Unvented Heaters in Greenhouses
Monday, 11 January 2010 16:19
Occasionally growers want to use an unvented heater as a back up heat source. The following was taken from an article that John Bartok, Agricultural Engineer wrote on using unvented heaters.
Tina Smith, UMass
An unvented heater is one that is designed without a flue connection so that the heat and products of combustion are exhausted into the greenhouse. Dumping these flue gases into the greenhouse may improve the overall efficiency rating as compared to a conventional heater but the pollutants and added moisture from combustion may put your plants in jeopardy.
Unvented heaters can be fired with natural gas, propane or kerosene. These fuels are advertised as clean-burning. This is generally true if the burner is clean and has been adjusted to top efficiency. A heater that has been serviced in the fall will probably not be at peak efficiency as spring approaches. Inefficient combustion can emit unwanted and harmful pollutants into the greenhouse that can affect the plants and the people working with them.
All fuels contain traces of sulfur, some more than others depending on its source. During combustion sulfur in the fuel is combined with oxygen to form sulfur dioxide. Levels as low a ½ part per million (ppm) can cause injury on some plants. Once the sulfur dioxide enters the plant through the stomates, it reacts with water to produce sulfuric acid that will cause leaf burn, flecking and general chlorosis. Tomatoes and white petunias are very sensitive to this and will show signs in as little as one hour. They therefore make good indicator plants.
Ethylene gas is another pollutant formed during combustion. It can also be formed from ripening or rotting plant materials. Levels as low as 0.01 ppm can create symptoms such as malformed leaves and flowers, stunted growth, bud abcission, epinasty and flower senescence. Levels are usually highest near the heater and can be diluted by air circulation.
Care should taken if you install unvented heaters. Be sure to have an adequate makeup air supply and provide frequent maintenance. Use indicator plants near heaters or commercially available indicator tubes to warn of excess levels of pollutants.
For the complete article see: Problems With Using Unvented Heaters
