How to Calculate Required BTU Output for Your Greenhouse Heating Needs
Your greenhouse is only capable of keeping the plants inside it warm, when there is enough direct sunlight hitting the covering daily, to do so. When the sun angles away from our hemisphere, resulting in colder temperatures and wintry conditions, the sun’s rays aren’t powerful enough to keep the plants inside your greenhouse warm. If you want your plants to survive the winter chills, you will need a heater that is up to the task.
Notice: This guide was built for the purpose of educating the public, as to how these figures are actually determined. If you’re not interested, in learning the extended mathematical process, EarthCareGreenhouses.com™ provides a free greenhouse BTU heating calculator and a greenhouse surface area calculator.
Table of Contents:
Part #1: Brief Introductory Statement
Introduction to Greenhouse Heating Elements:
Heaters aren’t given in size ranges. A larger unit doesn’t automatically mean more heat output. Heaters are measured by their output of BTUs. This number reflects the size of an area that a heater is capable of keeping warm. If you want a heater that will beat back the toughest winter freeze, you will need to do some calculations.
Part #2: Tools Required
Finding the right BTU output for your greenhouse is an easy task.
You will need a measuring tape, paper, something to write with, a calculator, and possibly a second set of hands and a ladder, depending on your greenhouse’s size.
What Do You Need:
- Measuring Tape (Required)
- Paper (Required)
- Writing Utensil (Required)
- Calculator (Required)
- Additional Help (Depends upon size)
- Ladder (Depends upon size)
Part #3: Calculations Equations, and Formulas
Step #1: How to Calculate Greenhouse Surface Area Sq. Footage
First, you need to find the total square feet of your greenhouse’s surface areas.
The equation for finding the square feet of a building with a sloped roof isn’t that difficult. On your paper, write down these letters, leaving space for measurements after each: H, L, W, R and S
For each of these letters, find the following measurements and write them beside the appropriate spot.
- H – Height of the greenhouse wall from the ground to the lowest point of the roof.
- L – Length of the longest greenhouse wall.
- W – Length of the shortest greenhouse wall.
- R – Height from the ground to the highest point of the greenhouse roof.
- S – Length from the highest roof point to the lowest roof point, measured along one eave.
Note: For the purposes of this explanation, the * symbol will mean you should multiply. Remember, anything that appears inside (…) needs to be the first math done in an equation.
Equation #: 1
First, plug the appropriate measurements into this equation: 2*(H+S)+L
Equation #: 2
Next, work the equation: (R+H)* W
Equation #: 3
Add those two results together, and you have your surface area in square feet.
Write this final calculation down in an unused corner of your paper.
Greenhouse Square Foot Calculator Tool: Input Your Values & Hit Enter!
Step #2: Temperature Expectations and Goal Calculations
Next, write down the temperature, in Fahrenheit, that you want your greenhouse to be at. Below that, write down the coldest temperature your climate experiences. Now, subtract the two. If your coldest temperature is a negative reading, add the two instead. Record the difference in Fahrenheit down by your square feet measurement.
Step #3: Converting Celsius to Fahrenheit Calculations
You will need to use Fahrenheit numbers for this math:
Equation #: 1
To convert Celsius to Fahrenheit quickly, double the Celsius reading, then subtract 1/10th of its value. Add 32.
Step #4: How to Find the Greenhouse Cover Heat Loss Values
The third thing you will need to know is the heat loss value of your covering material.
Some of the most popular types of greenhouse coverings are listed here, with their heat loss value. If your covering isn’t on this list, you will need to look up the heat loss value for yourself.
- Glass, single pane – 1.05
- Glass, double pane – .50
- Plastic sheeting – varies by brand
- Plastic films – 1.20
Next: Once you know your covering’s heat loss value, write it down alongside your square feet and temperature differences in the corner.
Step #5: Calculation of British Thermal Units
Finally, it is time to calculate the BTUs you will need in a heater. The equation for this is, square footage multiplied by temperature difference, multiplied by heat loss.
Equation #: 1
Square Feet * Temperature Difference * Heat Loss Value
BTU Calculator Tool: Input Your Values & Hit Enter!
Step #6: Shopping for Heaters using Formula Results
Take this number with you when you are searching for a greenhouse heater. Only purchase a heater that meets this minimum BTU output, but you don’t need to go much over it, either.
Part #4: Closing Statement
⇔ This article should help even beginning gardeners, to determine the proper greenhouse temperature management and control regulations, to implement. Prior to buying a greenhouse, I highly recommend doing the calculations, to understand and fully comprehend the cost, difficulty, and to create a much better planned outdoor greenhouse structure. If you have any questions or comments, please feel free to post them below, and we would be happy to help iron out any difficulties, you might run into along the way.
– Whitney Segura