Friday, December 13, 2013

Best Techniques and Equations for Calculating Greenhouse Heat Requirements

Calculating Your Greenhouse’s Heating Requirements

Both for novices and experienced greenhouse gardeners, there is one thing which is a perennial challenge – trying to determine exactly what size of heater is needed to keep a greenhouse at a desired temperature.

Obviously, there are a number of factors involved; the size of your greenhouse will make a difference. The kind of plants you’re growing is also an important consideration. Tomatoes, for instance, will thrive in warmer temperatures than say, lettuce.

One word of caution is in order before we go any further: it can be a bit on the expensive side to heat a greenhouse in the middle of winter, but it’s up to every gardener to decide for themselves what price they’re willing to pay for the pleasure of being able to pick tomatoes or watch tropical flowers bloom in January.

Before you begin to try to calculate how much heat you’ll need to generate, you should also go through your greenhouse carefully to seal any cracks which can allow heat to escape.

You may also want to use a few tricks to help retain heat in your greenhouse, such as large containers of rocks or water; these materials will absorb heat and then radiate it back into the environment, helping to keep your greenhouse a little warmer during the cold months.

The type of heater you use is up to you, but the following should help you begin figuring out your greenhouse’s heating requirements.

First, you’re going to need to take a few measurements. You may already have these figures handy, but if not, you’re going to need to head out to your greenhouse components, with a measuring tape and a stepladder.

You’ll need the following measurements:
  1. The length of your greenhouse (L).
  2. The width of your greenhouse (W).
  3. The height of your greenhouse, measured from the base of the greenhouse to the eaves (H).
  4. The height from the floor of your greenhouse to the highest point of your greenhouse’s roof (R).
  5. The length of the slope of your greenhouse’s roof (S).

Just as a note, these figures assume that your greenhouse is a single layer glass or plastic greenhouse whose glazing (or plastic sheeting) runs all the way down to the ground.

  • If the glass or plastic doesn't run all the way to ground level and you have brick or other solid materials covering the lower part of the structure, then take measurements of these parts of the greenhouse separately.
  • For further references, visit this plastic greenhouse manual by Cornell University Extension.
  • You’ll want to halve these measurements when you make your calculations to account for the lower heat loss of these materials.
  • If you have a double-layer greenhouse, whether the material is glass or plastic, then reduce the amount of heat you come up with by roughly 30%.

The figure you’ll arrive at aren't going to be accurate down to the millimeter, but it should be more than accurate enough to assess your greenhouse’s heating needs. This is the basic formula:

First, you’ll need to calculate the total surface area inside of your greenhouse. You can determine this by adding H and S, then multiplying by 2. Now, multiply this figure by L.

Next, add R and H, then multiply by W.

  • Add these two figures together to get the total surface area inside of your greenhouse.

If you’re more mathematically inclined, you can make this calculator slightly quicker using this formula: 

[2 x (H + S) x L] +

[(R+H) x W] = Inside Surface Area.

  1. The next step is to figure out the difference in temperature between the average winter low temperatures where you live and the minimum temperature you’d like to maintain inside your greenhouse.
  2. For instance, let’s say that you’re trying to grow tomatoes in your greenhouse. This means that you’ll want to maintain a temperature of at least 60 degrees.
  3. Now, let’s say that your average low temperature in winter is 32 degrees. This is a temperature differential of 28 degrees.

Now, multiply the temperature differential by the total inside surface area of your greenhouse and you’ll come up with a rough approximate of the number of BTUs you’ll need to keep your greenhouse at your desired minimum temperature.

If you’d rather have this figure in kilowatt hours (what your electric company uses to determine billing), you can divide the BTU figure by 3.413 to arrive at this number.
It’s as simple as that – which is to say, not exactly simple, but if you follow the directions here, you can find out how much heat you’ll need to provide for your greenhouse and a ballpark figure of what it will end up costing to keep your plants thriving in your greenhouse even in the middle of winter.

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