BetterGrow Hydro Blog

A BGH Guide to Grow Room AC Units

The BGH Guide to Selecting A/C Units for Indoor Gardens and Grow Rooms
Don’t let your plants get beat up by the summer heat-up

After a cold winter and a cool spring, most people are looking forward to the summer heat, when outdoor temperatures can rapidly shoot past 85 degrees F.  But in indoor gardens, where heat from lights and ballasts is already a potential problem, plants won’t be so happy about the heat -- and they’ll make it known, with symptoms ranging from wilted, curled leaves, stunted growth and even death if left unchecked.  High temperatures outside can easily push the temperature of indoor gardens past 85 degrees if you are drawing air from outdoors into your growing area. At that temperature, many plants halt life-sustaining photosynthesis and switch to survival mode, using more water for transpiration -- that’s right, plant sweat -- to keep cool, and that stunts their growth, reduces yields and can make them more vulnerable to insects and disease.  Luckily, growers have an array of options to protect their plants and yields from the unrelenting summer sun. From air conditioners to improved ventilation to boosting heat resistance with CO2 or silica supplements, there are a variety of ways for smart growers to beat the heat. And with the first day of summer falling on June 21 this year, there’s ample time to get your grow room ready. But where to start?


Size matters
One of the best ways to control heat in the grow room is to add air conditioning. And, like almost all things horticultural, that requires some basic calculations. Air conditioners are rated based on BTUs, or British Thermal Units, which is a measure of how much heat energy it takes to raise the temperature of one pound of water by one degree Fahrenheit. For reference, a burning match emits about one BTU, while one gallon of gasoline contains 124,238 BTUs.

Most air conditioners designed for a conventional 115-volt circuit are generally rated to handle between 5,000 and 12,000 BTUs per hour, and a number of websites let you calculate what size air conditioner you will need based on the dimensions and location of the room you want to cool. But nearly all of them fail to account for the intense heat grow lights and ballasts give off.

HID grow lights add 3.41 BTUs per watt, per hour (1,000 watts = 3,400 BTUs). In a sealed, windowless 4-by-8-foot grow room in a sunny apartment, about 3,000 BTUs worth of heat energy will build up over the course of an hour. But with two 1,000-watt lights and two digital ballasts* running in that room, that figure jumps to about 16,000 BTUs per hour, or 14,000 BTUs (20 percent less) if those lights are air cooled.

If the ballasts are less-efficient magnetic models, the total BTUs per hour increase by 2,000. A CO2 generator sized for that room will add 600 BTUs, and if the room is being dehumidified by a 40-pint dehumidifier, that will add up to 1,200 BTUs.

With all that equipment running, the room will be generating a total of 19,800 BTUs in one hour (15,800 BTUs for air-cooled lights).

One ton of air conditioning is equal to 12,000 BTUs, so to determine what size air conditioner you will need to fully counter that heat, divide the total number of BTUs your equipment is generating by 12,000 (19,800/12,000 = 1.65 tons or 15,800/12,000 = 1.31 tons for air-cooled lights).

To conquer the BTUs generated in the non-air-cooled scenario, you’ll need the power of the Klimaire DC Inverter Mini Split - 2 ton air conditioning unit, which is rated to handle 24,000 BTUs, giving the grower full control over grow room heat.

Klimaire DC Inverter Air Conditioner

But if the lights are air-cooled, the Klimaire DC Inverter Mini Split - 1.5 ton will tamp down 18,000 BTUs, far beyond the cooling power of most standard air-conditioning units on the market.

In fact, DC inverter technology allows Klimaire units to cool more efficiently by running the compressor at varying speeds to better match the temperature needs of your grow room, unlike standard units in which the compressor runs at the same speed all the time and draws the same amount of power regardless of the amount of cooling needed.

Another measure of an air conditioner’s quality is its Seasonal Energy Efficiency Rating, or SEER, which is based on how much cooling a unit provides and the power it consumes. The lowest rating on the scale is 8 SEER and the highest, or most efficient, is 23 SEER.

Weighing in at 20 SEER, the Klimaire DC Inverter Mini Split - 1 ton will have a big impact on your grow room temperatures, but its operating cost will have much less of an effect on your budget.

Klimaire units also keep maintenance to a minimum with an Auto Clean Function that purges the unit’s high-density air filter of 93 percent of dust and washes dust and grease from the gold-plated hydrophilic evaporator, keeping it clean and dry when the unit is not in use.

The three-speed, remote-controlled Klimaire units also have unique Follow Me technology that will match the unit’s cooling intensity to the temperature recorded by a sensor in the remote control, helping to minimize hot spots in larger grow rooms and letting the grower better dial-in temperatures.


