Indoor and ambient air quality testing at 2018 Stove Design Challenge

By Dan Ciolkosz, Assistant Research Professor in Pennsylvania State University's Department of Agricultural and Biological Engineering and John Ackerly, President of the AGH. Dan and John were judges and members of the Organizing Committee of the Wood Stove Design Challenge.

In addition to measuring particulate matter (PM) in the chimney stacks of stoves at the 2018 Wood Stove Design Challenge, the Alliance for Green Heat also measured levels of PM inside and around the tent.  The tent had 13 wood heaters in it, of which 10 burned cord wood, 2 burned pellets and one burned a densified log.  At any one time, eight to ten of the units were usually operating, providing an opportunity to look at the impact on indoor air and the air around the tent.

The 13 wood and pellet units were in an uninsulated tent the size of a basketball court that had considerable air leakage and cannot be compared with a permanent structure.  The tent was on an open field area on the National Mall.

PM Measurements were recorded at discrete times, in and around the National Wood Stove Design Challenge tent, on November 9 and 10. Measurements were taken using a Thermo Scientific pDR 1500 aerosol monitor, set to a 5-second integration interval (thanks to George Allen of NESCAUM for use of his meter). These readings correspond to total particulates in the air, though it is likely that the majority of the particulates are in the "fine particulate" range of less than 2.5 microns. Readings were taken at a height of 1m above the ground. Exterior readings were taken at a location 15 meters from the building, north, south, east and west of the building. When elevated levels were detected outside, additional readings were taken to track the length of the contaminant plume. 

We also used an indoor Speck PM monitor, and a Purple Air PA-II-SD monitor.  Often the Speck showed  good, moderate or elevated inside the tent.  Due to technical difficulties we were not able to get the Purple Air monitor online during the event, or download the data yet, but we should have that data soon.

Indoor and Outdoor Measurements, Nov. 9

Figure 1: Measured PM inside and near tent 

On November 9th, the day was drizzly with light or no wind. PM levels inside the tent were notably elevated, ranging from 28 to 142 micrograms per cubic meter. Outside the tent, PM levels were most commonly 9-13 micrograms per cubic meter, except for elevated levels measured west (downwind) at 11:40am, which were 95 micrograms per cubic meter. Additional measurements taken at that time on the west side of the tent indicate that elevated PM levels existed as much as 100m west of the structure.  The EPA short term PM standard is 35 ug/m3 for a 24 hour average. 

Figure 2: Plume on West side of tent

Indoor and Outdoor Measurements, 10 Nov.
On November 10th, the day was clear and blustery. PM levels inside the tent were not as elevated as the previous day, ranging from 21-23 micrograms per cubic meter. outside the tent, PM levels were most commonly 8-10 micrograms per cubic meter, except for elevated levels measured east (downwind) near midday, which were 118 micrograms per cubic meter. Additional measurements on the east side of the tent showed elevated levels more than 100m west of the structure.

Figure 3: Measured PM inside and near tent

Figure 4: Plume on east side 

Logged Indoor Readings, 10 November
The sensor was left in logging mode for several hours during the day on November 10th, allowing us to track trends and variability of PM levels within the tent. Measurements were logged every 10 seconds, from ~10am until ~3pm.

Figure 5. Measured PM inside tent Nov. 10.

The mean PM level in the tent during this time was 33.5 micrograms per cubic meter, with a 5th and 95th percentile of 14.5 and 63 micrograms per cubic meter. The overall trend was fairly steady, with the exception of fluctuations at the time of the manual outdoor readings (~10:45, 12:45, 14:45), as well as elevated values during the period from about 10:05 to 10:35. This elevated period may be due to either a period of lower wind (i.e. less fresh air infiltration in the tent) or unusual stove activity within the tent.

As we already noted, it's tough to draw any broad conclusions from these measurements, for two notable reasons:

- The tent is an unusual structure, in terms of its infiltration characteristics and airflow patterns. 

- There were many stoves in operation, some of which were in a beta test mode that may not be representative of typical use. 

That being said, these measurements are a good reminder that some stoves can create elevated particulate levels in the air inside as well as outside a building, and wood stove designers and users should take this into consideration when selecting and using wood burning appliances.  We expect that a few of these stoves, including but not limited to the pellet units, created virtually undetectable PM inside and outside of the tent.  Conversely, several prototype stoves spewed excessive smoke when being reloaded, and could have contributed a great majority of the indoor smoke.  For most of the event, there was no visible smoke coming from the chimneys, though during start up and at other times, several stoves were producing excessive smoke.  One expert estimated that smoke becomes invisible around 2 grams an hour.

As a final note, recent developments in sensor technology have made measurement and recording of particulate levels much less expensive, which should allow us all to do a better job designing and using wood heat systems in a clean and "neighbor friendly" way.