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N°8 – Monitoring filter cleanliness – HSC

For training at PB level, this section and the others will not be studied.
Article CH 38 concerning safety French regulations against fire and panic risks in buildings frequented by the general public, specifies that for:

  • Air handling units processing more than 10,000 [m³/h],
  • Air handling units supplying sleeping accommodation premises
  • All air handling units linked to one or more distribution networks common to these units, and processing more than 10 000 [m³/h],

A control device for head loss of filters must be fitted

This control of filter head loss serves to monitor their cleanliness.

A reminder that the head loss of a piece of equipment corresponds to the fall in pressure it generates for the circulating fluid.

Filter head loss can be determined by measuring the pressure on both sides.


What is the head loss of the filter shown below?

The head loss of the filter is the difference in pressure measured on both sides.
The difference here is 85 [Pa].

This pressure difference can also be measured by a “U Tube”.

The filter head losses can thus be measured and monitored.

As a preliminary approach we can see that a filter is contaminated when its head loss has doubled compared to its condition at installation, but for more precision it is necessary to consult the supplier.

To control the progress of filter clogging it is also possible to fit a pressure switch controlling head loss.

When the head loss of a (clean) filter has doubled, with the regulated setting of the pressure switch exceeded, a visual alarm could be displayed on the electrical panel of the AHU.


Knowing that the clean filter of the AHU below has just been fitted, propose a setting for the pressure switch monitoring its contamination.

The filter head loss is the difference in pressure measured on both sides.
The filter is clean and the pressure difference here is 65 [Pa].
We could imagine a setting in the region of 130 [Pa] (2 x 65).

Note : in fact to determine the maximum level of filter clogging efficiently, we should assess the variation of flow that its clogging would generate.
It will also be necessary to take into account the recommendations of the suppliers, for the performance of a filter (its efficiency) can vary according to the levels of contamination.
This subject goes beyond the scope of the current section.