The EPA measures cleanliness of stoves by measuring the emission rate of particulate matter (PM) and stoves as of 2015 have to emit no more than 4.5 grams per hour. PM is expensive and complicated to measure accurately. We used carbon monoxide (CO), which is a reasonable good proxy for PM1.
Figure 5. A sample of soot wiped off the glass
of each stove shows a variety of consistencies
and black and brown colors and build-up
collected during glass cleaning.
We chose to present CO numbers as the Testo recorded them, instead of correcting them to the same oxygen level since we were using CO as an indicator of cleanliness, and not as a modified combustion efficiency indicator.
Most of the stoves operated consistently in the 300 600 part per million (ppm) of CO, which is quite good for residential biomass combustion and far better than non-catalytic wood stoves. One notable achievement of the Quadra-Fire is that it burned exceptionally clean at its highest heat setting in our tests. It averaged only 114 ppm CO on its highest heat setting, when other stoves were between 350 ppm (Ravelli) and 1,290 ppm (Piazzetta). The Harman was far higher than the other stoves, and even higher than the Piazzetta. For consumers who care most about putting as few pollutants into the air and expect to be burning their stove mostly at medium or high, the Quadra-Fire is a clear winner.
In all the graphs produced in this report, we did not include the straight trend lines indicating the general course of tendency of combustion values for the Piazzetta because it missed the last week of testing, which would lead to unrepresentative trend lines compared to the other stoves. We did include the trend lines for the Harman even though the Harman could not be tested on its high heat setting. Our Testo combustion meter would go into shut down mode when the CO readings went above 4,000 parts per million. The stove completed the full four weeks but the CO and probably the efficiency trend line would have been even higher if we had been able to test the stove at its high burn.
Our database of CO readings over a month period provides an excellent opportunity to see how stoves performed compared to one another. When EPA accredited labs test pellet stoves, manufacturers are allowed to bring their own fuel (which is not allowed with wood stove testing), resulting in test data based on fuels with very different moisture and ash contents. The Quadra-Fire, for example was tested by OMNI Lab with 4.5% moisture pellets and received a 2.7-gram per hour certification. A month later, it was retested at a different lab, Intertek, with 2.3% moisture pellets and achieved a 0.5-gram an hour certification. It is unclear if there is a 2.3% moisture pellet on the commercial market and it is clearly not representative of the moisture content of commercially available pellets. These typically fall in the 3.5% to 7.0% range.This is one of the causes leading to variability in EPA testing of pellet stoves that we were able to drastically reduce in our testing.
Another important issue is that the method of testing pellet stoves has changed. As of 2015, the EPA endorsed the ASTM 2779 method, which is a shorter and simpler test that the previous Method 28 protocol. The ASTM method is likely to provide lower gram per hour readings, allowing more pellet stoves to pass the 2020 emission standards without redesign. The ASTM method is a continuous sampling method, where pulling filters in between burn rates is not required. This reduces the cost of testing, but data regarding the relationship between emissions and burn rate will no longer be obtained.
We took periodic photos of the vent pipe where the smoke exits, and found a noticeable difference in PM deposits, consistent with tested CO levels (Figure 7). Several had very little noticeable soot build up, whereas the higher emitting stoves had build up of dark soot nodules after several weeks. We burned up to 1 ton of pellets in each stove, and the build up on the pipe clearly showed a need for a T-cleanout, annual chimney cleaning, and inspection. While our tests only lasted 30 days, conceivably if a home used 5 or 6 tons, cleaning the chimney twice a year would be advisable for dirtier burning stoves. Some manufacturers recommend cleaning the stove every 2 or 3 tons of pellets.
Figure 7. Exhaust pipes after one week of testing
This NIH study found that CO is a good proxy for PM 2.5 with solid fuels. http://www.ncbi.nlm.nih.gov/pubmed/11351731
Other studies have also found CO to be a good proxy for PM, http://journals.lww.com/epidem/Fulltext/2011/01001/ Biomass_Smoke_and_Cooking__Can_Carbon_Monoxide_Be.810.aspx