I am often asked by growers to help estimate what size heater is needed for a greenhouse or what minimum temperature their high tunnel will reach at a certain outside temperature. Below are some tools to help you do this yourself. I have presented them in a range of complexity depending on how much you really want to get into the math. Enjoy.
1. SIMPLEST – Online greenhouse heat load calculator. http://www.greenhousemegastore.com/greenhouse_btu_calculator This online calculator allows you to enter the dimensions, construction material and temperatures you are interested in and it estimates the heat (and cooling) load.
2. LITTLE MORE COMPLEX – VirtualGrower – http://ars.usda.gov/services/software/download.htm?softwareid=309. This is a free software tool from USDA ARS that is a bit more complicated than the simple form above. But there is benefit to the complication. As with any analysis, the more you put into it, the more you get out of it. VirtualGrower allows easier management of multiple “what-if” scenarios, includes regional weather and light data automatically, and accounts for heating and ventilation systems. You may find it interesting and useful.
3. HEAVY LIFTING, but FULFILLING – Do the calculations yourself! The formulae behind all of the tools above are well described in “Greenhouse Engineering, NRAES-33” by R. A. Aldrich and J. W. Bartok. Available here as a PDF: http://host31.spidergraphics.com/nra/doc/Fair%20Use%20Web%20PDFs/NRAES-33_Web.pdf. See p. 65-71 specifically.
4. HEATING BENCHES, ROOT-ZONE or GROUND – When heating the root zone locally, different heater sizing approaches are needed. This is covered well by “Root Zone Heating Systems” in Wilkinson KM, Haase DL, Pinto JR, technical coordinators. National Proceedings: Forest and Conservation Nursery Associations—2013. Fort Collins (CO): USDA Forest Service, Rocky Mountain Research Station.Proceedings RMRS-P-72. 62-65. Available at: http://www.fs.fed.us/rm/pubs/rmrs_p072.htm.