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Check Furnaces and Flues Before Putting Crops in the Greenhouse

Thu, Jan 19th, 2023, created by Thomas Ford
Some growers shut down their greenhouses in December after their poinsettias have been shipped to their final destination and will fire up their heaters/furnaces again for the first time in late January. In most cases the resumption of heating a greenhouse should not be problematic, but in some situations the flue and heat exchangers may develop small pinholes in the metal that can leak damaging flue gases into the growing environment. The most common flue gas, ethylene, is odorless and colorless and may cause significant injury to sensitive crops when exposed. 

Prior to placing any crops in a greenhouse, the grower should examine the flue for cracks and pinholes. If pinholes or cracks are noticed in the flue, consider replacing or repairing the faulty sections. Some furnaces may develop pinholes or cracks in the heat exchangers or heater boxes. Either have a professional examine the heat exchangers or heater boxes for pinholes or cracks or do it yourself prior to firing up the heater for the first time. One small pinhole can allow flue gases to enter the growing area resulting in reduced crop yield and/or poor crop quality. If the heat exchangers or heater boxes are compromised, you may need to consider replacing the entire furnace if replacement parts cannot be located within a reasonable time frame. It is prudent to examine your heating system 2-3 weeks before you plan to begin heating the greenhouse so repairs can be facilitated if needed in a timely manner.

A flue cap at the end of the flue pipe is critically important in preventing flue gases from being blown back down the flue into the production area. If the flue cap is damaged or missing replace it right away so a sudden downdraft does not blow flue gases into the growing environment. For additional information please contact the author at

About the Author:

Thomas Ford

Commercial Horticulture Educator, Penn State Extension

Tom has worked for over 40 years with Cooperative Extension in Maryland, North Carolina, and Pennsylvania. During his career he has worked intensively with vegetable and fruit growers, greenhouse and nursery operators, landscape and turf professionals and area farmers with their production and pest management issues.


Tom is a native of Central Maryland and resides with his wife, Laura and their four sons in Duncansville, PA. Tom has a B.S. degree in Ornamental Horticulture from the University of Maryland and a MBA from Frostburg State University in Frostburg, MD. Tom currently serves as a Commercial Horticulture Educator with Penn State Extension and is housed in the Cambria County Extension Office in Ebensburg, PA.

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