Greenhouse heat conditions are based on the gap between the typical anticipated heat loss as well as the typical anticipated heat gain from the coldest portion of the year. These averages depend on climate, glazing material, heat storage capacity and measures taken to insulate the greenhouse. Efficient heating is dependent on matching the heating equipment to the heat condition of the greenhouse.
Determine the exposed surface area (A) of the greenhouse glazing and of different materials like curtain walls or solid north walls. Draw a diagram or set up a table in which every one of those areas is listed.
Locate the U value, or heat transfer coefficient, of the materials and construction kind used in the greenhouse; those are available in construction standards books or internet references.
Determine the biggest anticipated difference between inside and outside temperatures in degrees Fahrenheit. Use average lows to subtract the minimum external temperature from the maximum inside temperature to get the temperature differential (T).
Locate the conductive heat loss by multiplying the exposed area of each material times that the U value times the temperature gap (A x U x T). Add the heat loss for each material to find out the whole conductive heat loss.
Calculate the volume (V) of the greenhouse. Multiply the length (L) times the width (W) times the height (H) of vertical walls (L x W x H). If the greenhouse has a gabled roof, then multiply twice the height of the gable (h) times the width (W) of the greenhouse times the length, or 1/2 h x W x L. Add these numbers together. When the greenhouse has a curved roof, then multiply twice the height of the curved parts squared, times 3.14, times the length of the greenhouse and add it to the volume of the portion with vertical walls (1/2 r2 π) x L.
Determine the number of air exchanges per hour (E) dependent on the construction of your greenhouse. New ceramic or glass greenhouses have .75 to 1.5, new double-layer flexible plastic film greenhouses have .5 to 1.0. Old construction can be as large as 4, if it is poorly maintained or as low as 1 if it is well preserved. If the wind speed is low or your own greenhouse is protected from the wind, use the low number. When the average wind speed is high or the greenhouse is exposed, use the higher one.
Calculate the air infiltration heat loss by multiplying .22 times that the temperature gap, times the volume, times the air ducts. (.22 x T x V x E)
Calculate the total heat loss by inserting the conductive heat loss as well as the infiltration heat loss. This number is in BTUs and can be employed to determine the magnitude of heater required.