Amherst officials offer more insights on preventing house fires caused by pyrolysis

AMHERST – Of the six residential structure fires occurring in Amherst last year, five were directly related to the construction of the chimney.

More specifically, these fires were caused by wood building materials not separated from the chimney by two inches of clearance, drying out over time from exposure to chimney heat, until the heat in the chimney could cause combustion – a process known as pyrolysis.

According to Sarah Marchant, director of Community Development, whose department oversees building codes and inspections, “Today’s building code requires all new construction to provide a minimum two inches of separation between combustible materials and a masonry chimney.”

This code became standard practice in Amherst with the voter-approved 1983 Building Code.

“The residents of Amherst have always been proactive in ensuring quality construction in the community. As early as 1970, voters adopted the first comprehensive building code requiring a one-inch separation and invested in regular building inspections,” Marchant said. “The 1970 code was considered ‘best practice’ at the time. By comparison, the state of (New Hampshire) did not require residential structures to comply with a standard building code until its adoption of the ICC building codes in 2007.”

“It’s important to understand that construction of a chimney with less than today’s standard of two inches of separation may not be limited to structures in Amherst,” said Amherst Fire Chief Mark Boynton. “It’s likely that this type of construction can be found in residential structures across the region and the state.”

The two departments have created a Chimney Safety in Your Home Factsheet available at that offers steps for homeowners to follow if they are concerned about the potential for this type of fire in their homes. Residents can also call the Amherst Building Department at 673-6041 or Fire Department at 673-1545 for more information.

The town of Amherst has continued to make construction practices and enforcement a priority since the adoption of its first comprehensive building code in 1970. By ensuring construction adheres to current building codes, the town is protecting its structures and residents and reducing the number of property-related losses.

Recently, the Office of Community Development participated in an Insurance Services Office Building Code Effectiveness Grading Schedule review. The ISO is an independent statistical, rating and advisory organization that serves the property/casualty insurance industry, including Fire Department Protection Class Ratings. The review was based on an evaluation of:

1. Administration of Codes

2. Plan review and

3. Field inspection practices.

Amherst was found to have made significant improvements in all three categories, increasing its rating from a 6 in 2003 to a 4 in 2013 for both residential and commercial building services. This places Amherst among the top 25 cities and towns in New Hampshire for the quality of its building code enforcement.

Because the town’s rating has improved, Marchant suggests that residents undertaking new construction might benefit from lower insurance rates, and should check with their insurance agents before starting new construction projects.