Backdrafting a Concern for Homeowners, Deters Air Sealing

A water heater fails the exhaust spillage test, indicates backdrafting is occurring.

While air sealing retrofits reduce drafts and reduce a home's energy consumption, they also present potential health hazards to occupants. Air sealed houses depressurize more readily when using exhaust fans, including bathroom fans and those in range hoods and dryers.

Concerns about increasing backdrafts and spillage prevent people from air sealing their homes, driving costly fixes in other homes and requiring expensive testing in homes being considered for air sealing that also contain vented combustion appliances.

Residential Building Systems group of the Environmental Energies Technologies Division compiled 25 years worth of information regarding combustion appliance venting and safety diagnostics into a new report that shines light on research needs and opportunities to improve both residential energy efficiency and indoor air quality.

The report is expected to steer efforts to improve safety inspections for gas appliances in homes that are air tightened for energy efficiency and to reduce barriers to home air tightening and combustion appliance venting.

Previous studies attempted to assess the performance of current combustion safety diagnostics by identifying appliances and homes that might encounter backdrafting and spillage events, but much of the research focused on comparing results of short-term "stress" tests with results from in-home monitoring over periods of a week or more.

Because results from stress tests identified problematic appliances and homes for which there was no sustained spillage over the monitoring period, the compiled data suggests that backdrafting 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 clearly indicate spillage risks 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.

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.