Reducing Barriers to Residential Building Air Tightness: Assessing Safety Diagnostics for Atmospherically Vented Combustion Appliances

November 2012

Research Highlight

The following article is by Vi Rapp, a postdoctoral scholar in the Environmental Energy Technologies Division.

Air sealing of homes to reduce the uncontrolled entry of outdoor air is typically among the most cost-effective retrofit measures to reduce energy consumption and associated greenhouse gas emissions. Airtight envelopes are at the center of energy efficient upgrade practices promoted by the California Energy Commission and the U.S. Department of Energy's (DOE's) Building America and Low-Income Weatherization programs.

However, tighter houses present potential health hazards to occupants because they depressurize more readily when using exhaust fans, including bathroom fans and those in range hoods and dryers. Depressurization increases the likelihood that the ordinary, upward exhaust flow of a vented combustion appliance—such as a water heater or furnace—will be reversed in a process called "backdrafting." Backdrafting can cause combustion exhaust products to spill into the home rather than being vented upward and out through the vent. This spillage can expose residents to hazardous air pollutants that are produced by the combustion appliance burners.

Concerns about increasing backdrafts and spillage are limiting air sealing in many homes, driving costly fixes in other homes, and requiring expensive testing in homes that are being considered for air sealing but also contain vented combustion appliances within the living space.

For five months, we have been compiling 25 years worth of information regarding combustion appliance venting and safety diagnostics into a new report. The report shines light on research needs and opportunities to improve residential energy efficiency and indoor air quality. With funding from the California Energy Commission and DOE, the Residential Building Systems group of the Environmental Energy Technologies Division is working to reduce barriers to air tightening and combustion appliance venting.

The report is the first known document to compile and critically review literature related to combustion appliance venting. It is expected to steer efforts to improve safety inspections for gas appliances in homes that are air tightened for energy efficiency.

A number of studies have attempted to assess the performance of current combustion safety diagnostics in identifying appliances and homes that will encounter backdrafting and spillage events. Much of this research focuses on comparing results of short-term "stress" tests with results from in-home monitoring over periods of a week or more. Results from stress tests typically identified problematic appliances and homes for which there was no sustained spillage over the monitoring period. Available monitoring data support, but are not adequate to prove, the team's hypothesis that spillage is uncommon, since it requires the coincidence of environmental conditions, building characteristics, and exhaust fan usage.

Although the body of existing research provides some assessment of existing combustion safety diagnostics, the objectives of the diagnostics are not clearly defined and do not provide a clear indication of the risk of spillage during normal operation. Research also suggests that existing diagnostics are not reliable and repeatable predictors of venting performance.

A key deficiency of the research is that the test methods do not explicitly treat backdrafting and spillage as both physical and statistical phenomena. The statistical phenomenon associated with spillage is especially important because health risk depends on how frequently spillage occurs when an appliance emits a large amount of pollutants. Therefore, more research is required to quantify the frequency of test "failure" occurrence throughout the building stock, as well as to assess the statistical effects of weather (especially wind) on house depressurization, and in turn on combustion appliance venting.

In June 2012, DOE's Building America Program and the Partnership for Advanced Residential Retrofit (PARR) hosted an expert meeting on combustion safety, to identify gaps and barriers in the research and progress toward harmonization of existing combustion safety protocols. We presented the results of the team's research, and we believe them to be a valuable asset for advancing harmonization of those protocols. A number of meeting participants agreed that incorporating physical and statistical phenomena into existing combustion safety diagnostics is a valid approach to mitigating risk.

The next step in this project will be to use simulation software to identify health risks associated with combustion spillage. The goal of the project is to develop, by the summer of 2013, a more reliable diagnostic method that incorporates the physical and statistical phenomena associated with backdrafting and spillage to mitigate risk.

Vi Rapp is a postdoctoral fellow in the Residential Buildings Group of the Environmental Energy Technologies Division.

For more information, contact Vi Rapp at

Rapp, V.H., B.C. Singer, J.C. Stratton, and C.P. Wray. 2012. Assessment of Literature Related to Combustion Appliance Venting Systems. LBNL Report 82163.

Vi Rapp