So you’ve decided to build a new garden—oh, the choices you have to make!

  • What kind of reflectors, bulbs and ballasts?
  • How many plants will fit in the space?
  • Do you want to use a hydroponic system or traditional soil methods?
  • And what kind of cooling system will work best for you?

Of course the size of your garden will have a huge effect on the system you choose, but there are a lot of other variables, too—energy efficiency, geographic location and personal preferences will all be part of the decision.

The first thing that must be considered is the size of the system that you need. Sizing a cooling system for a garden is quite different from sizing an air conditioner for a home or commercial building. A garden has many more sources of heat than your home—all in use for long periods of time—and all of those heat sources must be considered in order to accurately size the cooling system.

Heat is measured in BTUs per hour and cooling systems are sized based on the BTUs per hour they’re capable of removing. Generally speaking, one ton of cooling capacity is equal to 12,000 BTUs.

We touched on this in great detail in a previous issue, but to briefly recap—the sources of heat in your garden are things like bulbs, ballasts, pumps, dehumidifiers, CO2 generators and of course the ambient outside temperature.

There is an excellent chart included in this article that can assist you with determining the specific heat load in your garden—keep in mind that the chart assumes that your garden is completely sealed with no air cooling of the lights and that insulation is moderate. Once your heat load is determined, the fun begins. What kind of cooling system should you choose? There are so many options available and they all have their pros and cons.

Evaporative coolers

A low-cost, energy-efficient method—particularly in greenhouse cooling—is the evaporative cooling method. When water evaporates it absorbs BTUs, resulting in general cooling. Evaporative cooling methods take advantage of this process by spraying a fine mist of water into the air—as the water evaporates, the temperature is reduced.

However, this method is far more effective in greenhouse applications than in a typical indoor garden, because the evaporation of water into the air results in a drastic humidity increase—most of us are battling humidity already and would prefer not to add to the problem by creating even more.

Also, as the humidity in the garden increases, the less likely the water from the evaporative cooler is to evaporate—so eventually you add so much humidity to the environment that you don’t get any cooling at all and you run the risk of your garden ending up hot and humid. Evaporative coolers are a great solution if you can make them work consistently in your application, but in the indoor gardening world their use is extremely limited.

Traditional air conditioning units in a sealed growroom

Traditional air conditioning systems are available in every imaginable size and application, ranging from the 5,000 BTU window system to the 240,000 BTU 20 ton commercial rooftop monster. Within this range of sizes are multiple options, upgrades, configurations and energy ratings to choose from.

When you choose an air conditioner for your cooling needs, keep in mind that you’ll likely be using it all year-round. Most a/c systems are intended to run only in summertime when the weather is warm, so you’ll need to verify that your choice can be used without damage to its components when outdoor temperatures are below freezing.

Modifications like compressor heaters, low-ambient start kits and other minor fixes might need to be made. You should also understand that most air conditioners will begin to lose efficiency when the outdoor temperature gets above 80°F.

If you purchase a five ton system, it’s likely only outputting that five tons of actual cooling when the temperature is no warmer than 80°F outside—if it’s any warmer, you might only get 4.5 tons and when it’s outrageously hot, you might only get 3.5 or four tons of cooling from it.

The amount of efficiency loss varies by brand and model, but when you live in a very hot climate you should always choose an air conditioner that’s at least 10 per cent larger than you really need to compensate for the efficiency loss in the very hot months—or you should plan on a backup method for those extremely hot days.

Window-mount air conditioners are a common choice for the small garden. They are convenient, lightweight, inexpensive and easy to install. The largest window-mount system commonly available is about 24,000 BTU (two tons), so unless you have multiple units, the most you can expect to cool with this size a/c is a 4,000 to 8,000 watt garden.

Obviously this figure covers a pretty wide range—the wattage that can be cooled with a two ton a/c varies wildly, based on whether you’re air cooling your lights, if your garden is sealed, what the outdoor ambient temperature is and what other equipment you might have running besides the lighting.

Window mount air conditioners don’t provide quite as many options with regard to energy efficiency as the larger units typically do, but their convenience and low cost will often outweigh this, especially in a very small garden.

In larger gardens or when energy efficiency is more important, a split or self-contained whole-house a/c is often a better choice, as there are a multitude of options available with this type of system. The higher the SEER (efficiency) rating the more energy efficient the unit will be. You can choose to go with a traditional home a/c, but these units will always require that an HVAC technician installs them.

Within our industry there are several manufacturers of DIY a/c systems and most gardeners choose to go this route. These units can be purchased (usually by special order) at any indoor gardening retail store. The split units with pre-charged refrigerant lines are typically available in sizes ranging from two to five tons.

They consist of an indoor air handler and an outdoor condenser/compressor; the cold refrigerant circulates through the air handler and the air from the garden circulates over the heat exchanger in the air handler—just like the central a/c system in most homes—except that end-users can connect the refrigerant lines themselves. In larger configurations there are also self-contained models available, typically from five to 20 tons.

These units have the entire refrigeration system located in one box and all you have to do is connect ducting to install the system—with no refrigeration connections needed, installation is made very simple.

Most very large commercial buildings use this type of set-up, except in cases where a chiller system is employed (more on that later). Consider that once you get over five tons you’ll be looking at a fairly sizable unit, so you’ll need to organize the equipment and manpower to get it properly placed in your facility.

Chiller systems

Chiller systems are the Cadillacs of cooling. They are generally more expensive than a/c systems, but they can be as much as 30 per cent more energy efficient than a high SEER a/c system. There are a few reasons for this—one is that the heat capacity of water is quadruple that of air, so it takes four times as much heat to increase the temperature of water than it does to increase the temperature of air.

A traditional air conditioner comes on when the air in the room increases in temperature, but the compressor on a chiller won’t come on until the water increases in temperature. Because it takes so much longer for water to heat up, the chiller will run less often to achieve the same cooling results—as long as there is cold water in the system, you get air conditioning even when no energy is being consumed by the chiller.

Because there is so much more heat contained in the water than would be contained in air, when the compressor of the chiller does come on, the capacity of the refrigerant to hold heat is maximized.

This means that chillers don’t typically start to lose efficiency until the outdoor temperatures are closer to 100°F, because the differential between the outdoor temperature and the temperature of the refrigerant is much higher than it is in a traditional air conditioner. Most large commercial buildings such as hospitals, hotels and major manufacturing facilities employ chiller systems for their cooling needs because of the extreme savings in energy consumption.

Chiller systems generally consist of an outdoor water chiller and indoor air-to-water heat exchangers with fans—these can be small heat exchangers with external fans for the small garden, or large air handler systems for larger gardens. They are simple to install, because no refrigeration work is needed.

The only thing traveling between the outdoor chiller and the indoor heat exchanger is a water line—this allows you to seal the garden completely and makes the system very simple to install. The downside is a slightly higher upfront cost, but when energy efficiency is at the top of your priority list, chiller systems are the most efficient solution.

When you consider the savings in electricity the operating cost of a chiller is far lower than that of an air conditioner, so you’ll nearly always make up the additional expense within a few months in the form of a lower electric bill.