Farming Magazine - January, 2009


Wood Chips

Heat a Year-Round Greenhouse
By Vern Grubinger

Fred Green, owner/operator of Stow Greenhouses in Stow, Mass., has 35,000 square feet of greenhouse space to produce cut lilies for the wholesale market year-round. The greenhouse must be maintained at 65 degrees Fahrenheit, and additional energy is needed to melt snow off the gutter-connected roof in the winter to keep it from blocking sunlight. Prior to converting to a biomass heating system, the operation used 30,000 gallons of heating oil annually.

The Hammermill grinder and poly-covered storage used to process and store the wood chips.

It became clear to Green that the rising price of heating oil posed a risk to the profitability of his business long before oil prices hit their historic high levels. In 2004, he came up with the idea of using waste wood from a nearby nursery operation. The nursery owner let his commercial customers dump wood chips from their jobs at no charge, and the nursery would then sell the chips to an electric power plant.

There was a good supply of waste wood and wood chips in the western suburbs of Boston, but Green had to figure out not only what equipment would be used to burn the wood, but also what was needed to handle it. He visited a dozen commercial wood burning installations in the eastern U.S. and Canada to get a better understanding of his options. Despite the rigor of his initial research, there was still a lot of trial and error until a satisfactory system was in place.

The first unit, a 1.5 million Btu unit designed to burn sawdust, was install-ed in 2005. The system presented several problems that Green worked to resolve before deciding to sell it.

First, the hot water heat exchanger wasn’t big enough, so even at full capacity it was necessary to burn oil, as well as wood chips, to maintain the greenhouse at the proper temperature in the dead of winter. A larger heat exchanger fixed that problem, but the 45-foot belt conveyor that fed chips into the boiler wasn’t designed for the weight of the chips, and it tore due to the friction between the belt and rollers. Sometimes, chips would wedge in between the roller and the belt, causing it to jam, but a screw auger solved that problem. However, the irregular shape of the chips that Green was using—especially longer pieces of branches that made it through the processing stage—caused the flow of chips to bridge and create a dam that shut down the supply to the boiler, causing it to shut off. Eventually, Green settled on a rail-type system to deliver the chips into the furnace.

Green decided to set up a system that would handle the fuel he had, so he purchased a used Conifer furnace manufactured by Hern Iron Works ( He connected the 3 million Btu model 49-S to a Kewanee wood-fired boiler (Kewanee is no longer in business, but used models are available, as are new parts, at and a 3,000-gallon water tank. Buying used components, then making several modifications to the furnace, like adding a new door and replacing the internal steps that guide the flow of fuel, Green was able to set the system up for about $30,000. He also installed a fan to pull rather than blow air through the furnace, which does a better job of forcing oxygen through the chips, leading to better gasification. He also installed a special screw used to force chips into the furnace when a bridge forms at the end of the auger.

There are challenges with using waste wood versus purchasing wood chips from a sawmill. You have to deal with branches, stumps, construction debris and other contaminants, such as plastic and metal trash. Green’s processing system separates out the worst of the contaminants, but he needed a combustion system that could handle irregular-shaped pieces of wood and a small amount of other debris.

The 3 million Btu Conifer furnace heats water in the Kewanee boiler; valves are set to control how much of the hot water is sent to heat the greenhouse or stored in the 3,000-gallon reservoir.

Keeping the furnace running, and running hot, is important to getting clean combustion and avoiding the small amount of smoke that’s created when the furnace is cold and then restarted. At full-bore, the temperature inside the furnace is about 2,000 degrees Fahrenheit, which is hot enough to cause near-total combustion and, thus, a minimum of pollution, including particulates. (Particulates are a big problem with some outdoor wood boilers that do not operate consistently at high temperatures.) When demand for heat is low in the greenhouse, Green keeps the furnace operating at no less than 1,000 degrees to avoid any smoke. The problem is what to do with all that heat.

Asiatic and Oriental lilies are planted and harvested all year long at Stow Greenhouses to sell to the wholesale cut flower market.

The 3,000-gallon tank of water is Green’s latest innovation. By connecting it to the boiler and hot water heating system, he created a large reservoir for excess heat that is not needed in the greenhouse during the day, but is needed at night. He also uses some of the excess heat to run a large steam sterilizer that treats his coir potting medium so it can be reused again and again to grow the lilies. Further, to control Botrytis in the greenhouse, there is plenty of heat to drive off humidity when necessary. When there’s simply too much heat, the reservoir boils off water that is later replaced. Green says, “It’s unbelievable that people are paying $4 and $5 a gallon for oil, and we have so much heat we can’t get rid of it.”

But, it comes with a price. He has process the chips by separating out the trash and grinding before storing the finished material to keep it dry.

Metal rails, bottom left, move chips that are dumped in the bin forward toward the furnace, where a smaller rail system then carries them into the furnace for combustion.

Green lets landscapers, arborists, phone line and power line crews drop off their wood chips for free, but doesn’t demand that the chips be of any particular quality. “You get all kinds of junk in with the chips—tools, barrels, trash—but we don’t look a gift horse in the mouth.” The unprocessed chips are loaded with a bucket and dropped off at a nearby location where large objects are removed before they are run through a Sundance Hammermill grinder, which can regrind 30 to 40 yards of chips an hour.

For the first three years of using waste wood chips as a fuel, Green didn’t grind it, he just used a screen to remove the larger pieces. That left a lot of unusable material, and some long, thin pieces of wood would get through the screens and later jam the delivery into the furnace.  Now, he grinds everything that will be burned.

Dry wood chips generate more Btu per cubic foot than wet chips, and they make less smoke. “We didn’t keep the chips dry the first winter, and that was a big mistake,” says Green. He set up a 28-by-80-foot wood structure covered with a layer of clear poly to store the chips and keep them dry. It holds 1,200 cubic yards of chips, enough to get through the coldest months, but only about half of the 2,500 to 3,000 yards that will be burned annually. The remainder is stored under plastic tarps. When there’s an excess of chips, Green sells them to landscapers.

Moving the chips has also involved a learning curve. “First we had a 30 hp New Holland with a bucket, but the front axle broke; then, we had a 50 hp Kubota, but the pistons on the bucket went and the tires came off the rims, and we finally realized that tractors were not designed for this type of continuous front end, work so we bought a 50 hp Cat loader ... and it has worked well. Burning is easy, boilers are easy; handling all those chips is a challenge.”

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