Calculating the Heating Needs for Your DIY Greenhouse
As covered in our other posts, heating your DIY Greenhouse can be one of the more expensive parts of owning it. Keeping several considerations in mind when you start planning your construction, namely your style or design, can reduce this cost greatly over the lifetime of ownership.
The flip side of that expense however is the benefit you receive of being able to control your growing environment to a greater degree, which in turn allows you better oversight of your plants growing patterns. It is this yearning for control, or ability to beat the natural weather patterns of your home climate that lead many to build a greenhouse in the first place.
We covered the types of greenhouse heating systems in another article. Please check there if you are trying to determine what heating source is best of your building.
But how do you make sure the heater you purchase and install is large enough to fit your needs without constantly running? It really just comes down to some simple math and understanding of heating principles in general.
To figure out what size heater you need to install, the first thing you need to know is that heaters are rated based on the amount of
BTUs they can supply to a space. In case you are not familiar with that term, a BTU, or British Thermal Unit is the amount of heat needed to raise one pound of water one degree Fahrenheit. A heaters BTU output will be shown on the packaging the heater comes in if you are purchasing a new unit. If you are reusing an old heater from another application, the BTU output will be shown on the heaters nameplate, normally a small tin plate on the side of the heater.
But how many BTUs of output do you need for your greenhouse space? This is where the math comes in. To calculate that, you need to know just a few simple things; Area, Difference, and the HLF (Heat Loss Factor) of the glazing and greenhouse design you have chosen.
For the Area of your greenhouse, multiply the length and height of each wall and roof panel, and then adding those smaller areas together.
For the Difference in the equation, you need to calculate the difference (or Delta) between the coldest nighttime temperature of the area that you live in, and the minimum heat level that you will allow in your greenhouse. For example if you are looking to grow in your greenhouse in the fall months in New Hampshire where the temperature can drop to 0 degrees Fahrenheit, but want to maintain a room temperature of 50 degrees, then your difference would be 50 degrees Fahrenheit.
For your HLF, we suggest using average factors as it keeps the math simple and works well for backyard gardeners. The sources we have found recommend a factor of 1.1 for the glazing used in greenhouses.
So for an example; let’s say that you had calculated that your greenhouse has an area of 450 square feet, and that the difference in your heating needs is 50 degrees. That would be 450 sq ft X 50 dg X 1.1 = 27,500 BTUs.
The above example assumes that this is a stand-alone greenhouse with single thickness glazing. If your greenhouse has double glazed glass or is double thickness polycarbonate, you can multiply that above factor by .70 (for 30% better efficiency), which would be a BTU requirement of 19,250 BTU in the example above. If you use triple glazed glass or three-walled polycarbonate you can multiply by a factor of .50 (for 50% better efficiency), which would be a BTU requirement of 13,750 BTU in the example above. If your greenhouse shares a wall with your home or other well-insulated building and is double glazed, your efficiency may even be improved by 60 %! (Multiply by .40)
As you can see, planning for efficiency upfront can save you a lot of money in the long run. Good luck with your planning and building your DIY greenhouse, for other great tips and helps, please be sure to browse our site.