Study suggets residential pellet offgassing a nonissue

By Anna Simet | August 12, 2015

According to results from a new study released by the University of New Hampshire, indoor storage of pellets in homes using the fuel for heat does not pose a risk of generating carbon monoxide (CO) levels above recommended thresholds.

The issue has been contentious in the Northeast, said Adam Sherman, executive director of Vermont-based Biomass Energy Resource Center.  “There has been some controversy there about whether it is okay to store pellets inside, or if it should be done in outdoor silos. It a bit of a tricky issue, and this study could really help put this debate to rest.”

Over a period of seven months, the CO concentration in the air of 25 residences in New Hampshire and Massachusetts were monitored. The fact that homes built in different eras possess a range of air tightness and dampness tendencies was taken into consideration, so construction varied from residences built in 1774 to 2013.

Out of the 25, 16 of the homes use wood pellet boilers with indoor pellet storage containers with a capacity of at least 3 tons; four of the homes use outdoor pellet storage, four use other heating fuels and the remainder is a university laboratory site.

The study was designed to obtain preliminary survey data of residential CO concentrations at the level of parts per million (ppm) in ambient indoor air in the immediate vicinity of wood pellet storage and heating systems compared to that of the homes using fossil fuel systems. It uses a threshold of 9 ppm of calibrated hourly averaged CO measurements, with data acquired at a time resolution of 5 minutes, to delimit concentrations that might adversely affect indoor air quality.

Throughout the duration of the study, no significant emissions source of CO from indoor wood pellet storage was found.

One of the study’s authors, Barbara Bernstein of the New Hampshire Public Utilities Commission, said feedback has been positive, and it has helped prove what many installers were already confident of. “These systems have been used extensively in Europe for quite some time and studied over there as well, and it didn’t seem there would be much difference between what you would find in Europe and find here,” she said

Sherman noted that although the study, which is not yet peer-reviewed, had favorable results, in 2013,  Clarkson University, under contract with NYSERDA, performed a similar study on bilk pellet storage and CO2 offgassing, and a different conclusion was reached—CO levels were too high.  

Philip Hopke, director for the Center for Air Resources Engineering and Science at Clarkson University, said that in CU's study, CO monitors were in the basement near pellet bins, and CO concentrations were found to be as high as 44 ppm, "well above the 9 ppm health-based recommendation," he said. "For the commercial bins, we put the monitors in the bins since we have concerns regarding the entry of personnel into the bins to perform maintenance on the pellet augers."

Hopke said the earlier study shows that  bins need to be treated as confined spaces under OSHA regulations, and that CU is working with NYSERDA to provide the appropriate confined space training to the operators of these systems. 

If continued study is to be done, Bernstein said, it would require extensive preparation and benchmarking before it begins, in order to study residence characteristics such as where the garage is in relation to the storage unit, and other variables. That can make a big difference in regard to air quality. “Where we did find high instances, there were explanations for that,” she said. “One gentleman kept his four-wheeler inside the garage near his pellet storage, and he would start it up and then open the garage door, causing quite a spike.”

Bernstein added that it is wise to move cautiously into any industry. “But this study shows that while there are plenty of basements with air quality issues, there is absolutely nothing there that shows it came from the storage of wood pellets.”

The study was joint work of the University of New Hampshire, New Hampshire Public utilities Commission, Earth Systems Research Center and the New Hampshire Department of Environmental Services.


This story has been updated from a previous version.