Getting Hot & Heavy: Dealing with Heat in Growrooms Part 2

By David Kessler
Published: July 1, 2016 | Last updated: April 21, 2021 10:15:14
Key Takeaways

Knowing how to deal with heat effectively can mean the difference between success and failure when growing indoors. In part one of this two-part series, David Kessler examined what equipment makes heat in a growroom, and how to design a space capable of handling high temperatures. In part two, he investigates growroom accessories that effectively reduce heat.

When building a growroom, functionality needs to be your primary concern. That means choosing the right equipment for the job. Here are some items to help you deal with the heat in your growroom.


Air-cooled Reflectors

One of the most common pieces of equipment used to help reduce heat in a growroom is the air-cooled reflector. The job of a reflector is to focus the light emitted from the lamp, maximizing the light available to drive photosynthesis. Air-cooled reflectors reduce heat in a growroom by running a stream of air, usually from a centrifugal fan, through the reflector via an air intake and exhaust vent.

When the air stream moves over the lamp, much of the heat created from the bulb can be exhausted before it can radiate into the growroom. An additional benefit is that if air is pulled from an adjacent room, passed through the sealed light and then exhausted out, heat is removed but CO2 is not.


There are three types of air-cooled reflectors: air-cooled cylinders; truncated prism with horizontally mounted lamps; and truncated prism with dual internal chambers. The air-cooled cylinder is the simplest design. Air is passed through a 6- or 8-in. glass cylinder in one end and exhausts out the other. Inside the cylinder is often a small reflector to focus the light towards the plants.

This design allows for efficient air cooling, but the tiny reflector does a poor job of maximizing light intensity and coverage area. If the reflector is removed and the air-cooled cylinder is mounted vertically instead of horizontally, it can be an ideal choice for a vertical garden. Air-cooled cylinders are also excellent choices for small, confined garden spaces and lower-wattage bulbs.

The truncated prism with a horizontally mounted lamp first appeared in the early ’90s with 4-in. vents, but now this style is more commonly seen with 6-, 8- or 10-in. vents. The larger the vent size, the greater the potential airflow.


The last style, the truncated prism with dual internal chambers, has a bottom chamber sealed in reflective aluminum and houses either a vertically or horizontally mounted lamp. The top chamber is between the steel housing and the aluminum insert.

This air gap allows the heat from the lamp to radiate through the aluminum and be carried away with the air stream. This type of reflector overcomes a design flaw in the other types, as it allows the lamp to operate at an ideal temperature, producing its optimal spectral output. HID bulbs are designed to run hot (as hot as 750°F), but when a bulb is exposed to a brisk, moving, cool air stream, it never reaches its ideal operating temperature. Operating a lamp below its ideal temperature can reduce PAR output by 7-10%.


Some people argue against the use of air-cooled reflectors due to the issues with the lamp reaching its ideal temperature, and because the tempered glass used inside these reflectors diffuses the light as it passes through it, which reduces light transmission by 6-8%. If we accept both of the above arguments, we are talking about losing a maximum of 18% of the light created.

But when you hang a reflector over the plant canopy, a non-air-cooled reflector has to be hung higher than an air-cooled reflector to prevent the heat created by the lamp from burning the plants.

The inverse square law states that light intensity is inversely proportional to the square of the distance from the source of that light, meaning if we compare an air-cooled reflector hung at 12-in. above the canopy and a non-air-cooled reflector hung at 24-in. above the canopy, the air-cooled reflector will provide significantly more usable light to the plants, even with the 18% reduction due to light diffusion and a lower lamp operating temperature.

Reflector Covers and Ducting

Including air-cooled reflectors in your growroom design will allow for better heat control, but you can take air cooling a step further, preventing even more heat from entering your growroom with insulated ducting and reflector covers. After heat from a lamp is picked up by the air stream, it is normally moved out of the room via thin aluminum ducting. If you want the coolest growroom possible, then spring for higher-end ducting that has several layers of insulation. This insulation will prevent heat from radiating into your growroom as the hot air is evacuated.

A reflector cover is a flame-retardant, fabric insulation custom fit to enclose a reflector. Reflectors are made of metal and the longer a bulb burns inside of that reflector, the hotter the metal housing gets, which radiates into your growroom. An air-cooled reflector can still have a surface temperature of 102°F, but when you install a reflector cover, the surface temperature can drop to below 70°F. This prevents a lot of heat from ever entering your growroom.

Heat Exchangers

Even a perfect growroom design and the proper equipment can’t overcome the laws of physics. The laws of physics, as we understand them at this time, do not allow for negotiation or manipulation, which means heat cannot be destroyed. Our growroom equipment makes lots of heat and even if we use the best equipment and designs, we can still only remove a fraction of that heat. This is where a heat exchanger comes in. Heat exchangers remove heat from one location and transfer it somewhere else. Three options are air conditioners, water chillers and evaporative coolers.

