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Tracer Gas Testing

Blower door and duct pressurization tests provide measurements of the leakage characteristics of the envelope and air-distribution systems at elevated and uniform pressure differences, but may not be accurate for predicting how much outside air actually enters a house under particular driving forces. Tracer gas measurements can provide such direct measurements [1]. Single zone tests measure the net whole-house air exchange rates under a variety of operating conditions. Multi-zone tests provide additional insight into how outside air is distributed room to room inside the house. Inside and outside temperatures, as well as outside wind speeds, should be measured during the tests.

Effective tracer gases should be detectable at low concentration, safe for humans, not naturally occurring, and have neutral buoyancy in air. The most common tracer gas for Building America projects is sulfur hexafluoride (SF6). In certain instances, multiple tracer gases are used to detect multi-zone interactions. NREL developed a test protocol for multi-zone tracer gas field tests [3].

Single-Zone Tracer Gas Decay Test

During a typical field test, six sample points are installed throughout the house and SF6 is injected in all areas of the house and mixed to a uniform concentration using the air handler or portable fans. The rate of decay in SF6 concentration is used to calculate the air exchange rate, stated as air changes per hour (ACH), due to infiltration or intentional ventilation. For single-zone tests, a combination of small portable fans, ceiling fans, and de-stratification fans (for multi-story homes) are operated to help maintain uniform mixing throughout the home. Portable heaters are used to control the interior temperatures during tests where operation of the space heating system is not desired such as natural infiltration or ventilation tests. The results from the six sampling points are averaged to calculate whole-house concentration at each time step. This method is documented in ASTM E741 [2]. For room to room distribution tests, interzonal mixing is not desired. The degree of mixing is inferred by examining the decay curves in each room under various test conditions. Mixing fans are used to create well mixed conditions inside each room to ensure that the measured concentration represents the zone average.

A bump test is used to determine the change in whole house air change rate caused by changing the operating state of a building system. Bump tests can be performed on the air handler or ventilation system during a period when natural infiltration is steady. During a tracer gas test, the air handler is turned on for a prescribed period of time and an increase in ACH implies there are leaks in the ductwork going to and from the air handler to outside. Measuring the duct leakage during a tracer gas test in the form of a bump test should be more reliable than a duct blaster test since the system is operating under normal pressurization conditions. A bump test can also be performed with the home’s ventilation system. For a balanced ventilation system, the increase in ACH should correspond to the volume rate of air being pulled out the house, but in unbalanced ventilation, the effective ventilation may not correspond to the volume of air being exhausted. A bump test on the ventilation system will measure the effective rate of ventilation in the home.

Quantities that can be measured using a single zone tracer gas test:

  • The natural infiltration rate of the house under a particular set of weather conditions
  • How infiltration is changed when the air handler is turned on in the heating/cooling/fan mode
  • How infiltration is changed when the ventilation system is operated

Limitations:

  • Results cannot always be generalized to different weather conditions.
  • The air handler bump test measures the change in air infiltration caused by pressure imbalances resulting from air handler operation, including the effects of supply and return duct leakage. Supply and return duct leakage cannot be easily separated from each other, or from local pressurization effects.
  • The location of duct leaks can skew the results of the air handler bump test. For instance, take a situation where the ducts are located in a vented attic and there is a leak on the supply side. The tracer gas will leak into the attic and mix with that unconditioned air. The air handler will also pull air from the attic in through the leak, which will contain some tracer gas, but also will be coming from an unconditioned space, unlike the rest of the return air. It is difficult to tease these details out of the bump test results and so the true effect of duct leakage on energy usage cannot be known precisely.

Tracer gas testing set-up, including a space heater, a small fan and a sampling tube
Ed Hancock

Multi-Zone Tracer Gas Decay Test: Reciprocal of Age-of-Air

Outside air distribution (natural infiltration combined with mechanical ventilation) can be non-uniform within a home causing ineffective dilution of contaminants and thus indoor air quality (IAQ) concerns in certain parts of the house. In evaluating the non-uniformity of outside air distribution within a home, room-to-room tracer gas test can be performed to examine the concentration decay or "age-of-air" in each room. For a detailed description of this test, reference "A Test Protocol for Room-to-Room Distribution of Outside Air by Residential Ventilation Systems" [3].

Quantities that can be measured using a multi-zone tracer gas test:

  • The reciprocal age-of-air in each zone under a particular set of operating conditions and weather conditions.

Limitations:

  • Interzonal airflows, which are necessary to analyze pollutant control when sources are non-uniform, cannot be calculated from a multi-zone, single tracer gas test. A multi-zone, multi-tracer gas test is necessary to quantify interzonal airflows.

References:

  1. Hancock, E.; Norton, P. and Hendron, R. Building America System Performance Test Performance Test Practices: Part 2, Air Exchange Measurements. National Renewable Energy Laboratory, August 2002. NREL/TP-550-30270
  2. ASTM E741, Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution. Conshohocken, Pennsylvania: American Society for Testing and Materials, 2000.
  3. Barley, D.; Anderson, R.; Hendron, R. and Hancock, E. A Test Protocol for Room-to-Room Distribution of Outside Air by Residential Ventilation Systems. National Renewable Energy Lab. Golden, CO. December 2007. NREL/TP-550-31548.
  4. ASHRAE (2002). ANSI/ASHRAE Standard 129-1997 (RA 2002). Measuring Air-Change Effectiveness. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.