In order to properly size a heating system for either an existing greenhouse or one still in the planning phases, you will need to perform a thorough heat loss calculation.

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## Heat Loss Factors

There are two main factors to consider in heat loss calculation. You must determine the heat loss through the glazing materials of the greenhouse, or the heat that will escape the greenhouse. You will also need to calculate the heat loss due to infiltration, which is the cold air that can come into the greenhouse.

## Calculations

The first two measurements to determine are the exposed surface areas of the actual greenhouse covering and then that of all the other exposed materials that make up the greenhouse exterior. From these measurements, you can determine the corresponding U-value, which is the heat transfer coefficient for greenhouse construction materials.

## Other Considerations

After you have determined the U-value, you will need to know the difference between the highest temperatures inside the greenhouse and subtract the minimum outside temperature from it to get your T-value. Multiply each of your U-values by your T-equation to determine your "QC," or conduction heat loss factor. From here, determine the volume of your interior space. Multiply that by your T-value and by .22 to determine the air infiltration heat loss, or "QA." Total heat loss for your greenhouse --- "QT" --- is equal to the sum of "QC" and "QA."

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#### References

- West Virginia University Extension Service; Planning and Building a Greenhouse; David S. Ross
- Texas A&M; University; Greenhouse Heating Requirements
- Auburn University; Greenhouse Heat Loss Calculations; J. Raymond Kessler, Jr.
- University of Florida IFAS Extension; Greenhouse Environmental Design Considerations - Florida Greenhouse Vegetable Production Handbook, Vol. 2; P.H. Jones; 1999