JARS v37n2 - Are Kerosene Heaters Safe in Greenhouses?

Are Kerosene Heaters Safe in Greenhouses?
Paul N. Reber

Reprinted from Green Scene, courtesy of Pennsylvania Horticultural Society.

Paul Reber is an ornamental horticultural graduate of Penn State University. As a county agent with the Montgomery Co. Cooperative Extension Service he conducts programs that benefit both commercial and private horticultural interests.

When my mother headed for the attic to uncover the sheet-draped "emergency" kerosene heater I knew that either the coal bin was empty or that I'd soon have delicious dried corn or schnitz (apples, the other half of knepp) that she would prepare on top of the heater's steam dryer.
According to present safety standards that kerosene heater of the 40's would never pass inspection as a heat source for living room or greenhouse. Not only did it burn fuel inefficiently, but the kerosene used had a high sulfur content.
Enter now a new breed of highly efficient unvented kerosene burning heaters that became a phenomenal success in 1981. With certain precautions these units can be used to supplement and partially replace oil, electric and gas heat in a home.
But what about the hobby greenhouse? Can plants be grown in a confined environment with an unvented heater? Let's evaluate the parts -then put it all together.

Kerosene Grades
To insure plant safety in a greenhouse the kerosene used should be "clear white" or 1-K grade. This kerosene will be 99.5% clean burning and contain a maximum sulfur content of 0.04 weight percent.* More about sulfur later.
Unfortunately it is difficult to find 1-K kerosene for sale. The 1-K is costly, probably twice as much per gallon to buy as standard kerosene. Distribution to the retailer should be made in clean or purged tank trucks; otherwise, it will be contaminated with fuel oil or other impurities. Kerosene refiners would have to sell 1-K in one gallon containers, not pumped, to make sure it remained pure.
The second grade of kerosene is 2-K. This is what is offered for sale by the majority of dealers. It has a maximum sulfur content of 0.3 weight percent and, according to a major oil company, should not be used in flueless (unvented) heaters.

Kerosene Heaters
Industry claims that kerosene heaters that are 99.5% clean burning are based on the use of "clear white" or 1-K grade kerosene, which is as clear as water. Using 2-K fuel will in time clog the wicks of unvented heaters and cause a significant increase in carbon monoxide and sulfur dioxide.

In any combustion process carbon dioxide (CO 2 ), carbon monoxide (CO), water vapor, nitrogen dioxide (NO 2 ) and sulfur dioxide (SO 2 ) (when using 2-K kerosene) are produced.
Carbon monoxide is dangerous because it is colorless and odorless. It accumulates in the blood and combines with hemoglobin (an oxygen-carrying constituent of blood) over 200 times more readily than oxygen does. Consequently CO robs the blood of oxygen and simultaneously prevents the disposal of waste carbon dioxide from the blood. Exposure to 0.05 percent for three hours is life-threatening.
Sulfur dioxide, although not life-threatening, can be injurious to plants. This air pollutant causes a bleaching or a leaf burning effect on foliage. The tissue between the veins is injured and the damage is often more prominent towards the petiole. Fully expanded leaves are the most sensitive.
Oxygen (O 2 ) is another gas present in quantity. If all the oxygen is removed by the heater the flame will be extinguished, there would be no heat, and the greenhouse crops could be lost.

Assembling the Parts
Finding 1-K grade kerosene to burn in an unvented heater is next to impossible; therefore, adequate ventilation must be available to reduce levels of gases when using the 2-K grade.
It could be argued that greenhouses are not tight and air leakage is common. But with the energy crunch the smart hobbyist has plugged those escape routes with polyethylene, bubble plastic or other insulation methods.
Under these conditions a rule of thumb is to provide one square inch of ventilation area for each 1,000 BTU's per hour of heater capacity. However in the dead of winter a louver open just one half inch can lose up to 12,000 BTU's of heat per hour, approximately the same heat output of a typical kerosene heater. Thus, the net heat gain will be zero.
Summing it up - I would not recommend using an unvented kerosene heater in the greenhouse. Although some hobby growers feel secure with theirs, the possibilities of malfunctioning to produce damaging pollutant levels are there.

*Author's Note: Consumer Reports (October 1982) states that burning 1-K grade could create sulfur dioxide levels up to 12 times the standard set by EPA for human exposure.