps keep down energy consumption, preserves unit service life and enhances activities or processes carried out in the areas they serve.

Major contaminants leading to air quality issues include dust and debris, lubricant oil residues from duct manufacture and harmful micro-organisms. Poor air quality is thought to be a contributing cause of the condition known as ‘sick building syndrome’, explained here by Craig Booth, director of cleaning equipment supplier Triventek and contributor to the new standard.

“The ill effects of so-called sick building syndrome are often minor but can cause chronic fatigue, headaches, lethargy, upper respiratory tract infections and irritation of mucous membranes such as the eyes. These are most significant to health and productivity in air-conditioned buildings where intellectual work such as banking, insurance, software development and customer service is carried out.

“Typically, dirt from the outside air and recycled from within the building builds up over years of operation, and it is also not uncommon for new HVAC systems to start off contaminated with materials associated with their manufacture and site installation.

“Since 1992 European workplace health, safety and welfare regulations have stipulated mechanical ventilation systems should be cleaned ‘as appropriate’. The purpose of these standards was to help building managers get some clarity as to what is appropriate, ie, what is clean and what is dirty.”

The scope of the CEN document is broad and includes system inspection for air quality and contamination issues, assessment of the need for cleaning, cleaning procedures and after-clean evaluation. Plus it sets out requirements for maintaining the cleanliness of ducted ventilation and touches on the design, build and installation of units for ease of cleaning. Installations for industrial processes are not included.

There is also flexibility for revisions and additions at a later date. It is envisaged that kitchen extract systems will be included in due course.

As well as building managers, target groups include system designers who specify the means of access to units, and services and maintenance companies.

For the first time, there are three classifications of building, with the quality of system cleanliness varying according to the site's vulnerability to contamination – low, medium and high. Typical examples of the cleanliness quality classes are shown below.

Low: rooms with only intermittent occupancy, eg, storage rooms, technical rooms.

Medium: offices, hotels, restaurants, schools, theatres, residential homes, shopping areas, exhibition buildings, sport venues, general areas in hospitals, general working areas in industries.

High: laboratories, treatment areas in hospitals, high quality offices.

The level of cleanliness required in these categories is aligned to the building’s use and type of ventilation system installed.

In a true spirit of co-operation, extensive work was undertaken by the European cleaning industry towards a consensus on measurement methods for cleanliness quality, where previously each country operated its own system.

The preferred method derives from the Finnish vacuum test model, whereby a result is achieved using a high volume air sampling pump and then measuring the weight of dust collected. This procedure is suitable for circular, flat-oval and rectangular sheet metal duct types. Other measurement methods for less common configurations such as internally-insulated ducts are also included.

To comply with the classification, samples should show results below acceptable cleanliness levels specified in the CEN document.

“More stringent limits are applied to supply and return ductwork where the air handled goes to the served area rather than to extract ductwork where the air is simply vented into the atmosphere,” Craig Booth said. “Measuring cleanliness or dirtiness according to an agreed protocol is an objective alternative to, or support for, simple visual assessment. After all, one man's dirty is another man's clean.”


Systems should be cleaned only when it is demonstrated to be necessary, rather than at an inflexible, pre-arranged time.

While basic and subjective, visual inspection provides a good low-cost indicator of the condition of a system and the need for cleaning. It can be effective in detecting extreme microbial growth such as slime in water reservoirs and obvious deposits from faulty filters, as well as point to lack of maintenance. This method can be supported by technical aids – cameras, endoscopes or robotic cameras with video capability.

For a more objective approach, system inspectors should have knowledge of a range of ventilation units. A trained person who has the use of special instrumentation is also better placed to evaluate the clean on completion.

Inspection intervals are recommended for each of the cleanliness quality classes in months, ranging from 12 to 48. Extremely hygiene critical components such as filters and wet areas of systems like humidifiers and cooling sections should be inspected more intensively at a six-month frequency.

The inspection plan should include information about the cleanliness quality class of the building and measurement methods being employed, plus confirmation that the inspectors have the qualifications and experience to do the job. There is also guidance for checking out new installations.

Cleaning need assessment

Assessing the nature of any contamination should be undertaken during the inspection process. This is important because it defines the required cleaning method.

System components are deemed polluted when visual inspection and analytical verification reveals unacceptable dust levels and microbial or other contamination. Dust accumulation and the type of pollutants present, such as bacterial growth, should determine the need for cleaning, cleaning methods and any environmental controls that will be required.

The standard sets out acceptable and unacceptable contamination levels according the preferred weight-based measurement method (g/m2) in three different scenarios relating to existing HVAC systems, new duct systems and cleaned systems.

"In all three cases, the standard provides the clarity and a benchmark that building owners, builders and service contractors have been crying out for," Booth said.

Where hazardous dusts such as asbestos and radiological materials are suspected to be present, a specialist investigation is required.


Prior to any work being carried out, a cleaning plan should be drawn up detailing the work, targets and tasks. It should include:

•A summary of the assessment, including the systems and parts that need cleaning

•Detailed time schedule for the work

•Cleaning methods to be employed

•Description of how access will be gained to ducts and equipment not visible in rooms

•List of components to be removed for cleaning and those to be replaced

•Protection of the rooms in which the cleaning equipment is to be used

•Briefing of building occupants about the work, including health and safety considerations

•Microbiological considerations when relevant, including the use of disinfectants

•Assessment of the need for specialist help regarding microbiological issues and targets for cleanliness after the work is completed

•Evaluation of the after-clean to confirm that the outcome conforms to an acceptable cleanliness level according to a defined measuring method.

Unless otherwise stated in system documents, cleaning intervals are dictated by the state of the unit highlighted during inspection. Cleansing methods must be sufficient to achieve the required level of cleanliness without damaging the system or impacting on building users and the environment generally.

The final evaluation to ensure the work meets acceptable post-clean levels should be made after all parts of the ductwork or system have been visually assessed as clean. An objective verification may also be required using the preferred vacuum test method detailed in the document.

A signed cleaning report setting out details about the system and the methods and equipment used to purge it is also required, along with the results of ‘before and after’ cleaning assessments and any recommendations that follow, perhaps relating to a time for the next clean.

Manufacturers of air conditioning, ventilation and air handling systems are singled out as having a key role in reducing health and hygiene issues in buildings by making life easier for those who have to inspect the units and carry out any cleaning work.

The CEN document stresses the need for systems to be designed, constructed and installed so they can be maintained to an acceptable standard during their entire lifetime. For this it is necessary for handing-over documents to identify the cleanliness quality class of the building for which a specific unit is intended and any design and installation issues, along with the means for keeping it clean.

The standard is a parallel document to EN 12097, which specifies the access components needed to permit effective cleaning.

•The document (prEN 15780:2008) was prepared by technical committee CEN/TC 156 ‘Ventilation for buildings’, the secretariat of which is held by BSI. It is due to be ratified on November 15 2011. The draft is available from each EU country’s national standards institution or from www.triventek-ductcleaning.com/news.asp


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