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Keepin' it cool, man! Thread Rating: 29 votes, 4.90 average.
Old 05-22-2010, 09:09 PM
Overkill is under-rated.
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Keepin' it cool, man!

Growroom cooling:

Due to the enclosed nature of the indoor grow room, and the significant heat produced by HID lighting, gas-fired CO2 generators, and dehumidifiers, every room large or small will require some form of cooling.

As a rule, flowering rooms need to be kept at approximately 75 degrees F with the lights on, 85 degree if supplementing with CO2, and at 50% humidity. Vegetative rooms should also be 75 degree with the lights on, and 50% humidity.

Nighttime temperatures should never drop below 60 degrees F or growth will slow, at 50 degrees F growth stops! If low nighttime temperatures are unavoidable, heat must be provided to bring the room, or at least the roots up to temperature to avoid stunting growth and delaying harvests. This can be accomplished with space heaters, heating cables, heat mats or even heated pots!

There are many ways to remove heat from a space, using refrigerants, water, or just moving the air out with fans. We'll take a brief look at each of these three methods and compare their advantages and disadvantages.

How much cooling do I need?

This is a common question, and though individual spaces vary, there are some formulas and considerations to help you come up with a figure. Essentially we need to add up the BTU's of heat produced, and counter it with at least the same amount of cooling BTU's.

The first consideration is for the ambient air temperature. If you live in a desert, or any place that routinely sees 100 degrees F+ temperatures, your cooling system will need to be larger than that for someone living in a sub-Arctic climate. When sizing cooling systems, go for the slightly larger option if living in a hot climate.

Calculating how many BTU's are needed to cool the empty space is more difficult, but according to user 00420 at, the following formula will be a good start:

Room Area BTU = L x W x 40 ( H = 8foot + 5btu per foot after that)

If you have a wall that is facing the sun add in for the extra heat

Sun facing wall BTU = L x H x 40

A non-air-cooled 1000W HID light will generate roughly 4000 BTU of heat per light, so a 3000W grow would need a minimum of 12K BTU of cooling just to counteract the heat from the lights. Again, if you live in a hotter climate, it would be good to have a slightly larger unit so it doesn't have to run constantly.

Air-cooling HID lights with ducted hoods and fans is a good way to remove approximately 50% of their heat, while only sacrificing 2% of the light due the the glass lens. For best results air should be pulled from outside the grow room, filtered with a simple dust/bug filter, then pushed through the duct work and lights with a centrifugal fan, and back outside. Keeping duct work straight is essential! A single 90 degree bend in the duct work will cut airflow by 60%! Seal the ducts and hoods with aluminum foil tape to prevent leaks, and clean the glass regularly of accumulated dust to keep light transmission levels high.

Gas-fired CO2 generators can create massive amounts of heat, but fortunately have a BTU output rating on their box. Add this figure to the BTU output of your lights.

Dehumidifiers can create quite a bit of heat as well. Check with your dehumidifier's manufacturer for a heat output rating in BTU's, and add it to your total.

HID ballasts, both magnetic and digital also produce heat. If these units cannot be located outside of the actual growing area, add another 1500 BTU per 1000 watts of ballasts and add to your total.

The insulation and sealing of the room will also affect how much heat it absorbs, and how much cooling is required just to cool the empty room. Uninsulated metal buildings get extremely hot due to the radiant heat of the sun, and can exceed ambient temperatures as the building cooks in the summer heat. Conversely, a tightly sealed basement with good insulation will remain cool even during a heat wave.

Timing of lighting schedules also plays an important part in cooling a grow room. Since most areas are hotter during the day, and electricity is often less expensive in the evening, most growers choose to run their lights at night to take advantage of the Earth's natural cooling. Whether you're drawing in cool outside air for cooling, or running a sealed room with an air conditioner, the lower outside temperatures will make both scenarios more feasible. Air conditioning condensers (the part outside the building) work best if not running in direct sunlight, as they have to exchange the heat from the grow room with the outside air too!

So now you should have a good idea of how many BTU's of cooling you need, but what system is best for you to provide that cooling?

Cooling systems:

Air cooling:

The simplest way to cool an indoor grow room is to exchange the hot air inside the room, with the cool air outside using fans. Fans are rated by how many cubic feet per minute, or CFM of air they can move at a certain pressure. For cooling most growers choose highly-efficient centrifugal fans from manufacturers such as Vortex, Elicent, or Solar and Palau. Fans are designed to PUSH air and are much more efficient than if they are configured to pull air.

