(How do you prove to a customer that their hvac system may be the source of their indoor air problems? How do you accurately assess the performance of the existing filtration system? How do you characterize the types and particles and their source within the hvac system? The National Air Duct Cleaners Association recently published ACR 2002, Assessment, Cleaning, & Restoration of HVAC Systems. The following excerpt is from the chapter on Aggressive Particle Profiling Procedures.)

Particle measurements must be taken at the return air and supply air registers. A laser particle counter that has the ability to enumerate individual particle sizes and show a printout of the results must be used. The measurements must be plotted on a filter chart, which may be found in ASHRAE documents.

Particle measurements should be taken using a 6-channel laser particle counter operating at a flow rate of 0.1 cfm.

Three (3) samples must be taken at the returning air entering the air-handling unit. Samples must also be taken at approximately 50% of the supply air outlets. For more aggressive sampling, one (1) sample should be procured at a supply outlet during start up of the indoor fan, and one (1) sample should be taken while lightly tapping on the side of the duct.

The particle size range to be studied includes the following: 0.3, 0.5, 0.7, 1.0, 2.0, and 5.0 micrometers. The coincidence error is less than 5% at 2X106 particles/cu. ft.

The particle counter must be properly prepared before sampling occurs. The following actions must be performed:

  • Check battery level to ensure unit does not become unoperational during sampling.

  • Program time and date to current settings.

  • Turn printer option on.

  • Turn machine on, attach purge filter, and run in concentration (CONCEN) mode for one minute. At the end of one minute, print the particle count taken with the purge filter on and stop the unit.

  • Replace the purge filter with the sampling probe.

The laser particle counter should be zero counted and purged each day prior to use (the manufacturer's instructions should be followed for this procedure). The laser particle counter should be factory calibrated and serviced in accordance with manufacturers recommendations (once a year).

Sampling procedures

When performing the sampling, the following procedures must be followed in sequence:

Set unit to desired number of samples (enough for a representative sample.)

Set period to one minute.

Set hold time based on accessibility to sampling locations.

Enable print mode.

Set location to a representative number.

Place counter in an area representative of the return air.

Run sampler in auto mode for length of preprogrammed time.

Remove sampling probe and replace with isokinetic probe.

Set the number of samples equal to the number taken in the return air.

Set location to a representative number.

Sample in the supply ducts by placing the isokinetic tube into the register openings, a different register per sample.

Continue this for the duration of the sample.

The data collected may be used to calculate the percentage increase or decrease in particulate measured in the air. The calculations must be performed using the following formula:

% Increase or Decrease = 100 (Supply air_return air -1)

Once the calculations are completed, the data should be plotted on a modified ASHRAE filter efficiency performance chart.

(Note: Aggressive Particle Profiling Procedures also includes visual observations of the hvac system, air sampling, surface sampling and air temperature/relative humidity readings. For the complete document, contact NADCA at 1518 K Street N.W., Suite 503, Washington DC 20005; 202-737-2926; fax 202-347-8847; www.nadca.com; e-mail: nadca@aol.com.)