How can I verify ductwork cleanliness after cleaning?

If you have ventilation air ductwork, it’s a very good idea to get them professionally cleaned periodically by a B&ESA (Building and Engineering Services Association) accredited company

Health and safety regulations stipulate that employers must ensure that their ducts are thoroughly cleaned and maintained so having professional cleaners in is the only real way to guarantee this is the case.

The penalties for not complying with this legislation are stiff and employers or property owners could face prosecution if they fail to follow the rules.

But after having your ductwork cleaned professionally, how will you know that it’s been done? We examine the differences you could expect to see and find out what proof you could obtain.

Personal Visual Inspection   

You can of course take a look yourself. Presumably you will have seen the internal condition of the ductwork prior to engaging a cleaner and therefore be in a good position to judge the difference when the work is finished and ready to be signed off. This is a good method as any irregularities can be tackled whilst the operatives are still on site.

Before and After Photographs

Probably the most common form of verification particularly if the client engineer/contact cannot be present on site.The before and after situation can be clearly seen in the images. A typical situation is shown below.

Report Sheet

One step up from before and after photographs where a little more detail is added to produce documented evidence of the location, date and time that the cleaning work has taken place. Comments regarding the overall condition of the system and recommended cleaning frequency can also be added.

Dust Thickness Tests

There are three recognized testing procedures for measuring the thickness of debris on internal ductwork surfaces.

Electromagnetic Thickness Gauge – This device will give an instant readout when the probe is placed on the ductwork surface. The device should be calibrated before each session so that it may operate within + or – 3 microns. At least 20 readings in any one location should be taken and an average obtained.

Preferred Vacuum Test – This test utilizes an air pump to suck dust from the surface of the ductwork guided by a plastic template. The collected dust is deposited on a sample media which is then sent to an independent laboratory for analysis which can take about 7 days to produce the results.

Wet Film Thickness Test – To be used on greasy/sticky surfaces where the above two methods become unusable. This consists of a precision gauge which can measure from 50 microns up to 800 microns at suitable increments. The gauge should be cleaned after each measurement.

Microbiological Testing

Where ductwork systems are serving sterile or clinically clean areas in hospitals, laboratories or food processing plants there is often a requirement to sterilize the ductwork surfaces with a biocide to kill any bacteria that may be lingering after the cleaning process has been completed. Swab samples can then be taken from the surface of the ductwork and sent to an independent laboratory so that cultures can be grown and identified. The report normally arrives 7 days after submission of the samples.

Full Written Report

This is where some of the testing methods previously described are combined into a comprehensive written report which could be used as a basis to determine a future cleaning, maintenance or testing schedule.


Establishing the facts about the internal condition of your ventilation ductwork system need not be a difficult task. Many of the methods mentioned are recognized within the B&ESA (Building & Engineering Services Association) TR19 Document.

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