Following are step by step instructions on how to calculate your greenhouse’s heat loss and help you determine the size of heater which will be needed to keep temperatures at an optimal level. Another great tool is this greenhouse heater calculator which does the math for you.
Calculate Greenhouse Heating:
1. Determine the exposed surface area (A) of the greenhouse glazing and of other materials such as curtain walls or solid north walls. Draw a diagram or set up a table in which each of these areas is listed.
2 .Find the U value (heat transfer coefficient) of the materials used in the greenhouse; available here.
3. Determine the largest expected difference between inside and outside temperatures in degrees Fahrenheit. Use average lows to subtract the minimum outside temperature from the maximum inside temperature to get the temperature differential (T).
4. Find the conductive heat loss by multiplying the exposed area of each material times the U value times the temperature difference (A x U x T). Add the heat loss for each material to determine the total conductive heat loss.
5. Calculate the volume (V) of the greenhouse. Multiply the length (L) times the width times the height (H) of vertical walls (L x W x H). If the greenhouse has a gabled roof, multiply one-half times the height of the gable (h) times the width of the greenhouse times the length, or 1/2 h x W x L. Add these numbers together. If the greenhouse has a curved roof, multiply one-half times the height of the curved portions squared, times 3.14, times the length of the greenhouse and add this to the volume of the portion with vertical walls (1/2 r2 π) x L.
6. Determine the number of air exchanges per hour (E) based on the construction of your greenhouse; available here. Old construction can be as high as 4, if it is poorly maintained or as low as 1 if it is well maintained. If the wind speed is low or your greenhouse is protected from the wind, use the low number. If the average wind speed is high or the greenhouse is exposed, use the higher one.
7. Calculate the air infiltration heat loss by multiplying .22 times the temperature difference, times the volume, times the air exchanges. (.22 x T x V x E)
8. Calculate the total heat loss by adding the conductive heat loss and the infiltration heat loss. This number is in BTUs and can be used to determine the size of heater needed.