A natural source for small-scale use

Photos by Vern Grubinger.
Kevin and Laura Channell installed a 100,000-Btu wood pellet furnace to heat their 24-by-48-foot organic greenhouse.

Kevin and Laura Channell, owner/operators of Your Farm in Fairlee, Vt., (www.yourfarmonline.com) started their small, 20-acre farm only a few years ago. The land was all in hay when they purchased it, and since that time they’ve transitioned it to organic production and established 6 acres of a mixed vegetable and cover crop rotation. They constructed a 26-by-48-foot greenhouse for starting transplants in the spring and raising greenhouse vegetables in the summer and fall. They market through a CSA, farmers’ markets and restaurants. Their CSA features an online ordering system for members.

They wanted to avoid dependence on fossil fuels as much as possible, and after much research they settled on wood pellets as a renewable fuel for greenhouse heat. They bought a Harman PF 100 forced hot air pellet furnace (Harman Stove Company, Halifax, Pa., www.harmonstove.com) from Upper Valley Stove Company in Lebanon, N.H. (www.uppervalleystoves.com). The cost in early 2007 was about $3,200 for the furnace.

Channell did the installation on his own with a dolly and a trailer and help from one other man. He hooked up the direct vent himself and installed baffles, and he assembled and attached the blower fan. It took a few hours to unload and install/assemble. He had the plenum fabricated by a sheet metal shop, and assembled it after the furnace was in place. The vent pipe is double- walled and a little more expensive than normal, so a short run is desirable.

The furnace is located in a 12-by-16-foot potting shed attached to the greenhouse. There is shared air between the greenhouse and potting shed through an 8-by-7-foot opening in the shared wall. However, the furnace is behind the end wall and the plenum protrudes through the end wall 7 feet high. The blower fan is always kept on the “High” setting.

There is an HAF fan that circulates the forced hot air 12 feet (3rd bow) from the plenum outlet throughout the house. A second HAF fan is located at the other end of the house. The thermostat is in the middle of the greenhouse, 4 feet above the ground. The potting shed is insulated with 2-inch rigid foam (R-9 or R-11 value.)

During the first season, the furnace used 2.75 tons of pellet fuel from March 17 through May 30, when it was shut down. Only half of the greenhouse was heated (using a sheet of plastic as a partition in the middle of the house) from March 17 through April 28, when the partition was taken down to heat the whole house.

The second season, the furnace was fired up on March 5 and shut down on May 30 again. They partitioned the greenhouse from March 5 to April 1, but cracked it at night to let heat into the area where early greens for market were growing in the ground. They used 3.5 tons of pellet fuel during this time period.

Wood pellets can be purchased in 40-pound bags, or in 1-ton totes, shown here. It is critical tokeep the pellets dry in storage; if they get wet, they’ll fall apart.

Channell maintains the furnace carefully. “I give the heat exchanger a good cleaning after every ton, about three times a heating season, and I empty the ash pan, scrape the firebox and clean out the fines from the igniter box after each ton. This takes 30 to 45 minutes. At the end of the season, a more thorough cleaning gets done including cleaning the combustion fan, the complete firebox, auger mechanism and the sensor (the brains of the furnace). It takes two hours to do that well.”

They used Energex premium pellets (www.energex.com) purchased from Upper Valley Stove Company, which come from Quebec or Pennsylvania. Premium is mostly hardwood and some softwood pellets. This gives high Btu with hardwood and good high heat exchange with softwoods. It also has a minimum amount of “fines,” which can clog up the auger mechanism in the furnace.

The price of wood pellets has been volatile in recent years, along with many other fuels. In 2007, the Channells paid $210 per ton, in 2008 they paid $230, and in 2009 they prebought at $275 per ton due to fuel surcharges, “which will hopefully tail off now that fuel prices are down again,” says Channell.

Like all biomass furnaces, a pellet furnace requires regular maintenance,including cleaning the heat exchangers and emptying the ashpan, shown here.

The system has worked well overall. “The only trouble we have faced is a failed auger motor. The auger feeds pellets as the thermostat calls for heat. So, one morning I came in to find the temp at 37 degrees. It was easily replaced in 30 minutes once I did the troubleshooting and retrieved the part. Thankfully, it was a sunny day and the stove company had them in stock. This is a common failure with this unit. Keep an extra on hand if you are going to go this route,” says Channell.

“Down the road I am concerned about the rust building up on the heat exchangers. I’m keeping a close watch on that and the efficiency and durability of the unit in the long run. The manufacturer said this is their first greenhouse application … we’re all holding our breath, but thankful to be using renewable fuels.”

The author is a vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.