Farming Magazine - January, 2012


A Corn Furnace for Greenhouse Heat

By Vern Grubinger

Greenhouses and high tunnels give vegetable farmers a jump on the growing season in the spring and protect against cold temperatures in the fall. In northern climates, extending the season significantly for warm-season crops comes at a cost: some kind of a heating system. The vast majority of such systems burn fossil fuels like heating oil or propane. However, in recent years, more and more farmers have been experimenting with renewable fuels such as wood, wood chips, used vegetable oil or shell corn.

The Intervale Community Farm uses this corn furnace to heat its greenhouse. When you switch to a new heating system that uses biomass fuel, it's a good idea to keep your old fossil fuel system in place until you work out the kinks.
Photos by Vern Grubinger.

In Vermont, Massachusetts and elsewhere, cooperative extension has obtained and provided some cost share funds for growers to help them try out renewable energy heating systems. The ultimate goal is to help growers save money on heating costs while at the same time reducing reliance on fossil fuels. In response, some growers have installed shell corn furnaces, in part because there are nearby farms that produce that fuel at a reasonable cost. However, as is often the case with new technologies, there can be a pretty steep learning curve associated with switching heating systems.

"For the first couple months we thought this was a huge lemon," said Andy Jones, farm manager at the Intervale Community Farm (ICF) in Burlington, Vt. The ICF grows 25 acres of mixed vegetables for their 525-member CSA. They start transplants for many of their vegetable crops in a 30-by-100-foot double-poly greenhouse, firing up the furnace in mid-March. Until a couple of years ago, the greenhouse was heated with propane. Then, as part of the University of Vermont Extension biomass furnace project for greenhouses, they installed an LDJ A-Maize-Ing Heat furnace. The unit is rated at up to 165,000 Btu per hour and comes with a 14-bushel hopper from which corn is augured into the combustion chamber.

"Our first year we had a lot of challenges getting the unit set up so it was burning right. We had the chimney installed according to spec in the owner's manual, but it didn't end up working well in a greenhouse environment, so there was a lot of tinkering to make the system run properly. By the next year it was working great, and now we couldn't be more excited to have it," Jones explained.

"Fuel quality is just as important with biomass heating systems as it is with conventional furnaces. To operate properly, corn furnaces require fuel that has been cleaned and dried down to the proper moisture level for optimum combustion."
"The lessons we learned that first year were: buy good-quality fuel with low 'fines' content; route a straight chimney inside the greenhouse using double-wall stovepipe and outside the greenhouse use triple-wall chimney pipe; and finally, be sure to set the greenhouse ventilation louvers to open prior to the exhaust fan switching on in order to prevent back drafting of flue gasses into the greenhouse," he said.

"By using the corn furnace we are now reducing our propane use by 80 percent or so. We used to heat the greenhouse with a couple propane unit heaters. We've kept one in place since the corn furnace doesn't like to idle in pilot mode, which any furnace has to do when the sun is out a lot and the days are relatively warm. So late in the heating season we shut off the corn furnace and burn propane when it's needed," Jones said. "Still, the 80 percent reduction in propane use is significant, especially when you look at the cost of propane versus the cost of other fuels. Shell corn is about half the cost of propane on a Btu basis, so we end up shaving 40 percent off our fuel cost, and that's pretty attractive."

If you are considering a change in fuels for heating your greenhouses, be sure to compare the true costs, which should account not only for the Btu content per dollar of fuel, but also for the typical combustion efficiency of different fuels. In the case of corn or wood fuels, you also have to account for burning off the portion that is water. Penn State University has an easy-to-use calculator online ( that can help you make true cost comparisons.

Penn State also has a lot of useful information about using shell corn as a fuel, located at Topics include: shopping for a corn stove (or furnace), locating a corn supplier, quality and storage considerations, and disposing of the corn ash. Apparently corn ash has some modest value as a fertilizer and as a liming agent, with no evidence of heavy metals or any other contaminants.

According to the Penn State site: "Burning shelled corn as a fuel can be a feasible way of dealing with the high prices of more conventional fuels such as fuel oil, propane, natural gas, coal and firewood. Using corn as a fuel does not compete with the food supply needed for nourishment throughout the world. While it is recognized that malnutrition is a serious global problem, the world is not experiencing a food production problem. Instead, the world faces political challenges associated with providing infrastructure systems for food distribution and storage."

Jones is heating his greenhouse with shell corn specifically grown and processed to be used as fuel. That means it is thoroughly cleaned and dried down to the appropriate moisture content, which is about 12 or 13 percent. It's supplied by Vermont Golden Harvest Biofuels, a family farm in adjacent Addison County. Other farmers participating in the biomass furnace project with the same type of furnace have successfully used wood pellets instead of shell corn or a mix of the two fuels. Both fuels must be stored properly to prevent them from absorbing moisture, and shell corn must also be sealed well to protect against hungry rodents and birds. In addition, materials handling is more of an issue with biomass fuels than fuels that are pumped into your tank. For example, it takes 15 pounds of shell corn to provide the same amount of heat as one gallon of propane, so more storage space is needed.

"The biomass unit here at the Intervale Community Farm cost around $6,000 installed" said Jones. "That's three times what a similar propane unit would cost. It's a lot more money, but consider that fossil fuel prices have about doubled in the last few years, so the payback is a lot faster than it would have been in the past."

In addition to reducing your fuel costs, working with a local supply of biomass fuel can provide some measure of security about future fuel costs and availability. Your farm could also gain a "green" marketing benefit if your customers are excited about the effort to use less fossil fuel. For farmers like Jones, these things all come together in a package that has multiple benefits, despite some extra hassles compared to simply setting the thermostat and letting the gas company fill the tanks.

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