Clogged filter: your pressure switch might be lying to you

A classic reflex in specifications for air handling units: fitting a differential pressure switch to detect clogged filters.

On a constant-volume installation, that solution works perfectly well. But as soon as the air flow varies, a single fixed threshold becomes barely representative.

Why a fixed threshold is not enough

Because a filter’s pressure drop depends both on how clogged it is and on the air flow passing through it. And the relationship is not linear: as the flow rises, the pressure drop increases sharply. The same filter can therefore show very different pressure readings depending on the installation’s operating point.

What it means in operation

• At low flow, a heavily clogged filter can stay below the switch threshold and never raise an alarm.
• At high flow, a still-clean filter can briefly exceed the threshold and trigger a false alarm.
• During load changes, the operator cannot tell whether the rise in ΔP comes from the filter or simply from the air flow.
• Filters get replaced too early, too late, or on a fixed calendar unrelated to their actual condition.

A pressure switch set at nominal flow therefore ends up blind at part load and hypersensitive at full load.

Our recommendation: a differential pressure sensor

At Workswell we recommend a differential pressure sensor with continuous ΔP measurement. Combined with flow measurement, fan speed or a reference operating point, it makes it possible to tell a normal pressure variation from an increase genuinely caused by filter clogging.

Concrete benefits

• Reliable detection across the whole operating range.
• Alarm threshold adapted or compensated according to actual flow.
• Pressure drop displayed on the supervision system.
• Trend logging and monitoring of the clogging rate.
• Detection of abnormal changes, for instance after works or an incident.
• Replacement planned before a critical fault appears.
• Fewer false alarms and fewer unnecessary site visits.
• Better use of the filter’s real service life.
• Reduced additional electricity consumption caused by overly clogged filters.
• The option of several alarm levels: pre-alarm, maintenance to be scheduled, and maximum threshold.

A measured reference from commissioning onwards

The sensor also provides useful information at commissioning. It allows the initial pressure drop of the clean filter to be checked and compared with the manufacturer’s data. Any later drift can then be assessed against a measured reference rather than a generic threshold.

Constant or variable flow?

The pressure switch remains relevant as a simple device or as a last line of safety. On a variable-volume installation, however, it should no longer be the only means of monitoring.

• Constant flow: a pressure switch may be sufficient.
• Variable flow: continuous measurement, compensation and trend monitoring become essential.

Let’s talk about your installation

Are you drawing up specifications for an air handling unit or an MCR/BMS installation? Let’s discuss it.