Air Conditioners

As the most common heat exchangers used in growrooms, air conditioners have some basic components to manage refrigerant and move air indoors and outside. The cold side of an air conditioner contains the evaporator and a fan that blows air over the chilled coils and into the room.

The hot side of an air conditioner is made up of the compressor, condenser and another fan to vent hot air coming off the compressed refrigerant. In between the two sets of coils, there is an expansion valve that regulates the amount of compressed liquid refrigerant moving into the evaporator.

Once in the evaporator, the refrigerant experiences a pressure drop, expands and changes back into a gas. The compressor is actually a large electric pump that pressurizes the gaseous refrigerant as part of the process of turning it back into a liquid.

Most gardeners will use a window unit, split component unit or ductless air-conditioning unit. The biggest drawback of using a window air-conditioning unit is that it removes air from the growroom, making a sealed room or the use of CO2 nearly impossible. Larger air conditioners with split components can also make a sealed growroom or the use of CO2 problematic if the return air vent is located in the growroom.

The third option, ductless air conditioners, separate the components similarly to the larger split air conditioners, but as their name implies, they do not use any ducts. The internal unit is mounted to the wall with only a small hole needed to connect the refrigeration and low-level electrical lines to the outside portion of the unit, which can be located anywhere from 15-50-ft. away.

One of the most significant benefits of the ductless units is they do not transfer any air between the internal and external components, allowing a grower to construct a truly sealed room.

When choosing an air conditioner, its efficiency rating should be factored in. Air conditioners are rated in BTU or British thermal units; the higher the BTU, the more cooling capacity a unit will have. Efficiency, however, does not correlate to size; you have to look for its energy-efficiency rating.

The EER rating is calculated by dividing a unit’s BTU by its wattage. The higher the EER, the more efficient it will be. Another efficiency rating seen on air conditioners is SEER, or seasonal energy efficiency ratio. The SEER is similar to EER, but instead of being evaluated at a single operating condition, it represents the expected overall performance for a typical year’s weather in a given location, using a range of outside temperatures.

Water Chillers

Water chillers remove heat from water, not air. Many people use water chillers in growrooms to maintain hydroponic reservoirs at the ideal temperature of 68°F, but thanks to the ingenuity of some hydro companies, we can also use water chillers to keep rooms cool with water-cooled reflectors, water-cooled CO2 generators and radiator-style air handlers.

All of these pieces of equipment circulate cold water through a radiator made of thin, highly conductive metal with lots of surface area to absorb the heat. As heat passes over the radiator’s surface, the cold water absorbs the heat and becomes hot water.

The hot water is then sent back into the water chiller to be cooled and the process starts over.

Water cooling can be up to 50% more efficient than using air conditioners. A water-cooled CO2 generator is able to remove as much as 86% of the heat created by the combustion reaction (burning of natural gas or propane) that creates CO2. The heat removed from water by a water chiller cannot be destroyed, it is merely moved or transferred elsewhere. This means you cannot place a water chiller in your growroom or you will remove heat from the water only to heat up the air!

Evaporative Coolers

More commonly found in greenhouses, evaporative coolers provide a low-cost, energy-efficient method of cooling. When water evaporates, it absorbs heat, which results in a cooling effect in the surrounding vicinity. One type of evaporative cooling system uses misters to spray small water droplets into the air. These water droplets quickly vaporize, dropping the surrounding temperature.

The reason this type of cooling is not used for indoor growing is because along with the temperature drop comes a sharp rise in humidity. Even more challenging is the fact that the higher the humidity level, the less effective this type of cooling becomes.

The second type of evaporative cooler is known as an aspen cooler or swamp cooler. This style of cooler drips water across thick 4-6-in. treated cardboard pads that cover an air intake; large exhaust fans at the opposite end of the room or building pull large volumes of hot air from outside across the wet cardboard pads.

As the hot air moves across the wet cardboard, water evaporates and provides a localized cooling effect. This type of cooler also adds humidity to the air and is similarly not suited for indoor cultivation unless the growroom is in an arid region, and even then it would be unlikely to keep the room at ideal temperatures.


Remember that sometimes heat problems have simple solutions. Adding a silicon fertilizer can increase your plants’ heat tolerance. Silicon is actively transported into the plant, similar to macronutrients like potassium. From there it moves up the xylem and is distributed out to the growing shoots.

There, the silicon forms larger polymer chains, allowing plants to deposit silicon in the form of solid amorphous, hydrated silica, which is then incorporated into cell walls, armoring plant cells and allowing them better control over their rate of transpiration. This affords plants improved internal temperature regulation.

Dealing with growroom heat starts with design, and continues with getting the right equipment. Follow this guide to ensure your plants not only survive but thrive. If you already have 99 problems, make sure that heat ain’t one!

Don't Miss: Getting Hot & Heavy: Dealing With Heat in Growrooms Part 1


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Written by David Kessler

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David Kessler heads research and development at Atlantis Hydroponics and writes for their popular blog. David has more than two decades of experience and multiple degrees from the State University of New York. An accredited judge for the American Orchid Society, he travels the world judging events. Follow his blog at

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