Excel Air Compact Split A/C -- 3000 watt will cool the heat radiating from as much as 3,000 watts worth of lighting. These split-system designs feature an indoor air handler with a squirrel-cage blower that cools the growing area while the outdoor condensing unit removes heat from your garden to the fresh air outside.

If you have a much larger growing space with six to 15 lights and you’re looking for a way to discretely bring cool air to your grow room, you will need a unit capable of quietly combating much more heat, like the Excel Air Stealth Cooling System, which offers six models rated from 24,000 BTUs to a whopping 60,000 BTUs. These plug-and-play units come with a 40-foot pre-charged refrigeration line, so all the user needs to do is thread the fittings together to get started. And since the fittings are re-sealable, the unit can be dismantled, relocated and re-installed without losing refrigerant and without an expensive certified installer, saving you lots of money.

Excel Air Stealth Cooling System

The Stealth series’ triple-split design also features a sound-deadened box for its outdoor condensing unit so you can keep your grow room temperatures in check without waking up the neighbors.

Of all the variables a grower has to consider, underestimating the need or under-budgeting for air conditioning to combat the summer heat can be the most detrimental. While outdoor temperatures may be favorable most the year, many inexperienced growers overlook the six or so weeks the weather easily reaches 100 degrees -- and that’s when plants are subject to life-or-death decisions the grower makes about cooling.

In high temperatures, plants will die or struggle to produce even the most meager of yields. But with an inadequately sized air conditioner, the electricity bill may grow but the plants won’t, as the air conditioner constantly grapples with heat but does not have the power to properly contain it. That’s why price can be a major driver for a grower considering air conditioning, as well as a major pitfall if it forces a decision that only appears to be a cost-effective solution.

On paper, portable, window- and wall-mounted air conditioners might be rated to handle 12,000 BTUs. But, in practice, the small compressors and low-force blowers that come with most models make it so they can only successfully handle roughly half that amount of heat or less, so these units really need to be over-sized for the job they are intended to handle.

Many growers are sealing their grow rooms to gain complete control of environmental conditions. However, most portable air conditioners make it impossible to keep the grow room completely sealed since they require an exhaust duct to remove hot air, which will also transmit unwanted odors from the grow room to the outdoors. The blowers in most portable units are not powerful enough to move exhaust through more than a few feet of ducting. While larger commercial portable units perform closer to their specifications, and often rely on intake and exhaust ducts that will keep the room sealed, they cost far more than mini-split units.

If cost truly is a consideration, a grower operating a less efficient, undersized window-mounted or portable unit could easily end up spending more money on power in a year than they saved by buying a lower-cost air conditioner.

Swamp coolers, also known as evaporative coolers, are probably the least-desired solution for grown room heat, as they evaporate water to cool a room, increasing the humidity and multiplying the chances that mold will take control of your crop. A swamp cooler’s cooling function works best in a dry environment, so as humidity levels increase, its cooling ability is greatly impacted.

Other options include operating lights after dark when ambient temperatures fall. For the 4-by-8-foot grow room listed above, running the lights at night could shave up to 1,000 BTUs off the total amount of heat energy generated each hour. Removing the two ballasts from the growing space can cut heat buildup by another 5,000 BTUs per hour*.


In hot water
Air temperature is far from the grower’s only concern. During summer, high outdoor temps and hot grow lights can easily conspire to push hydroponics reservoir temperatures past the 80-degree mark, increasing the likelihood of disease and root rot, which will be evident when plants begin to wilt in even the slightest heat. This is especially important when using systems such as Deep Water Culture (DWC), as higher temps will have a direct and dramatic impact on oxygen levels in the nutrient solution as well as the growth of pathogens.

EcoPlus 1/4 HP Water Chiller

To keep your reservoir within the ideal temperature range of 65 to 75 degrees, try the EcoPlus 1/10 horsepower Water Chiller. This energy-efficient, easy-to-use chiller features a micro-computer with a digital display and is capable of lowering the temperature of 93 gallons of water by 10 degrees or 35 gallons by 30 degrees. Pair the 1/10 HP chiller with an EcoPlus Submersible Pump -- 264 gph for best results.

For larger applications, the EcoPlus ¼ horsepower Water Chiller will cool 170 gallons of water by 10 degrees or 100 gallons by 30 degrees. The 633 gph EcoPlus Submersible Water Pump or the 1056 gph EcoPlus Submersible Water Pump are excellent matches for this chiller.


Gassed up

Photosynthesis slows to a halt when grow room temperatures reach 85 degrees, but not in a CO2-enriched grow room. With a CO2 tank, a REG-1 CO2 Regulator / Valve and a PPM4 - CO2 PPM Monitor/Controller, growers can attain a concentration of CO2 that may allow plants to tolerate temperatures as high as 90 degrees before experiencing the adverse effects of heat stress. That requires CO2  levels in the growing area to reach 1,500 PPM, about three times what’s found in typical urban air.

