While the growth spurt is continuing, it is also showing signs of slowing down. In the last six years, wood heat showed its greatest uptick in 2005-2006, growing 8.5% in one year. On average from 2000 to 2010, wood grew 4.5% each year, so a 1.7% increase last year may indicate a softening of the market. (Click here for update from 2012 census.)
The US Census does annual surveys based on sample populations which are then extrapolated to estimate trends in the entire country. The margin of error is higher during these annual surveys than during the decennial surveys where the Census tries to contact all households. The slower rate of rise in wood heat in 2010 may not be connected to the warm winter of 2011-2012 as that winter occurred mostly in 2012.
The ACS does not track secondary heat use, but it is likely that more Americans are using wood to supplement their main fuel as well. According to the 2009 EIA Renewable Energy Consumption Survey, 7.7% of American homes used wood as a supplemental source of heat and wood was the second most common secondary home heating fuel behind electricity. When more homes use wood or pellets as primary heat, the number of homes using it as a secondary heat source grows as well, as far as we know.
Wood heat changes at the state level
During the past decade, several states in the northeast as well as Michigan, Ohio and Nevada saw the number of homes mainly heating with wood or pellets increase by more than 90%. But of these eight states, only Massachusetts and Maine continued the upward trend in 2011. Instead, Hawaii (52.5%), Florida (30.5%), Delaware (23.2%), Wyoming (21.6%), and Idaho (19.8%) had the greatest gains.
The top five states for wood heating as a percentage of households are Vermont (16%), Maine (12.2%), Montana (8.8%), Idaho (8.3%) and New Hampshire (7.3%). The states with the greatest total number of households heating with wood are California, New York, Pennsylvania, Michigan and Washington.
Residential fossil fuel use declines while solar thermal experiences greatest growth
In 2000, only 1.68% of U.S. households heated mainly with wood or pellets, but between 2000 and 2010, wood experienced the greatest growth of any fuel with the number of primary wood heat households increasing an average of 4.5% per year since 2005. Rising fuel prices and a federal tax credit offered during the latter years of the decade may account for wood heat’s dramatic upswing. The economic downtown may have also contributed to more people making the switch from oil or propane to wood or pellets, two of the most inexpensive fuels available in America.
While the residential use of wood rises, the use of natural gas, the most common primary heating fuel in America, has been declining since 2006. Once the main source of heat for over 50% of U.S. homes, it fell nearly one percent last year and is now at 49%. Other less common fossil fuels such as propane, oil and coal also experienced declines this past year, continuing a multi-year trend.
Other than wood, household fuels that grew during the year were electricity with a 2.5% increase over 2010, and solar thermal with an 18% increase, the highest of any heating fuel. While overall solar residential numbers are still tiny and there has been in overall decrease in solar heated homes since 2000, the renewable has shown the highest growth rate of any heating fuel since 2007. More than half of the approximately 50,000 U.S. homes heated primarily by solar are concentrated in California, Puerto Rico, and Hawaii.
 These numbers reflect the absolute per capita change in the number of homes heating with wood or pellets. They are not adjusted for population growth. See: http://www.forgreenheat.org/resources/press.pdf