Residential wood heating: An overview of U.S. impacts and regulations.

Air pollution from residential wood heating poses a significant public health risk and is a primary cause of PM nonattainment in some areas of the United States. Those emissions also play a role in regional haze and climate change. While regulatory programs have focused on emissions reductions from large facilities, the residential heating sector has received limited attention. The failure to develop effective programs to address this emission source hampers the ability of state and local air quality programs to meet clean air goals. An updated New Source Performance Standard (NSPS) for Residential Wood Heaters was promulgated in 2015, which includes more stringent emissions standards for wood stoves and broadens its scope to regulate additional types of wood heating appliances. However, weaknesses in the test methods and programs used to certify compliance with the NSPS limits hamper the efficacy of those requirements. Current emissions certification tests measure stove performance under defined laboratory conditions that (1) do not adequately reflect operation and performance of appliances in homes, (2) are not sufficiently repeatable to allow for comparison of emissions of different appliances, and (3) allow manufacturers leeway to modify critical test fueling and operating parameters which can significantly impact performance outcomes. These foundational regulatory issues present substantial challenges to promoting the cleanest and most efficient wood heating systems. This paper provides an overview of the air quality and public health impacts of residential wood heating and discusses the weaknesses in the current emission certification approaches and work by the Northeast States for Coordinated Air Use Management (NESCAUM) and the New York State Energy Research and Development Authority to develop improved testing methods. Other articles in this issue discuss the development and testing of those methods in detail.Implications: Air pollution from residential wood heating poses a significant public health risk and is a primary cause of PM nonattainment in some areas of the United States. Those emissions also play a role in regional haze and climate change. While regulatory programs have focused on emissions reductions from large facilities, the residential heating sector has received limited attention. The failure to develop effective programs to address this emission source hampers the ability of state and local air quality programs to meet clean air goals. This paper provides an overview of the issue.

Impact of fueling protocols on emission outcomes for residential wood-fired appliances.

Many believe that certification testing of residential wood heat appliances should provide data indicative of installed performance. Operationally, test methods typically only assess steady-state emissions and fail to include other typical conditions for batch appliances such as start-up. From a fueling perspective, protocols should ensure a consistent approach reflecting common use practices. Ensuring representative conditions and accurate quantification of emissions requires assessing the impact of different start-up conditions and whether or not start-up conditions affect appliance operation during start-up and beyond. This study evaluated the impact of modifying fuel piece sizes and configurations using a "smart" wood-fired hydronic heater (WHH) cordwood appliance. The appliance represents technologies using software and oxygen sensors to improve performance. Since the study used a "smart" appliance, the results likely reflect the least amount of variability found in a WHH cordwood appliance. The analysis consisted of a series of tests that involved changing one fuel variable per series, including: (1) kindling fuel arrangement in the firebox; (2) fuel piece size; and (3) the amount of kindling and starter fuel used. A goal of the study was to determine how each variable affects emissions performance during start-up and the following steady state load. Testing used a dual-stage combustion cordwood WHH equipped with external thermal storage. Particulate matter (PM), carbon monoxide (CO), and delivered heating efficiency were measured, and visible emissions from the stack and secondary combustion chamber were observed. Replicate tests were conducted for each protocol series to evaluate WHH performance reproducibility. These tests found that for a low-mass staged combustion WHH with external thermal storage, the use of different fueling protocols can substantially affect PM and CO emissions.Implications: As test methods move to incorporate measurements beyond steady-state emissions, fueling protocols must be assessed to determine (1) if they reflect typical field procedures and (2) the impact of start-up procedures on the complete test run. This paper assessed how changing start-up conditions affected run variability and PM emission impacts.

Impacts of wood species and moisture content on emissions from residential wood heaters.

Homeowners burn wood of a wide range of species and moisture content (MC) in residential cordwood and pellet stoves. An effective emission certification test protocol must account for and accurately measure the impact of those variables in order to ensure a reasonable correlation between laboratory results and in-use emissions and to promote the design and manufacture of cleaner burning appliances. This study explored the effect of wood species and MC on emissions and efficiency in four cordwood and four pellet stoves. PM emissions were consistently lower with pellets manufactured from softwood than for hardwood species and were highly correlated with ash content. Higher MC oak fuel substantially increased PM emissions in a non-catalytic cordwood stove; however, a hybrid cordwood stove was able to meet federal emissions limits even with the higher MC fuel. The results of this study, in combination with previous research, suggest that certification tests that use softwood fuel likely report lower emissions than tests that use hardwood. Requiring hardwood and higher MC cordwood fuel in certification tests would enable the assessment of an appliance's ability to operate well even when fuel conditions are not optimized.Implications: The emission testing results reported in this paper call into question the adequacy of the fuel moisture content and fuel species specifications in testing protocols approved for certifying compliance with EPA's New Source Performance Standards for cordwood and pellet stoves. We recommend changes in those specifications, including the prohibition of testing with Douglas fir and other low ash softwood species, requiring the use of cordwood test fuel with a higher moisture content, and requiring pellet stoves to be tested using hardwood pellets. Adoption of these measures would increase the replicability of tests. allow for the identification of stoves that are unlikely to perform well in the field when fuel conditions are not ideal, and, ultimately, result in the design of cleaner burning stoves.