Pressure Drop in Commercial and Industrial Facilities

For an introduction to the concept pressure drop, please read our previous post: Pressure Drop Basics.

Pressure drop is the best indicator of the status of a filter and when it should be removed.  We highly recommend that facilities with many filters in use rely on a manometer to schedule their filter changes,

When Pressure Drop is Too Low

Every filter has a specified initial pressure drop (IPD), when installed.  For example, at 500FPM, a Taff Pleat MERV 8 has an initial pressure drop of 0.2”.  If your manometer indicates a pressure drop lower than your filter’s IPD, this could indicate an issue such as:

  • The filter has been damaged, and air is passing through a perforation, rather than the filter media.

  • There is not a good seal around the filter, and too much air is able to pass around the filter, rather than through it.

  • The manometer is broken, or improperly calibrated

  • The filter has been removed

If a filter has been in use for a high percentage of its typical useful life, and the pressure drop is still close to the IPD, this could indicate one of the above issues, or another issue such as:

  • The filter is not efficient enough – most of the particles in the air are smaller than the filter is designed to remove, and are passing through the filter, rather than being removed by the filter.

  • The prefilters are preventing most particles from reaching the final filter.  This is not necessarily a problem.

When Pressure Drop is Too High

Each filter also has a Final Pressure Drop (FPD).  For a Taff Pleat MERV 8, this is 1.0”. Generally, a filter will arrive at its FPD after approximately the same amount of use as it has before.  At that point, it should be changed.  However, if the filter arrives at its FPD too soon, this can indicate issues such as:

  • A new source of particulate, such as debris or rust in a duct

  • Airflow has become unbalanced and is only going through some of the filters in a bank

  • The filter has become obstructed by an object, or “blinded” by an oil or liquid.

  • The manometer is broken, or improperly calibrated

The Importance of Changing at Final Pressure Drop

Using a filter beyond its intended FPR requires your fan motors to work harder, which has several negative consequences:

  • Increased energy usage

  • Increased carbon footprint

  • Increased wear on equipment

Here is an example:

An HVAC system with 60% motor blower efficiency has 6 Taff-Pleat MERV 8 filters in a bank.  The system is in operation 24 hours/day, for 92 days, about 3 months.  In that time, the filters go from 0.2” to 0.75” of pressure drop.

Since the filter has not achieved its FPD of 1”, it is left in the system.  However, rather than checking the pressure drop, the filters are left in the bank another 92 days. The pressure drop is 1.30, when the filter is changed.

Here is a comparison of the energy consumption and carbon output that will result.

 

Energy Consumption (kWh)

  • Period 1: 2466

  • Period 2: 5316

CO2 Produced (lb.) *

  • Period 1: 3327

  • Period 2: 7180

*CO2 emissions from fossil fuel burning power plant

 

The energy consumption and the carbon emissions are more than doubled, in the second period, because of the increased restriction of using the filter past its intended final pressure drop.

Jeff Rosen