Pellet Stove Endurance Testing
The Alliance for Green Heat is conducting a month-long test of 6 popular pellet stoves at its office in Takoma Park, Maryland. The side-by-side test of 6 stoves is a project within the Pellet Stove Design Challenge and is designed to explore how different stoves perform in a controlled environment. The testing also provides an opportunity for consumers and other stakeholders to better understand how pellet stoves can be tested, pellet stove technology and environmental issues around their use. This pellet stove testing regimen also provides a glimpse into some of the issues in EPA certification tests of pellet stoves and the implications for how they perform when they are not properly cleaned and maintained.
The Study
This was a side-by-side test of 6 popular residential pellet stoves. Four of the 6 stoves are freestanding pellet stoves and the remaining 2 are pellet fireplace inserts. The stoves were being burned 24 hours a day for 30 days and were all set at the same heat settings for the same amount of time. A flue gas analyzer was used to measure stack temperature, oxygen (O2) and carbon monoxide (CO) each day, and an efficiency value was calculated using a stack loss method. At the conclusion of the test, we will provide comparative data on CO, O2 stack temperature and efficiency for all the stoves, along with observations on their respective strengths and weaknesses.
The Stoves
We chose six popular pellet stoves that exemplify a range of price points, styles and manufacturers. To identify the 6 stoves, we interviewed scores of retailers about their top selling North American and European stoves. We then purchased them off the shelf to help assure independent, unbiased testing and assessment.
The six stoves being tested exemplify a range of designs. Two are under or side feed systems, where pellets are pushed out into the burn pot and four of the stoves are top feeding, where pellets drop down into a burn pot. The stoves employ a wide assortment of sensor technology to maintain safe conditions, to help the consumer trouble shoot issues, to maintain optimum combustion and for other goals. Some stoves may prioritize reliability and the air wash (to keep the glass clean) over high efficiency. Some have automated features that can help the consumer maintain constant room temperatures. All have automated ignition.
The pellet stoves are:
Enviro M55 insert
England Stove Works 25-PDCV
Harman Accentra 52i insert
Quadra-Fire Mt. Vernon AE
Piazzetta Sabrina
Ravelli RV80
Goals
One of the main goals is to see how well stoves maintain their efficiency over a month-long period without cleaning or routine maintenance. Some experts predict that some stoves may emit more pollution and lose up to 15% efficiency or more, as their burn pots and heat exchangers get dirty. Many, if not most people do not clean their stoves on a daily and weekly basis, as recommended by the manufacturers. We want to help consumers assess how stoves perform and how efficiency is impacted in sub-optimal conditions. In addition, the project will help in understanding how stoves operate in real world settings in consumer hands, compared to test labs that certify stoves for the EPA. The Alliance submitted a Freedom of Information Act (FOIA) and received additional details about the lab certification tests to better understand how the stoves were originally tested and why anomalies may exist in subsequent testing.
The Test Equipment
We are using a Testo 320Flue Gas Analyzer, a $1,000 piece of German test equipment designed for oil and gas heaters and then adapted for solid fuel appliances. The Testo can test for Carbon Monoxide (CO) ppm (parts per million), oxygen,ambient temperature, stack temperature and provides a “combined efficiency” from an internal calculation using these parameters.The Testo takes readings every second that download onto a computer with Testo Easyheat software. Other test equipment, including the Wohler 550 flue gas analyzer, was used to verify readings.
The Fuel
We are using a Curran premium hardwood softwood blend pellet that is PFI certified.To be certified by PFI under premium, pellets must be equal to or below 8% moisture content, 1%inorganic ash content and meet a variety of other quality assurance metrics. Prior to being used as fuel for testing, pellets should be 3rd party tested to get exact measurements. The Curran brand pellets used for this experiment were third party tested by Twin Ports Test Lab and were found to have a moisture content of at 3.77% moisture content and .67% ash. We selected this pellet because it is a popular and commonly used pellet in the northeast, available at big box stores at an affordable price. We paid $250 a ton and will be using about 7 tons to conduct the study.
Carbon Monoxide and Pollution
Carbon Monoxide (CO), a key gas that results from incomplete combustion, is often used as a proxy for PM 2.5 (particulate matter). Some European test methods use CO instead of PM to test for pollution. Pellet stoves will often have an average of 200 – 500 ppm for CO in any 15-minute period. We found that unlike other parameters that we have the ability to test, CO had a wider range of variation and needed a 15-minute test to ensure that we were testing at a steady state. Wood stoves usually have much higher CO readings than pellet stoves, due to a variety of factors, including a less controlled combustion chamber and much less fuel consistency than is available with pellets.
Stack temperature
We test temperatures in the exhaust stack at 6 feet above the floor. A key part of efficiency calculations is stack temperature, since heat that leaves the house through the stack does not end up heating the home. High stack temperatures can reduce efficiency but also can be an indicator of hotter, more complete combustion. Normal stack temperatures in these stoves can range from 275℉ to 475℉. Lower stack temperatures are partially the result of effective heat transfer from the stove to the home through heat exchangers.
Oxygen
Measuring the oxygen content of the flue gas reveals how close the stove is to optimum burn conditions. If the excess air is too low, higher emissions result. If the excess air is too high, the extra heat required to heat this air results in lower efficiency. Oxygen sensors are used on larger residential biomass heaters as a way to optimize combustion, just as they are in automobiles. Optimum stack oxygen can be in the 8 – 13% range in a pellet stove, but most pellet stoves have more than that, reducing their combustion and thermal efficiency. Pellet stoves that have more than an average of 15% oxygen are unlikely to be high efficiency appliances.
Efficiency
The efficiency values we are measuring are a combination of combustion efficiency and heat transfer efficiency and is not equivalent to, or comparable with, the efficiency values reported by EPA accredited stove testing labs (using CSA B415). Testo uses a proprietary equation based on stack temperatures, CO, CO2 and O2. It is also an estimated efficiency, because exact efficiency numbers require much more detailed input of data. Thus, the efficiency data we gather is primarily to compare each stove with itself, over a one-month period, and secondarily to compare them with each other. We plan to release this info in a generic format, and not as exact numeric values, to avoid confusion that could otherwise arise.
The report on endurance testing of popular pellet stoves is funded by the Osprey Foundation, the US Forest Service and the Alliance for Green Heat and is not part of the Design Challenge that is funded by NYSERDA.
