Air pollutants from a faulty heating system can affect greenhouse plant production by damaging leaves or flowers and reducing plant growth. Although the visible effect on the plants may be quite obvious, the cause may be difficult to find.
Based on the number of calls that I receive, the problem is quite prevalent. A survey of 58 growers by Michigan State University about 15 years ago showed over 30% of the heating systems sampled had potentially harmful levels of pollutants in the area around them. The most comment pollutants are sulfur dioxide, ethylene and fumes from escaping fuel.
All fuels contain sulfur, some more than others do. During the combustion process, sulfur is converted to sulfur dioxide. If this leaks into the greenhouse and combines with the moisture there, sulfuric acid is formed. In excess, this can be toxic.
At high levels, which might occur when using unvented heaters during a power outage, severe leaf burn can occur. Young leaves seem to be more susceptible. Long term, low levels, which may occur from a cracked firebox or leaky exhaust pipe, may result in flecking and premature leaf drop. Sulfur dioxide concentrations as low as 0.5 ppm can cause injury. When purchasing fuel, specify sulfur content of less than 0.02% by weight to help reduce this potential.
This clear, odorless gas is a byproduct of the combustion of fossil fuels. Ethylene can be damaging at levels as low as 0.05 ppm. Even a few hours of exposure can cause devastating effects on the growth and flowering of plants. Injury includes leaf distortion, abortion of flower buds, defoliation and chlorosis.
Leaks of raw fuel can affect plants. Propane or natural gas at levels of 50 ppm can have damaging effects. Also, a fuel oil, if it volatilizes on a hot surface, can put harmful vapors into the air. Check the piping frequently for leaks.
Indicator plants are a good way to monitor for the presence of sulfur dioxide and ethylene in a greenhouse. Tomato seedlings are often used because they germinate quickly and can be grown on a year-round basis. They respond very quickly, in as little as three hours.
Following are areas of the heating system that frequently cause pollution problems:
Continual expansion and contraction of the metal in the heat exchanger of a furnace can stress the welds, resulting in cracks. These cracks are a prime source of pollution, especially in older units.
Placing a furnace candle or smoke bomb inside the firebox and observing any escaping smoke can be an effective way to check a furnace. An alternative is to insert a trouble light into the firebox at night and look for light rays in the heat exchanger area.
Cutting into the outside of the metal furnace enclosure and welding the split seam can repair some units. In other cases the whole firebox must be replaced.
The stovepipe or connector pipe should be severely fastened to prevent leaks. On stovepipe, sheet metal screws can be used to fasten the joints. If the sections do not fit tightly, fill the cracks with pipe cement.
To get adequate draft for combustion and to reduce the potential for backdrafts, the top of the chimney must extend above the peak of the greenhouse and any nearby obstructions. Heating codes recommend a height of three feet above the ridge of the greenhouse or two feet above a 10-foot horizontal line to any part of the structure. A cap on the chimney can help to reduce down drafts, a common cause of fumes inside the greenhouse.
Today's tight greenhouses require an outside source of makeup air to feed the combustion process. On a cold night with the heating system operating almost continuously, the oxygen can be depleted in two or three hours in a tight house if no makeup air is provided.
Some of the newer furnaces and boilers have a built-in port to connect a pipe to an outside air source. For heating units without this port, outside air can be brought into the area of the burner using PVC drainpipe or galvanized stovepipe. Pipe size should be at least as large as the vent on the furnace. The pipe should extend from the furnace through the side or endwall and up above the expected snow line. Attach the pipe to the greenhouse for support. A cap and screen should be placed on the exterior end to shed water and keep out animals.
Before the heating season begins, take the time to make an inspection. It can improve the efficiency of the heating system and reduce the potential for pollution problems.
Some Heating System Problems That Create Air Pollution in the Greenhouse
|photos by John W. Bartok, Jr., University of Connecticut|
John W. Bartok, Jr.
Extension Professor- Emeritus, Natural Resources Management and Engineering Department
University of Connecticut
From: Yankee Grower, Volume 1 - 5, September/October issue, 1999.
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