Photos by Vern Grubinger.
Mike Collins of Old Athens Farm has two 350,000-Btu Clean Burn furnaces in his greenhouses that he runs on waste vegetable oil from restaurants.

Mike Collins and his wife Rebecca Nixon own and operate Old Athens Farm, located in southeastern Vermont. Fresh-market organic vegetables and berries are grown on 2 acres, and 10,000 square feet of greenhouses produce tomatoes, cucumbers, lettuce, eggplant and other crops. Sales are to local farmers’ markets and wholesale accounts.

The three greenhouses require heating from late winter through late spring, at different times depending on the crops they hold. In the past, over 3,000 gallons of No. 2 heating oil was required, but over the years Collins has worked to reduce his heating costs by installing alternative energy systems, including a homemade wood boiler. In 2005, he started using waste vegetable oil as a fuel. By 2008 it was his primary heat source, and he burned approximately 4,000 gallons.

In 2005, Collins purchased two Clean Burn CB 3500 waste oil furnaces ( for about $5,000 each; he suspects they cost about $7,000 now. These units are rated at 350,000 Btus maximum. They are designed to burn waste motor oil but have performed satisfactorily with waste vegetable oil, although they require frequent maintenance to keep them running consistently.

“If you get one of these units I wouldn’t throw out your old heater,” says Collins. “You might need it as a backup.” Installing the units cost about $1,000 each for the plenum to distribute the hot air, the stovepipe stack, copper fuel line and fittings.

After he collects the waste vegetable oil from local restaurants, mostly in 5-gallon plastic jugs, Collins pours it into 275-gallon plastic totes and allows it to settle outdoors. Later, he pumps oil from the top of these totes into 55-gallon drums, leaving behind the sludge that can cause problems in the heating system. These drums are later moved inside and the oil is allowed to settle once again before being pumped into the 275-gallon end tank that feeds the furnace. The sludge is burned in the wood-fired boiler that is used to provide ground heat.

The Clean Burn system delivers oil under pressure from the tank to the burner, unlike conventional fuel oil furnaces. A pump on top of the end tank sucks up oil and pushes it at 8.5 PSI through the fuel line to the furnace. There is a screen on the pick-up, then a stainless steel filter, and another filter in the pump.

Collins says some sludge still comes through the line once in a while. He cleans the filters about twice a year; more often if any waste oil is poured directly into the end tank without settling. A vacuum gauge in the line after the filter tells if it’s getting clogged.

The vegetable oil is collected from restaurants in 5-gallon jugs. It is then allowed to settle twice, once in 275-gallon totes outside, and again inside in 50-gallon drums, before the clean oil is pumped off and burned.

Once the oil gets to the burner, a compressor forces it through a nozzle and a spinner head that vaporizes it for combustion. A squirrel cage blower moves air into the combustion chamber and provides a forced draft. You can adjust the amount of air going into the chamber and thus the flame length. “You want it about 30 inches long, all the way to the back of the burn chamber without touching the back of it,” says Collins, “but you don’t want the flame to sparkle with unburned material, so you might have to make it a little shorter and hotter. The manual tells you how to make these adjustments.”

One problem Collins has had with these units is clogging on the inside of the burner as a result of the “shellac” that forms from the waste vegetable oil, gunking everything up. After having the system go down several times, usually in the middle of the night, Collins now performs preventative maintenance by cleaning the spinner head and nozzle every week, or after 75 hours of operation. He scrapes off the gunk with a knife and washes parts in hot water, which takes about an hour. He also keeps spare burner nozzles on hand. This has prevented sleepless nights tending the furnace, although once the pump itself failed and there was nothing he could do about that on short notice.

To avoid extra trips, Collins collects most of the waste vegetable oil during the growing season as part of his vegetable deliveries. The oil comes from restaurants within a 10-mile radius of the farm. Collins only works with restaurants that change their fryolator oil frequently, so it is relatively clean, and that use unhydrogenated oil, so it remains a liquid at relatively cold temperatures, down into the 20s. However, the oil is dense when it is that cold, so an electric barrel heater is needed in order to move it around. The oil to be fed into the heater must be stored inside so it is warm enough to flow easily. Collins keeps about 900 gallons of oil inside during the heating season.

Collection of the waste oil takes about 100 hours of additional labor and management per year. The heaters require about 2 hours of maintenance each week during the peak heating season. This results in extra labor costs between $1,000 and $2,000 annually, depending on hourly labor rate. “Right now I don’t have to pay for the oil, but I figure it costs me about 50 cents a gallon to handle it, and another 50 cents a gallon in system maintenance.”

Burning used vegetable oil creates a residue on the furnace nozzle and spinner head that vaporizes the oil. These must be cleaned off on a regular basis to avoid malfunction.

Assuming that the waste oil costs $1 per gallon to deal with, and that it replaced 3,000 gallons of fuel oil, Collins saved about $3,600 in winter 2005-06, compared to paying $2.20 per gallon for No. 2 oil. In winter 2007-08, with the cost of fuel oil at $4 per gallon, he saved about $9,000. The additional cost of the oil furnaces must also be considered, amortized over their life expectancy of more than 10 years, but “the payback is pretty quick,” Collins says.

The wood-fired boiler used to provide hot water for heating the soil can also be used to heat a couple of the greenhouses in an emergency. In addition, Collins keeps a backup fuel oil furnace in another greenhouse.

One constraint to this system is the limited availability of high-quality waste vegetable oil in many communities. Collins works closely with just a few small family restaurants that are dedicated to managing and saving their oil for him to use as fuel, in part because of their good relationship as buyers of his produce. In the beginning, restaurants have to be educated about the quality and handling of waste vegetable oil to be used as fuel.

“This system is worth it for me because I have a reliable supply of good oil based on solid relationships,” says Collins. “But you need to think twice about it because the supply is limited and demand is growing. It’s pretty depressing when you go to pick up your oil and someone’s stolen it.”

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