Is Your Heater Hurting Your Plants?
November 4, 2009 
Greenhouse heating is essential to overwinter vegetables in all but the extreme southern latitudes of the U.S. Rather than just keeping plants alive, which can be done at any temperature above freezing, maintaining a temperature set point of 64°F or above will help to produce a good yield of high-quality fruit from your greenhouse.
There are several rules a grower needs to follow to be certain the heating system is beneficial to the bottom line.
Follow The Rules
Whether your heating fuel is natural gas (methane), LP gas (propane), No. 2 diesel oil, wood, or some alternative fuel type, the same rules apply. If you have a heat pump or an electric heater, you can ignore this advice.
1. First and foremost, vent all burners to the outside. Never allow gases to remain inside the greenhouse. Tomato plants are very sensitive to certain pollutants found in the exhaust from fossil fuels. In fact, 1 ppm (part per million) of ethylene or carbon monoxide can cause serious damage. All fuel-burning heaters need to be vented with a stack to the outside.
2. The exhaust stack (flue) should extend at least 2 feet, preferably 4 feet, above the ridge (highest point) of the greenhouse. The high temperature of the exhaust gases helps to draw pollutants out of the greenhouse.
3. When designing the greenhouse layout, make sure the intake vent from one greenhouse is not close to the exhaust stack of another one. This will likely cause one greenhouse to pull in contaminated exhaust from the other.
4. Avoid space heaters in a tomato greenhouse. Sometimes a grower may be caught by a sudden cold snap in the fall and not have the heater in a new greenhouse installed yet. Another scenario is that he may not have tested a heater since last winter and will find that it doesn’t work.
With kerosene and propane space heaters (unvented), the potential exists to poison plants with toxic pollutants. Also, space heaters may consume oxygen and deplete it so that incomplete combustion may result, producing harmful by-products. Or the lack of oxygen may cause the flame to go out and the burner to shut off, leading to freeze damage. In any event, the use of unvented space heaters is too risky for the commercial greenhouse grower. Test your main heating system a few weeks before the expected heating season to be sure it is working properly.
5. In a tight, plastic greenhouse, it may be necessary to bring fresh outside air to the burner via a duct to ensure complete combustion. You can size the air intake for the heating unit with a simple formula: use 50 square inches of intake for each 100,000 BTUs of the heating system.
For example, if your heater is 160,000 BTUs, use 160,000/100,000, or 1.6 times 50 equals 80 square inches (or about half a square foot) of air intake for the burners.
6. Other less likely, but possible sources of ethylene are from combustion engines outside the greenhouse (cars and trucks parked just outside, forklifts, etc.) and fuel supply lines with leaks. If you suspect a leaky fuel line, try painting soapy water on the joints and seams and look for bubbling.
Recognizing Ethylene Damage
How do you know if you have ethylene damage on your greenhouse tomatoes? Common early symptoms are drooping tomato leaves, although they are still turgid and not wilted.
As damage continues, flowers form, but then fall off. In severe cases, nearly every flower in the greenhouse can be
lost in a matter of hours. Also, the young, upper leaves on the plant will twist around, sometimes turning upside down.
If any of these symptoms are occurring, it is time to investigate the source of the problem. Providing that the heaters are not unvented, often the leak is due to an old, worn out heater with cracks in the heat exchanger or fine holes that formed in the exhaust pipe between the heater and the greenhouse wall or roof.
Also, be sure the flame is adjusted properly to get complete combustion. The flame should burn clear blue. If it is yellow or orange it needs adjustment.
Repairs should be made as soon as possible. Ethylene damage seems to be most common on new heaters not installed properly, and on heaters more than six to eight years old.
Remember, ethylene is colorless and odorless. You can’t detect it without specialized equipment. If you suspect a gas leak, try to get your utility company (gas, diesel) to come test your exhaust with an ethylene tester. It is important that the tester can measure as low as parts per million.
Proper Testing Tools
A common problem occurs when the company sends out a rep with a tester that measures in percentages rather than ppm and states that there is no reading, meaning that the air is ethylene free. However, keep in mind that 1% is 10,000 times bigger than 1 ppm. So, if a tester reads less than 1%, all that means is that there is under 10,000 ppm of ethylene. So be sure the utility company is prepared with proper equipment to help you.
How much is too much?
Even 1 ppm of ethylene for just three to four hours can be toxic. On the other hand, much lower levels, 0.01 to 0.05 ppm, can cause problems if this level is maintained for long periods (chronic exposure).
With tighter greenhouse coverings, especially those with double poly or double walled polycarbonate, heat leaks through the glazing are nearly eliminated. However, tighter greenhouses are more likely to allow buildup of toxic gases. So, be on the lookout for ethylene symptoms and fix the problem before it’s too late.
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