Greenhouse Heater Size Calculator

When the weather gets realy cold, it sneaks up on us sometimes, doesn’t it? The temperature drop fast, and then there we are in our greenhouse with a thermometer wondering how to make sure our seedlings don’t freeze. Heat escapes incrementally, gradually building up toward disaster, and intuition’s no help; it builds slowly, silently. Think of the greenhouse more like a thermal system different than merely a shelter, and think of getting the heating correctly sized as a matter of life and death for plants inside, not you.

Once you input the size of your roof and what kind of glazing it has, the calculator do the math. First it considers the envelope, total surface area exposed to the cold. That’s the roof, plus walls beneath the roof (and panels over those walls). The bigger the hoop house, the more energy it require compared to a little lean-to. Here, the shape of the roof also count: A steep gable will add some vertical wall area, so it increases total square footage to be heated.

How to Size Your Greenhouse Heater

The second big variable is type of glazing. This refers to the material itself and determines how quickly heat escape through it. Inexpensive single polyethylene film has a high U-factor (i.e. Low resistance to heat flow). Double-inflated poly and twinwall polycarbonate does better at trapping air layers that reduce rate of loss. Single cover may be enough if your climate is relatively mild; otherwise upgrading the glazing tends to cost less then stepping up to a bigger heater during tough winters. The tool includes those differences, and you don’t need to remember thermal conductivity numbers yourself.

Many growers forget about this invisible cost: Air infiltration. Unless it’s a very tightly sealed greenhouse, some amount of cold air leak in and forces out warm air continually. One figure requested by the calculator is the “air change” rate, meaning number of times the interior air exchange with outdoor air during each hour. A drafty tunnel might have three or four changes. A super-tight hobby house may have fewer than one. This can multiply heat load considerably, making air sealing even more important then turning up the thermostat.

Secondly, what’s the temperature increase that you’re trying to achieve? This means finding difference between coldest night you expect and the lowest temperature your plants will survive. Maintaining a 35-degree greenhouse in 10-degree weather is doable; maintaining a 50-degree one isn’t so easy. People typically miscalculate for their anticipated “average” winter night rather than the worst-case minimum for which they are designing, leaving themselves insufficient power for super-cold nights. It’s smart to factor in some safety buffer since heaters turns on and off, and no real-life system is ever perfectly efficient because of duct losses.

What does that translate into? The answer is in final output: the wattage and BTUs your heat needs. That is just a straight-up number. It reflects the combination of your own goals, your climate, and particular structure where you are growing. And once you have that number, you know what size heat to get, whether it be a propane unit, an electric element, or a water-based system that meets the load.

When you’ve sized right, you’ll worry less about frost, your plants will thrive and your energy bill stays predictable. Winter gardening becomes ordered instead of challenging. You should of used this tool sooner.