Friday, January 18, 2008

Greenhouse - Check for Malfunctioning Greenhouse Unit Heaters

Be sure that your greenhouse heaters are operating properly. Malfunctioning heaters can lead to poor plant performance. The following is an article on the subject.

Now is the time of the year when growers need to be especially aware of the working condition of their gas-fired unit heaters and the impact of other activities in the greenhouse that may contribute to some form of air pollution. The following article and web sites will be of interest as you begin to open up houses and re-fire unit heaters to warm up houses.
Crops grown in greenhouses that utilize gas-fired unit heaters can be susceptible to ethylene injury. Ethylene (C2H4) is an odorless, colorless gas that acts as a plant hormone. Plants are very susceptible to ethylene injury at levels from 0.01 to 1 ppm or more. No other air pollutant causes a greater range of symptoms than ethylene gas. Symptoms range from misshapen leaves and flowers, thickened stems, stunted growth, flower or leaf abortion to epinasty. See these examples for photos:

The effects on greenhouse crops will vary with the plant species and growth stage, temperature, length of exposure, and the concentration of the ethylene. I have noted plant injury symptoms more often in plastic greenhouses compared to glass greenhouses, due to the airtight nature of poly-greenhouses. A good bulletin on the subject was written by faculty at North Carolina State University and is titled Ethylene: Sources, Symptom, and Prevention for Greenhouse Crops. It can be downloaded free at:
An indicator plant to use for ethylene is a tomato plant. They are highly sensitive and will twist or wilt when exposed to ethylene. Tomatoes will exhibit injury within 24 hours if ethylene is present.

To avoid ethylene injury, unit heaters need proper ventilation and intake of fresh air from the outside. One square inch of vent cross section (of outside air) for every 2,500 Btu’s of heater output is recommended. Consider using a laundry dryer vent hose as a fresh air intake. Thus, if you have a 125,000 Btu heater, you would need an 8-inch diameter fresh air inlet pipe that would give you the 50 square inches you need.

Also, unit heaters need to be maintained so that the heater itself is running properly, and the distribution tube, vent stack, ventilation louvers, and fuel line are all functioning correctly. Try this checklist for proper maintenance.

Annual heating unit checklist

Heat exchanger - Check for cracks. While the furnace is running, inspect for light penetration.
Furnace - Check for leaks. Place a smoke bomb or furnace candle within the firebox.
Gas lines - Check for leaks. Paint soapy water on the joints and seams.
Exhaust chimney - Check for leaks and obstructions.
Pilot light - Clean pilot and orifice.
Flame - Make sure the burner flame is clear blue. Yellow or orange flames represent impurities or a wrong setting.

If you suspect ethylene injury is occurring, contact your local UD Horticulture agent. We can look at the crop and obtain air samples to verify if ethylene is the problem. Also, call your furnace maintenance firm to inspect the unit.

Most plants will recover from ethylene injury. However, those plants that bloom once (like lilies, tulips and hyacinths) that were exposed to ethylene when they were in the flower bud stage will likely not bloom.

Other activities can also lead to greenhouse plant damage caused by air pollution besides faulty unit heaters. These include ozone (smog),unburned fuel in your gas lines, herbicides that were used in and around a greenhouse, dibutyl phthalate (plasticisers used in manufacturing semirigid plastics), use of wood preservatives and even emissions from propane fired hi-lo's. A great article on this subject was written by Dr. Theo J. Blom from the Vineland Research Station, University of Guelph in Canada. Go to: for more details. Again feel free to contact your greenhouse Extension educator to help you diagnose any of these problems if you are seeing plant injury.

From "Malfunctioning greenhouse unit heaters and other air pollutants equal greenhouse crop problems" by Thomas Dudek, District Extension Horticulture and Marketing Educator in the January 19, 2007 edition of the Greenhouse Alert newsletter from Michigan State University Extension.

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