Flue Gases Can Wreak Havoc in Greenhouses

Carnations have long been known to be highly sensitive to ethylene gas, and its presence in very low concentrations will cause the carnation flowers to senesce prematurely.

Ethylene gas is colorless and odorless and is a frequent byproduct of the combustion of fossil fuels. Well-maintained, vented natural gas and propane furnaces evacuate flue gases through the stack or flue pipe. If there are holes or cracks in the heat exchanger or the flue pipe, ethylene gas and other flue gases may enter the greenhouse structure resulting in plant injury.

In some greenhouse operations where flue gas injury has been observed, the cap to the flue pipe is missing. When this occurs, a sudden downdraft can push the flue gases into the greenhouse structure resulting in crop damage. Flue gas injury can also be observed when a grower has not extended the vent or flue pipe a few feet above the greenhouse roofline. When the flue pipe is not extended a few feet above the roofline, flue gases are expelled closer to greenhouse air intakes and vents, resulting in the infiltration of the flue gases into the greenhouse production area.

Figure 2. The position of the flue pipe is critical to the prevention of flue gas injury. Photo: Tom Ford, Penn State

In some greenhouses, wood and coal stoves are used to heat greenhouses. Often these stoves are located in the center of the greenhouse and must be stoked from inside the greenhouse structure by the operator. Cracks and pinholes in the flue pipes can lead to flue gas injury, but almost all stoves leak a little bit, allowing the flue gases to circulate inside the production area. Ethylene injury is routinely observed in greenhouses with wood/coal stoves located inside the greenhouse structure.

Figure 3. Making sure there are no cracks or pinholes in the flue pipes will help decrease flue gas circulation. Photo: Tom Ford, Penn State

Tomatoes are one of the most sensitive vegetable crops to ethylene exposure. Tomato plants exposed to low levels of ethylene gas will appear wilted or display symptoms of epinasty. While these plants will recover if the ethylene gas issue is corrected, the plants will take several weeks to grow out of it, which will result in increased production times and a delayed tomato harvest.

Figure 4. Epinasty is a classic symptom of ethylene damage. The tomato plant pictured displays symptoms of ethylene damage. Photo: Tom Ford, Penn State

Before firing your greenhouse up this spring, consider the following best management practices:

1. Inspect the heat exchanger for cracks and holes that lead to flue gas leakage into the greenhouse.
2. Make sure that your flue pipe extends a few feet above the roofline of your greenhouse.
3. Ensure that a flue cap is used on the flue pipe to prevent flue gases from blowing down the pipe and into the greenhouse.
4. Ensure that exhaust/ventilation fans are not running when the furnace is operating since they could potentially suck flue gases into the greenhouse.
5. Make sure that you have a fresh air inlet in place to provide enough oxygen for proper combustion. Generally, a minimum of one square inch per 2000 BTU of heat output is required.