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Greenhouse IPM Report: Evaluate Unit Heaters Before Firing Up Your Greenhouse

Greenhouse IPM Report: Evaluate Unit Heaters Before Firing Up Your Greenhouse
By Thomas Ford
With plugs arriving soon and the 2021 greenhouse season starting it is essential that you evaluate the unit heater components carefully to ensure that they will function properly when you light the pilot or turn on the furnace for the first time this winter.
Flue gas injury is an unfortunate reality in many greenhouses. Small pinholes in heat exchangers and cracks in flue pipes can allow ethylene gas and other flue gases to escape into the production areas resulting in plant injury. Ethylene gas which is colorless and odorless can cause geranium flowers to abort and tomatoes to develop epinastic growth (a downward bending of the stems/leaves).
Some of the most susceptible greenhouse crops to ethylene gas are listed in the table below:
CarnationNarcissusRoseSweet pea
Ethylene gas is a byproduct of incomplete combustion in gas-fired heaters/furnaces. Ethylene gas concentrations as low as 1-2 ppm in the air have been shown to prevent or delay flower bud initiation in poinsettias and chrysanthemums. African marigolds and sunflowers are considered to be two of the most sensitive floral crops to ethylene gas with injury being observed at levels as low as .0005 ppm.
When carbon-based fuels are burned completely they produce carbon dioxide and water vapor. If incomplete combustion occurs due to a malfunctioning heater or due to insufficient oxygen levels in the greenhouse, ethylene gas can be produced. New and experienced growers should be aware that a supply of fresh air to the back of each gas-fired unit heater is required for complete combustion to occur. Greenhouse operators should install a plastic duct through the greenhouse wall that has a minimum cross-sectional area of one square inch for each 2000 BTU heat output rating of the heater/furnace.
Flue gases, a byproduct of the combustion process of carbon-based fuels must be safely transported to the outdoor environment through flue or vent pipes as they are burnt. Flue pipes should be carefully examined for cracks and pinholes to ensure that flue gases are not vented directly into the production area. The exhaust or flue should extend a few feet above the roofline to aid in the dissipation of the flue gases into the atmosphere. A proper fitting cap should be installed at the top of the flue to prevent downdrafts from taking flue gases back into the growing area.
Sulfur dioxide is another flue gas that has been responsible for plant injury in greenhouses. Sulfur dioxide injury tends to be observed when unvented heaters are deployed in greenhouses or when a downdraft forces flue gases back into the greenhouse. Sulfur dioxide injury historically was observed in greenhouses that burnt low-quality soft coal in their boilers or stoves. On broadleaf plants, sulfur dioxide causes necrotic spots or lesions to form between the major veins. The injured tissue between the veins turns tan in color and will seem papery in texture as depicted in the photo below.
Sulfur dioxide
Figure 2: Sulfur dioxide injury on tomato due to a downdraft of flue gases from a coal-fired boiler into the greenhouse. Photo: Tom Ford, Penn State
Plants that are very physiologically active are the most susceptible to sulfur dioxide injury. Research has shown that susceptible plants can be injured when exposed to 0.48 ppm sulfur dioxide for four hours or 0.28 ppm for 24 hours (Thomas, 1961). Some of the greenhouse crops that are susceptible to sulfur dioxide injury are listed in the table below.
AgeratumChina asterFour O'clockMorning gloryTomato
Brussel sproutsCosmosHibiscusPepper 
CentaureaEndiveMallowSweet William 
With the growing season upon us, all greenhouse operators should take the following steps to prevent flue gas injury this winter/spring season:
  • Inspect and service all unit heaters prior to lighting them this winter.
  • Provide fresh air inlets for all gas-fired heaters.
  • Install a flue cap on all flues to prevent downdrafts
  • into the greenhouse.
  • Make sure that flue pipes extend a few feet above the greenhouse roofline to allow flue gases to dissipate. 
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