21 Hot Grow Tips for Cool Growrooms

By Danny Danko

Photos by Nico Escondido

“Hot Fun in the Summertime” might work for Sly and the Family Stone, but if you’re growing pot, high temps are simply not an option. Here are 21 ways to control the heat, thus keeping the heat from controlling you.

Use the Right Fan

Fans are rated by CFM (cubic feet per minute), which refers to how much air a fan will remove in 60 seconds at full strength. Measure your grow space to determine its total volume in cubic feet (i.e., length x width x height), then get an outtake fan capable of moving that air out in three to five minutes or less. For example, a 2,000-cubic-foot room needs, at minimum, a 400-CFM fan to push out the hot, spent air.

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Thermometers and Hygrometers

These are crucial because you must keep track of temperature and humidity at all times and do your absolute best to keep the daytime temps around 75°F tops and the humidity as close to 50% as possible. The best digital versions of these tools keep track of highs and lows and can also alert you when the conditions are less than optimal so you can take the proper measures to correct them.

Running Lights at Night

Setting the timer so that your plants’ 12 hours of “daylight” in the flowering stage happen at night can keep your room cooler and, in some places, help you save on energy costs as well. Plus, running your lights from 6 p.m. to 6 a.m. (or 8 p.m. to 8 a.m., etc.) will still give you plenty of time to work in the growroom.

Remote Ballasts

Ballasts for high-intensity discharge (HID) lighting give off a bunch of heat as they convert high-voltage electricity to a lower level. Even digital versions can raise a growroom’s temps several degrees, so try to keep all of the ballasts in a separate room (thus “remote”). If the ballasts must be in your growing space, hang them up high, close to your outtake fan and carbon-filtration unit.

Air-Cooled Lighting

Using special hoods with vents for intake and exhaust can greatly reduce the amount of heat put out by your bulbs (especially if they’re right above sensitive plant tops). Several such lights can be strung inline at once for even better efficiency. Keep in mind, however, that the glass lenses reduce the amount of lumens reaching your plants by about 10 percent, so be sure to lower the lights accordingly.

Central Air

If you’re lucky enough (or smart enough) to have central air conditioning built into your house or apartment, take full advantage of it by simply setting the temperature in your grow area and then letting the central air unit maintain the proper levels at all times. However, larger grow facilities will require heavy-duty, industrial-sized units capable of providing over 100,000 BTUs (British thermal units) of cooling.


Heat builds up very quickly in dead spots that fresh air doesn’t reach. Several oscillating fans in your room will create enough of a breeze to spread the cooler air around and also move plant leaves that can use up CO2 at surface levels quickly. Not only is this kind of air agitation good for your plants’ growth, but it has the added benefit of making them stronger as well.

Air Conditioning

Whether you invest in a window or wall unit, a stand alone portable or the pricier split air conditioners, it’s very important to purchase the right one for your space. Remember that you’re not just striving to cool the air, but also to counter the heat buildup created by bulbs, ballasts, pumps, CO2 generators and other assorted electronics.



Water-Cooled Lighting

These days, some grow lights even use chilled water to stay cool. These lights require a separate reservoir and take advantage of the water’s ability to conduct cold temperatures quickly and efficiently. Some people might cringe at the thought of water being so close to their bulbs, but the units are actually quite safe when used properly.

Insulate Properly

A well-insulated room is much easier to keep cool than one without insulation of any kind. An attic space that’s exposed to the roof is much more vulnerable to heat and cold spikes due to its proximity to the outside world. By insulating your grow space properly, you’ll gain better environmental control.



Reservoir Chiller

Hydro growers learn quickly that warm temps (exceeding 80°F) exacerbated by heat from water pumps in the nutrient reservoir can lead to root rot and issues with oxygen levels that can harm or even kill your pot plants. Keeping the nutrient solution cool by means of a plug-in chiller solves this issue altogether.

Frozen Water Bottle

One easy, short-term way to cool a reservoir is to freeze a 2- or 3-liter plastic bottle of water and then drop it into your tank. The water temps will slowly drop to more manageable levels, but keep in mind that they’ll creep up again once the ice has melted.

CO2 Enrichment

While CO2 generators produce additional heat, the tanks filled with cold gas cool the room. Plants that receive CO2 enrichment (up to 1500 PPM, or parts per million) can live in hotter environments and thrive. Rooms with CO2 gas added regularly can produce great pot even at 85°F.

Portable Swamp Coolers

If you can connect to a water supply, then a portable swamp cooler may be the answer to your heat-reduction needs. These units use evaporation to your advantage by absorbing moisture and outputting cooler wet air without the use of chlorofluorocarbons like Freon.

Digital Cooling Humidifier

One inexpensive way to reduce heat and add moisture is a digital cooling humidifier. These units utilize tanks that you refill with water to put out a cooling mist that will help you reduce your energy costs. They’re available in many different sizes and applications for use in almost any growroom.

Less Hose, Less Resistance

Keep the ducting in your growroom straight and down to a minimum. Any bends in the venting tubes will slow down air expulsion considerably and thereby reduce the efficiency of fans and filters, since CFM rates drop sharply with any resistance or obstructions along the way. Insulating the ducting helps as well.



Reservoir Placement

Putting your reservoir tank on a concrete floor will keep the nutrient solution much cooler. Water conducts cold as well as heat, so a reservoir placed directly on a basement floor will substantially reduce your water-cooling costs. Alternately, you should raise the reservoir and add an aquarium heater if the solution gets too cold (below 60°F).

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White Tubes

Use white, opaque tubing and hoses instead of black, since the darker-colored tubing absorbs more light (and thus more heat). Hydro shops and most nurseries sell high-quality white tubing and hoses that will deflect heat instead of letting it in. Also, never use clear tubing for any purpose.



Detect Hot Spots

Get yourself an infrared laser thermometer to check for hot spots or thermal leaks. For less than $50, you can point the “gun” anywhere and get a digital readout of the exact temperature at that spot. If you want success every time, putting one thermometer in the middle of the room just won’t cut it.

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Supplemental Lighting

Adding supplemental lighting such as compact fluorescents (CFLs), T5 fluoro tubes or LEDs instead of another HID light works great to keep a space cool. Typically arrayed either in an alternating checkerboard pattern or around the outside of a vertically lit space, these kinds of lights can drastically reduce both heat production and your energy costs.

Light Movers

Whether you put your lights on a track or a spinner, light movers raise your growing footprint while also reducing heat. The light is spread around more, which means fewer leaves and tops are shaded out. Also, using a light-moving unit allows you to lower the bulbs a bit, improving the amount of lumens your plants receive at the canopy level.