One quick and easy way to boost the CO2 level is the Excellofizz CO2 Kit, which can raise the amount of CO2 in a 10-by-10 room by about 1,600 PPM. After purchasing one or two Excellofizz kits to test the impact that CO2 will have on their yield, many customers learn that, in the long run, it’s much less expensive to purchase a CO2 tank or CO2 generator and a controller rather than purchasing Excellofizz every month. Once your garden responds to CO2 and you are convinced of its impact, we strongly recommend making the investment on a more permanent and controllable solution that CO2 tanks, regulators and generators offer.

But heat can be an issue even when CO2 is being used. Remember that the optimal temperature for harnessing explosive growth via CO2 is below 85 degrees, and even lower during the last two weeks of flowering. To determine the proper CO2 level and flow rate for your grow room, use the BGH CO2 Calculator.


Inside job

High temperatures in the garden are made even worse considering that the temperature inside a plant’s leaves can be 5 degrees higher than the temperature of the surrounding air. That’s not to say you can’t help your plants fight the heat from the inside out. Silicon, one of the most commonly found elements in natural soil, can be a key element in the fight against heat stress and a powerful weapon in the quest for bigger yields.

Silica Blast strengthens your plant’s stems and stabilizes the pH in recirculating hydrogardens.

Botanicare’s Silica Blast strengthens your plant’s stems and stabilizes the pH in recirculating hydrogardens. It’s ideal for use when growing indoors in containers or in soil-less hydrogardens, since indoor plants don’t receive the benefit that natural breeze and wind provide. Outdoors, wind causes plants to move, creating tiny tears in stem tissue. Much like in body building, when these micro-tears are repaired the plant tissue becomes stronger and the stems and branches are able to handle a heavier fruit or flower load later on. The silicate in Silica Blast also encourages the formation of a silicate matrix that reduces transpiration and may allow plants to continue photosynthesis when temperatures surge. Since silica is found in the walls of every plant cell, Silica Blast -- now available in a 2.5-gallon size -- will also increase the weight of the finished product, an added benefit that most growers don’t realize. Even if heat isn’t an issue for your garden, the increased weight Silica Blast delivers is reason enough to add it to your normal feeding schedule.


Under cover
Not all gardens will face the same heat challenges this summer. For growers looking to reduce grow room temperatures by only a few degrees, Silver Jacket Insulated Ducting and Sun Shield Reflector Covers can cut down on the amount of heat escaping back into gardens through ducting and the tops of reflectors. Don’t have a tempered glass lens for your reflector? Add one to control the heat beaming down from above. It will also let you position your lights closer to the tops of your plants.


How hot is it?

EcoPlus Temp/Humidity Gauge

Given work and other duties outside the garden, it may be difficult to keep tabs on exactly how hot your growing space is getting if you are not there to check the thermometer during the peak of the day’s heat. Position the EcoPlus Large Display Temp/Humidity gauge near the tops of your plants and they will store the maximum and minimum temperature and humidity recorded during the day.

Indoor growers may have their hands full in combating the summer heat, but with the right combination of cooling technology and know-how, they can ensure their plants -- and their harvests -- aren’t feeling the heat. For more information on the array of cooling technology and products available to the indoor gardener, visit BetterGrow Hydro.

* BTU ratings for lighting and ballasts are based on test data compiled by Hydro Innovations.

  • Rich

    “1,000 watts = 3,400″

    “two 1,000-watt lights and two digital ballasts* running in that room, that figure jumps to about 16,000 BTUs per hour”

    how do those numbers add up? 3400×2 + 3000 for the room itself = 9800. how many btu’s do the ballasts produce?

  • Jake A

    Hey Rich,
    According to Hydro Innovations heat tests using a laser
    thermometer, a 1,000-watt magnetic ballast generates 3,500 BTUs per hour and a
    1,000-watt Lumatek Digital Ballast  generates 2,500 BTUs per hour. The two
    electronic ballasts used in the grow room example above would generate a total
    of 5,000 BTUs, bringing the total for the room to 14,800 BTUs. The remaining 1,200
    BTUs are to compensate for higher amounts of heat other brands of digital
    ballasts produce, since it is better  to be on the safe side when it comes
    to grow room heat.

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  • Willy

    I see all this associated equipment to manage heat and the lights and fans, and more….

    Makes me glad that new technology in lights is still happening.

    New lights called AC Plant Phasers are solid state with no fans, power supplies or ANY electronic components. NO PCB. 22 Watts. Same effective frow power as 400 W HPS. Saw them on a craigs list ad.

    They have 96 light diodes in a 1 inch diameter. Now THAT is a breathrough. At $300 bucks they seem to be a good buy. Save money on AC and HEAT issues and replacing bulbs.