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 obvious, the cause may be difficult to find.

The most common pollutants are sulfur dioxide, ethylene and fumes from escaping fuel.

Sulfur dioxide

All fuels contain sulfur, some more than others. 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 should you have to use unvented heaters during a power outage, severe leaf burn can occur. Young leaves seem to be more susceptible. Long term, low levels — which can stem 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 a sulfur content of less than 0.02% by weight to help reduce this potential.

In tight greenhouses, makeup air is required to get complete combustion of the fuel.
Photo courtesy of John W. Bartok, Jr.


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 exposure can cause devastating effects on the growth and flowering of plants. Injury includes epinasty, abortion of flower buds, defoliation and chlorosis.

Indicator plants are a good way to monitor the presence of sulfur dioxide and ethylene in a greenhouse. Tomato seedlings are often used as they germinate quickly and can be grown on a year-round basis. They respond very quickly, in as little as three hours. Test equipment is expensive and frequently doesn’t measure accurately at the low gas levels that affect plants.

Fuel fumes

Leaks of raw fuel can affect plants. Propane or natural gas at levels of 50 ppm can have damaging effects. Also, if fuel oil volatilizes on a hot surface, it can emit harmful vapors into the air. Check piping frequently for leaks.

The following are areas of the heating system that frequently cause pollution problems.

A chimney height above the peak of the greenhouse and a cap will prevent flue pipe backdrafts.
Photo courtesy of john w. bartok, jr.

Firebox leaks

Continual expansion and contraction of metal in a furnace’s heat exchanger can stress the welds and result in cracks. These cracks are a prime source of pollution, especially in older units that are used only for spring production.

A furnace can be checked by placing a furnace candle or smoke bomb inside the firebox and observing any escaping smoke. An alternative is to insert a trouble light into the firebox at night and look for light rays in the heat exchanger area. Some units can be repaired by cutting into the outside of the metal furnace enclosure and welding the split seam. In other cases, the whole firebox must be replaced.

Chimney connector

The flue pipe should be securely fastened to prevent leaks. Sheet metal screws can be used to fasten the joints. If the sections do not fit tightly, fill the cracks with a pipe cement.

Chimney height

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 3 feet above the ridge of the greenhouse or 2 feet above a 10-foot horizontal line to any part of the structure. A cap on the chimney can help reduce drafts, a common cause of fumes inside the greenhouse.

Makeup air

Today’s tight greenhouses require an outside source of makeup air to feed the combustion process. On a cold night, when the heating system is operating almost continuously, the oxygen can be depleted in two or three hours if no makeup air is provided.

Many high-efficiency furnaces and boilers (separated combustion and condensing) are piped to an outside air source. For heating units without this connection, outside air can be brought into the area of the burner using PVC drainpipe or galvanized stovepipe. The pipe size should be at least as large as the flue on the furnace. The pipe should extend from the furnace through the side or end wall and above the expected snow line. A cap and screen should be placed on the exterior end to shed water and keep out animals.

Now is a good time to inspect, correct any problems to improve the efficiency of the heating system and reduce the potential for pollution.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.