Assessment of Literature Related to Combustion Appliance Venting Systems

Abstract

In many residential building retrofit programs, air tightening to increase energy efficiency is constrained by concerns about related impacts on the safety of naturally vented combustion appliances. Tighter housing units more readily depressurize when exhaust equipment is operated, making combustion appliances more prone to backdraft or spillage. Several test methods purportedly assess the potential for depressurization-induced backdrafting and spillage, but these tests are not robustly reliable and repeatable predictors of venting performance, in part because they do not fully capture weather effects on venting performance. The purpose of this literature review is to investigate combustion safety diagnostics in existing codes, standards, and guidelines related to combustion appliances. This review summarizes existing combustion safety test methods, evaluations of these test methods, and also discusses research related to wind effects and the simulation of vent system performance. Current codes and standards related to combustion appliance installation provide little information on assessing backdrafting or spillage potential. A substantial amount of research has been conducted to assess combustion appliance backdrafting and spillage test methods, but primarily focuses on comparing short-term (stress) induced tests and monitoring results. Monitoring, typically performed over one week, indicated that combinations of environmental and house operation characteristics most conducive to combustion spillage were rare. Research, to an extent, has assessed existing combustion safety diagnostics for house depressurization, but the objectives of the diagnostics, both stress and monitoring, are not clearly defined. More research is also needed to quantify the frequency of test "failure" occurrence throughout the building stock and assess the statistical effects of weather (especially wind) on house depressurization and in turn on combustion appliance venting. Incorporating weather variations and house ventilation system characteristics in existing simulation software may assist such analyses and with developing a more reliable diagnostic for use on-site.

A Field Study of Wall Furnace Venting and Coincident Exhaust Fan Usage in 16 Northern California Apartments

“…Study results support the suggestion by (Rapp et al, 2015) and others that many combustion appliance hazards in residences can be identified by test procedures that focus on visual inspection methods and/or non-worst-case testing. In these apartments, the greatest hazards were: (1) high appliance CO (>1,000 ppm in Apt A_2_2), which was identified and corrected prior to monitoring, (2) appliances that were spilling and tripping spill switches under 'natural' test conditions (low-flow continuous bathroom exhaust, in this case), and (3) occupant efforts to defeat the engineered safety features included in their systems (in this case, the intentional blocking of combustion air vents using t-shirts and repeated manual resetting of spill switches).…”

“…The frequency and long duration of downdrafting is notable since the wall furnace being in a downdraft condition immediately preceded each unambiguous spillage event. All burner cycles eventually established draft within the 1-5 minutes currently deemed acceptable by the gas industry (Rapp et al, 2015), but uncertainty remains about the possibility that draft may not always have been complete and robust.…”

“…In the past several years, some consensus has developed around the belief that worst-case testing is overly conservative and that it may result in many "safe" residences failing CAS inspections (Rapp, Less, Singer, Stratton, & Wray, 2015). These purported false-positives lead to reductions in the efficacy of energy retrofit programs, either through diversion of program funds towards remediation of perceived (but not real) combustion hazards, or by limiting energy-saving air-sealing in apartments deemed to be at risk of failing a CAS test.…”

Combustion Safety Simplified Test Protocol Field Study

“…The difficulty in finding houses was surprising. The original work by Rapp et al (2012) suggested that a significant frequency of homes failed combustion safety testing but were unlikely to have problems in practice, and these were the homes that this project was targeting.…”

“…Although the testing standards have recently moved away from WC conditions, few field data remain to correlate the test result to the actual combustion safety incidents in the home. In fact, the general assumption is that the combustion safety test is too severe and results in many false positives that are not supported by the actual operation of the house (Rapp et al 2012;Cautley et al 2012). The greatest needs are: Hard data about field failures A simplified test procedure (STP) that is more predictive than the commonly used procedures of failure under real operating conditions in occupied homes Data from the field to verify the effectiveness of the proposed test procedure.…”