You’ll have to heat your greenhouse if you intend to keep the structure running all winter long—in most areas at least. There are several kinds of heaters to choose between, some are better than others for small hobby greenhouses. Since heating can make up as much as 80 percent of your greenhouse expenses, you want to balance your need to heat your plants with your budget and ability to maintain and upkeep your equipment. After all, you can install the most efficient unit available, but if it doesn’t get regular care, you’ll be replacing it in short order.
Types of greenhouse heaters
Before you spend your hard-earned money on the newest greenhouse heater on the market, it’s a good idea to understand what you’re buying. Greenhouse heaters come in four common designs, many of which can be powered by either electricity or a combustible fuel like gas or wood. You’re likely to encounter the following types of heaters while shopping:
Space heaters – Available at hardware stores everywhere, space heaters are great choices for small hobby greenhouses. Unlike the bigger systems, space heaters are inexpensive and can be set up temporarily in the greenhouse during the winter and moved out as temperatures warm to make extra space. These units come in a variety of configurations, with or without fans to increase circulation. Space heaters can get you a lot of bang for your buck, especially if you choose one that has a built-in thermostat and safety features like automatic shut-offs.
Forced-air heaters – Forced-air heaters are another good option for hobby greenhouses. They incorporate a duct that runs the length of the greenhouse and a blower to evenly distribute heat across the structure. These units are permanently installed and fitted with thermostats to better control the heat in the greenhouse. If you have a large hobby greenhouse with an eight foot or taller ceiling, a forced-air heater may be perfect for you.
Hot water/steam heaters – You don’t usually find hot water or steam heaters in a hobby greenhouse, but if you’re big on sterilization, you should give them a look. They can be configured in different ways, from simply functioning as bench warmers to being set up to heat the entire greenhouse. The big advantage here is that you also have an on-demand steam sterilizer in your greenhouse at no additional cost. Hot water and steam heaters are probably overkill for a small hobby greenhouse, but if you have a very large structure or several smaller buildings tied together, it may be worth the expense.
Infrared heaters – If you only plan to keep a few plants in the greenhouse over the coldest parts of the winter, an infrared heater could solve your heating problems. These units are powered by electricity and are used to create a small area of warmth in a zone directly beneath them. When hung properly, infrared heaters can provide an excellent growing environment for plants and when coupled with heating mats, they’re helpful for starting seeds in an otherwise cold greenhouse. These heaters are relatively inexpensive and come in a variety of sizes, perfect for small to medium buildings.
To vent or not to vent
Another important consideration when choosing a greenhouse heater is whether or not you’re willing or able to vent away the by-products of your heating efforts. Gases like ethylene can build up when natural gas or liquefied petroleum-burning heaters aren’t properly vented, leading to problems with plants in the greenhouse. You can’t smell ethylene, so it’s hard to know if you have a problem until your plants begin to suffer from unusual symptoms like wilting despite being turgid, dropping flowers and twisted and deformation of upper leaves. Other by-products like sulfur can cause mysterious and sudden leaf burns and widespread chlorosis.
If have a small greenhouse, it might not make sense to vent, or there may be no good location for a properly sized vent for a gas heater. In these cases, look to electric heating, or select a heater that allows you to place the combustion unit outside the greenhouse. Only install gas or wood heaters inside a well-ventilated area to prevent the build-up of toxic fumes and damage to the plants in your greenhouse. They can begin to show signs of poisoning via air pollution in as little as an hour, even a little exposure to some combustion by-products is too much.
Greenhouse heater sizing
Once you’ve decided what kind of greenhouse heater you’re looking for, you need to know what size unit is right for your building. This is trickier than it sounds. A lot of people try to guess, and sometimes they get it right, but it’s better to be sure when it comes to such an important investment. So, get out your calculator, because we’re going to do the math.
You’ll need to know what the surface area is of your greenhouse floor if you’re going to figure out how quickly it loses heat—to figure that multiply the length of the building by its width. This is a very simplistic way to calculate heat loss, but is usually close enough. The table below can be used as a guide to sizing your new heater once you have your surface area figured in square feet. Just look across the table until you find a temperature that corresponds to your area’s coldest nights and multiply the number in the row that best describes your greenhouse by the surface area to get your estimated maximum BTU usage. This table assumes you only want to heat your greenhouse to 60 degrees Fahrenheit on those coldest nights.
So, let’s say that my freestanding single-glazed greenhouse is six feet wide and eight feet long and my coldest nights are around -10 degrees Fahrenheit. If I multiply 48 by 280, I find that I need a heater capable of outputting at least 13,440 BTUs. I don’t need much to keep that little building warm. However, if I’m also going to heat my 10-foot by 15-foot greenhouse, I’ll need an additional 42,000 BTU heating system for that building, which means a bigger investment.
Freestanding, Single Glazed
Freestanding, Double Glazed
Lean-To, Single Glazed
Lean-To, Double Glazed