To calculate how many CFM's you need, calculate the total volume of your grow room by measuring Length X Width X Height in feet. That is the total number in cubic feet of your grow room. For best results, your fan system should be configured to be able to exchange 100% of those cubic feet in 5 minutes or less, should the need arise. So a 1000 cubic foot room would need a 200CFM fan at a bare minimum for an air exchange in 5 minutes. Smart growers will use a larger fan than is required, and hook it to a speed controller to slow it down, reducing the substantial noise a full-blast fan can generate. My personal preference is to have the fans connected to a thermostat, which will enable those fans to maintain a set temperature. Using timers is a clumsy way to regulate temperatures, as they may run when the room is already too cold, making a problem worse!

When exhausting air outside, it is critical for security that this air be filtered using a high-quality carbon charcoal filter to remove suspicious odors. More than a few lazy growers have been caught and arrested for neglecting this simple fact! Have the exhaust fan pull through the air filter, then blow outside with as little and as straight a ducting as possible.

This will induce negative pressure in the room, meaning the air outside the room will be sucked into crack and crevice that it can find. This is certainly better for security, as odors cannot escape outside this way. However, if you have more exhaust than intake air, not enough cold air will be coming in to cool the grow.

Intakes can be passive, such as a duct, hole, or even an open window. It’s best to screen or filter the incoming air to keep pollen, mold and insects out of the grow. On larger systems, it’s usually necessary to add a powered intake fan to help push cold air into the grow. This should be smaller than the exhaust fans so negative pressure is maintained, and the air should also be filtered for the same reasons. It’s best to have approximately 50% of the exhaust fan CFM for an intake or intake fan. These fans can be connected to the same thermostat that drives the exhaust fans, so they work in tandem.

Air conditioning:

Refrigerants, or air-conditioned systems is one of the most popular methods for grow room cooling today. Since air conditioners range from 5000BTU to over 120K BTU, there is a model and system for every growers need and budget.

There are three types of air conditioners that growers use today, portables, window/wall units, and split air conditioners.

First we'll cover the portable unit, which range in size from 9K BTU to over 20K BTU for the Kwikool spot cooling units. These are typically not very efficient at cooling, as their exhaust heat must be pushed through a duct, and on larger units with dual hoses, an intake air duct as well. They are convenient and portable however, and the unit is completely inside the grow room, which alleviates many security concerns. Expect to pay more for a good portable unit than a comparable window or wall unit, and get one size bigger than you think you need to compensate for the inefficiency. Portable AC units also commonly leak CO2 and some odors, particularly the single-hose models. Be careful and smell your exhaust often!

Next in line is the window or wall mounted AC unit. You often see these hanging out a window or bedroom wall, chugging away all summer. They are typically inexpensive, range from 5K to 24K BTU, and usually quite efficient. Some models do allow inside air to escape outside, which can cause security concerns over odor. When selecting a unit, find a model that will meet your cooling needs first, then look for a feature to close off the outside air vents so it will recirculate inside air and cool it. If you don't have a window, most contractors can cut a hole in a wall for a permanent installation if desired. Get a standard sized unit if doing this! Most AC units over 15K BTU will require a 240V AC circuit for power, and even a 12-15K BTU unit will require a dedicated 20A 120V circuit to operate.

Lastly is the split, or min-split air conditioner. This system is just a scaled-down version of the central AC units that often cool entire houses. These range from 12K BTU (1 ton) to 20 tons and beyond. A compressor unit sits outside the building on a concrete slab or pad, and connects to the indoor evaporator unit with a fan and thermostat. Split air conditioners often require an HVAC technician to install them, but companies like Excel Air make units that are DIY install, with the refrigerant pre-charged inside the copper lines. Just place the components, power them with 240V power, connect the 2 copper lines and turn the thermostat on! These units are more efficient than other types of air conditioners, exchange no air with the outside environment, and are quieter than most AC units. They are the most expensive, but they are also the most worthwhile!

All air conditioners provide at least some dehumidification, and will accumulate water that must be drained or managed. Condensate pumps will help move this drain water to where it's needed, some growers even dump it into their reservoir as it's usually good, clean water. Check the instructions to see if your planned unit has a drain, or collector tank that needs to be emptied.


Water-cooing systems have surged in popularity in recent years, due to the new technology and water chiller packages designed for growers. From water-cooled grow lights, to water-cooled CO2 generators, and water-cooled heat exchangers, more and more grow equipment can be cooled with water today.

Unless you live in an area where the climate is cold year-round, you will probably need a water chiller to remove the heat from your grow and equipment. By pumping the warm output water from the grow into the chiller, it will sense that it needs to cool the water, and the compressor will kick on and cool the water. Most growers use an insulated reservoir to maintain a stockpile of cold water so the chiller can run less often, and saving electricity.

The chillers range in size from 1/4 ton up to 30 tons in size, and everything in between. A pump is used to pump hot water in one side, and another pump on the outlet to push cold water through the system. The chiller sits outside the grow like an air-conditioning compressor, and uses the same 240V power lines. Another benefit to water-cooling is there are no refrigerant lines to run, just water lines! This means you can install a fairly large cooling system with no HVAC technician help.

