In Colorado, it has been a beautiful indian summer, yet blowing snow and sub-freezing temperatures are just around the corner. It is time to make sure that your heating systems are ready to function efficiently.
Greenhouse growers who use boiler systems typically have had all their annual inspections and have completed their annual maintenance, but growers who rely on gas fired unit heaters are often not quite so conscience of the conditions of their heaters. It is way past time to inspect your heaters. Regular maintenance will easily pay for itself this season with high gas prices predicted.
Inspect the flue pipe. Wind is responsible for most damage to flue pipes, however, one can always expect some degradation of the joints in the greenhouse. Check for rusting unions and for any debris that may have collected in the flu pipe. If the flue pipe has an exhaust fan to move exhaust gasses through the flue, make sure that it is operating properly.
Inspect the heat coils. Greenhouses are humid environments and metal equipment is subject to rust. If the heat coils are rusted through, exhaust gasses can contaminate the greenhouse environment.
Inspect the gas manifold. Dirty gas orifices will cause incomplete combustion of the fuel, which will result in exhaust gasses that will contaminate the greenhouse environment.
Check the ignition modules and gas valves. These devices do wear out and require periodic service. These devices should be inspected and serviced by a licensed technician. Inefficient operation of gas fired unit heaters can lead to a lot of problems in the greenhouse. Primarily we think of carbon monoxide, which is deadly to the staff, but one must also think about ethylene gas as well.
Ethylene levels as low as 20 ppb (that is parts per billion) have been shown to damage Cattleya species and 500 ppb are sufficient to cause flower abortion in tomatoes. Concentrations of 50 ppb for extended periods (how long? two to four hours) are just as deleterious as high concentrations.
There has been some interest in using carbon monoxide (CO) detectors for estimating ethylene in a greenhouse. In Holland they have studied these detectors. They conclude that the ethylene level would be less than 0.1 of the critical 50 ppb if the CO content of the undiluted flu gasses did not exceed 50 ppm. The presence of CO, however, does not guarantee the presence of ethylene and vice versa. But they are cheap. Some growers use tomato plants underneath their unit heaters and if the leaves exhibit epinasty, they assume that there is ethylene contamination. Tomato plants are typically more sensitive than other floriculture crops to ethylene. (you can read more in Dr. J.J. Hanan's text, Greenhouses: Advanced Technology for Protected Horticulture)
Prevention is the key to ethylene gas control in the greenhouse. Maintain your gas-fired heaters in good condition. Clean the manifolds regularly and check for cracks in the heat exchangers. Flue pipes must be clean and free of debris. They must also have the correct clearance over the building if they are not connected to a forced air exhaust system. IR-radiant heat systems are not immune to ethylene contamination. Mount the exhaust fans as close to the end of the flue as possible to prevent any back pressure from a prevailing wind. Finally, make sure that your gas supply is adequate for the unit heater and that you are supplying adequate oxygen for combustion.
If you suspect that you have an ethylene gas problem, contact the Floriculture faculty at North Carolina State University for testing.