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Perfluorocarbon Tracer (PFT) Ventilation Testing

The PFT test method [1] is a relatively simple, very approximate method of testing ventilation rates in a home.  Up to six different types of tracer gas are continuously injected into the home via small, passive emitter capsules.  Capillary adsorption tube samplers (CATS) detect the average concentrations of each tracer present at various sampling locations.  Then, laboratory analysis of the CATS indicates the matrix of airflow rates among the emitter and sampling locations as well as the atmosphere (outside air).

The minimal apparatus is unobtrusive, allowing testing for long periods while the home is occupied. Thus, the effects of occupant behavior, as well as averaging over a variety of weather conditions and HVAC operating modes, can be included in the measurement. It is also an inexpensive way of diagnosing airflow patterns, such as entrainment of pollutants from a garage or crawl space into the conditioned space. However, despite the  many advantages to a PFT test relative to alternatives like tracer gas tests, it is an inaccurate test method due to the many sources of uncertainty. 

Limitations:

The PFT method is subject to several significant sources of error, including the following:

  • Miscalibration of the laboratory equipment that analyzes the CATS.  This has been an issue in past field tests.
  • Uncertainty in estimating the volumes of zones within the home, among which air is exchanged, with the assumption of uniformity within each zone.  This is a rough approximation of actual tracer distributions throughout the volume.
  • Possible contamination of the CATS during handling and storage.  Errors may be larger in short-term PFT tests than in longer-term tests, because of the lower signal-to-noise ratio in the CATS samplers (less of the tracer gas compared to the contaminants).
  • Reciprocal averaging:  Because ventilation rates are inversely related to tracer concentrations, the measurement of time-averaged tracer concentrations systematically underestimates the average air-change rate (ACR) when the rate varies significantly during the test. When considering indoor air quality, the reciprocal ACR is regarded as more meaningful than the mean ACR, because it more closely corresponds to the exposure of occupants to pollutants over the time period. However, for heating and cooling calculations, the actual average ACR is more meaningful.

“Even avoiding the worst situations of assumption violations, [continuous-injection, long-term sampling] should be considered as having a something like a factor of two uncertainty for the broad field trials that it is typically used in.” [2]

Because of these accuracy concerns, a recommended practice is to include a number of control samples with the test samples in order to analyze systematic and random error trends.  The control samples may be exposed during side-by-side testing with tracer decay tests over several days in preliminary short-term testing.  The tracer decay method is not subject to the same sources of error and is considered more reliable.

References:

  1. Dietz, R.N.; Goodrich, R.W.; Cote, E.A.; Wieser, R.F. (1986).  Detailed Description and Performance of a Passive Perfluorocarbon Tracer System for Building Ventilation and Air Exchange Measurements.  Measured Air Leakage of Buildings, ASTM STP 904, H.R. Trechsel and P.L. Lagus, Eds.  Philadelphia, PA: American Society for Testing and Materials, pp. 203-264.
  2. Sherman, M.H.; Walker, I.S.; Lunden, M.L.  (2014).  Uncertainties in Air Exchange using Continuous-Injection, Long-Term Sampling Tracer-Gas Methods.  International Journal of Ventilation.  LBNL Paper LBNL-6544E.