How should I heat my greenhouse? - Ask Extension
the greenhouse is all plastic with vents a mandoor and roll up sides. We plan on raising garden veggies and flowers from seed. I am currently giving...
Knowledgebase
How should I heat my greenhouse? #889792
Asked November 13, 2024, 3:00 PM EST
the greenhouse is all plastic with vents a mandoor and roll up sides. We plan on raising garden veggies and flowers from seed. I am currently giving thought to propane (vented) or near infrared. the greenhouse is not well insulated except that the roof has 2 layers of plastic with spacers in between.
Yoder Produce says I need 30,000 btu vented propane. I have no idea how to size infrared. Greenhouse is 12'X12' with 6'sidewalls and an arched cieling.
Would like to know your recomendations and the pros and cons of each. Will be running water drain and electir lines soon and need to know if infrared is an option in order to bury the right size wire. Radiant heat could also be an option but I am not sure it can heat up fast enough.
Wayne County Ohio
Expert Response
Hi Ben,
Ultimately, sizing the heaters comes down to doing some math about your greenhouse. First you will calculate the surface area of the greenhouse. Here is a helpful tool for that: https://greenhouseemporium.com/greenhouse-surface-area-calculator/
Then you take the calculated surface area and use it to calculate needed BTU's. The formula for this is BTU=((desired inside temperature - minimum outside temp) x calculated surface area) / R Value.
We are zone 6a in Wayne County, so our typical minimum outdoor temp is -10 F. It is up to you as far as your desired temperature maximum in the tunnel. I would suggest somewhere around 60-70F. The R-value of 6 mil double layered coverings ranges slightly from 1.25-1.42. This is only accounting for the double layered surface area footage though. If you have single layer side walls, this drops the overall R-value of the greenhouse. The end walls are also a factor. If they (and the sidewall curtains) are single layer, 6 mil poly, the R value is .85. I know sometime the end walls are installed as a hard double wall polycarbonate, and that has an R value of 1.7. As you determine the R values for the various components of the structure, for simplicity sake in the calculation, taking the average of the values will give you general R-value of the structure.
More on R-values - https://www.youtube.com/watch?v=dNZAhQyeMwo
Here is a calculator tool to help: https://greenhouseemporium.com/greenhouse-heater-calculator/
and video: https://youtu.be/tF-drNAhZsM?si=UBK1K8uFRKgzXqQn
Just use the "Enter Manually" option on the dropdown bar for the greenhouse brand.
Ultimately, it comes down to what you are willing to invest in for the installation. Any system will work, so long as it is correctly calculated for the space. Too, it is about your production goals. Are you wanting to grow all year round? Or, is your goal to get an early start in the season, in which case, you really don't need a lot of supplemental as your goal would be to basically keep the crop from freezing. The other factor in a heating system working effectively and efficiently is added insulation. This could be some extra foam board insulation on the sidewalls, or bubble insulation on the end walls, as examples, which could increase the overall R value and allow for more heat retention, slower temperature fluctuations and improved heater efficiency.
Additional Resources
https://www.nrcs.usda.gov/sites/default/files/2022-10/Controlling-High-Tunnel-Environment.pdf
https://www.tunnelvisionhoops.com/blogbest-greenhouse-plastic-comparing-through-first-hand-experience/
https://www.solawrapfilms.com/greenhouse-plastic-blog/greenhouse-r-value-what-does-than-mean-for-energy-savings
https://ag.umass.edu/greenhouse-floriculture/fact-sheets/insulating-sidewalls-endwalls-has-short-payback
If you have any other questions, let me know. Yoder's is normally pretty accurate with their calculations. You may also be interested in talking to CropKing in Lodi. They are a greenhouse company that would also have experience with these systems. Hope this information is helpful!
Ultimately, sizing the heaters comes down to doing some math about your greenhouse. First you will calculate the surface area of the greenhouse. Here is a helpful tool for that: https://greenhouseemporium.com/greenhouse-surface-area-calculator/
Then you take the calculated surface area and use it to calculate needed BTU's. The formula for this is BTU=((desired inside temperature - minimum outside temp) x calculated surface area) / R Value.
We are zone 6a in Wayne County, so our typical minimum outdoor temp is -10 F. It is up to you as far as your desired temperature maximum in the tunnel. I would suggest somewhere around 60-70F. The R-value of 6 mil double layered coverings ranges slightly from 1.25-1.42. This is only accounting for the double layered surface area footage though. If you have single layer side walls, this drops the overall R-value of the greenhouse. The end walls are also a factor. If they (and the sidewall curtains) are single layer, 6 mil poly, the R value is .85. I know sometime the end walls are installed as a hard double wall polycarbonate, and that has an R value of 1.7. As you determine the R values for the various components of the structure, for simplicity sake in the calculation, taking the average of the values will give you general R-value of the structure.
More on R-values - https://www.youtube.com/watch?v=dNZAhQyeMwo
Here is a calculator tool to help: https://greenhouseemporium.com/greenhouse-heater-calculator/
and video: https://youtu.be/tF-drNAhZsM?si=UBK1K8uFRKgzXqQn
Just use the "Enter Manually" option on the dropdown bar for the greenhouse brand.
Ultimately, it comes down to what you are willing to invest in for the installation. Any system will work, so long as it is correctly calculated for the space. Too, it is about your production goals. Are you wanting to grow all year round? Or, is your goal to get an early start in the season, in which case, you really don't need a lot of supplemental as your goal would be to basically keep the crop from freezing. The other factor in a heating system working effectively and efficiently is added insulation. This could be some extra foam board insulation on the sidewalls, or bubble insulation on the end walls, as examples, which could increase the overall R value and allow for more heat retention, slower temperature fluctuations and improved heater efficiency.
Additional Resources
https://www.nrcs.usda.gov/sites/default/files/2022-10/Controlling-High-Tunnel-Environment.pdf
https://www.tunnelvisionhoops.com/blogbest-greenhouse-plastic-comparing-through-first-hand-experience/
https://www.solawrapfilms.com/greenhouse-plastic-blog/greenhouse-r-value-what-does-than-mean-for-energy-savings
https://ag.umass.edu/greenhouse-floriculture/fact-sheets/insulating-sidewalls-endwalls-has-short-payback
If you have any other questions, let me know. Yoder's is normally pretty accurate with their calculations. You may also be interested in talking to CropKing in Lodi. They are a greenhouse company that would also have experience with these systems. Hope this information is helpful!