Inside the grow, you can pass cold water through a heat exchanger like those from online. These consist of a large metal box with between 1 and 4 heat exchangers and fans inside the box. As the cold water moves through the exchanger, the fans blow across the exchanger, which blows out air that is the same temperature as the water coming in. By the time the water leaves the exchanger, it will be warmer than when it went in, and is sent back to the chiller/reservoir for cooling again.

There are three big advantages a water-cooling system has over air-conditioning:

Water is 4 times more efficient at transferring heat from air than an air-to-air system, meaning the system can provide the same amount of cooling with less electricity used.

Water-cooling systems in cold climates, or winter weather can often provide substantial cooling without even running a chiller. Warm water from inside the grow is pumped through a radiator outside the grow, allowing the heat to be quickly transferred away.

DIY installation means less security concerns and easier repairs if something should falter.

It's also possible to use cold tap water to provide cold water to the system in the event that the chiller should malfunction. It is very wasteful to run a water-cooled system to waste, just dumping the warm water down the drain. But in an emergency situation it's nice to at least have an option besides turning off the grow!

Several companies now make a water-cooled lighting fixture, where cold water is pumped into a sleeve that encapsulates the HID bulb and sends the hot water back to the chiller. This is a very efficient means of removing heat, but requires more plumbing and fairly expensive light fixtures to operate properly.

Water-cooled CO2 generators like the HydroGen Pro from Hydro Innovations are essentially small tank-less water heaters, and can produce copious amounts of CO2. They work best when plumbed to a tap water line, and a 120V valve controls when the unit operates. Hot discharge water is too hot for a chiller to deal with efficiently, so most growers will dump this water down the drain. Since the units run for a very short period of time, the water waste is minimized.

For long-term and efficient water-cooling solutions, some growers are starting to connect their indoor heat exchangers to geothermal cooling loops. These are long coils of PEX tubing buried deep underground in vertical or horizontal arrays, and use the Earth's stable cool temperatures to remove the heat from their indoor grows. If you have a pond, lake or stream, these coils can be placed underwater to provide the same effect. Though digging the trenches or holes for geothermal cooling may be expensive, they ensure that the grower will always have a stable and reliable cooling system that does not require a water chiller to operate. Since no chiller is required, the energy savings are substantial, since the only items that require power are the water pumps and the heat exchanger fans. These systems typically pay for themselves in about 5 years of use, so they are best for permanent installations.

Last edited by Skip; 05-22-2010 at 09:27 PM..
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Old 05-22-2010, 10:40 PM

Join Date: 01-01-1970
Great article!
Old 05-23-2010, 03:02 AM
Overkill is under-rated.

Join Date: 06-06-2009
Location: Norcal
Posts: 5217
Hehe thanks MT! I figured I'd try my hand, but I think I can do better. Stay tuned for a new article coming soon!
*I'm fairly busy so please excuse my absence or slow responses to PM's!

All about Growroom cooling: Keepin' it cool man (my 1st published article)
How to automate reservoir top offs or most anything water-related
The only UV-B lighting thread worth a damn
CO2 for dummies

Big grow:
The 36KW medical/perpetual basement

Old 05-23-2010, 06:27 AM

Join Date: 11-06-2009
Posts: 109
nice post lazy, I'm sure this is going to help a lot of people.
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Old 05-23-2010, 06:49 AM
Oh, So Interesting!

Join Date: 05-17-2010
Posts: 565
Evaporative cooling can also be useful. Swamp coolers, wall-of-water for greenhouses and fine mist nozzles work in drier climates.

I like the geothermal heat pump idea:
Old 05-23-2010, 11:59 AM

Join Date: 01-01-1970
Good article but i feel 75 degrees is to low I find plants will grow faster at 85 degrees even without co2.
Old 05-23-2010, 10:17 PM

Join Date: 11-08-2009
Location: Weed Hell
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Super-Great article along with great writing. This should be included in The indoor/outdoor Medical Grower's Bible by Jorge Cervantes.

Thanks for this contribution.
Life is hard, you must be harder.
Old 05-24-2010, 03:16 AM
super chato

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Cool read lazyman. Easy to understand. Even while baked.Thanks .
I dont know why you always have to be judging me , because I only believe in mota.
Old 05-24-2010, 03:28 AM
Agent 420

Join Date: 05-17-2010
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Very nice article Lazyman.
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Old 05-24-2010, 09:53 AM

Join Date: 11-24-2007
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Real nice read Lazyman! Much respect for bringing knowledge and know how to the people of Icmag.
Old 05-24-2010, 01:17 PM
I cough up honey oil

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Good job Lazyman!
Old 05-24-2010, 05:18 PM

Join Date: 01-26-2009
Posts: 291
well put info
Old 05-25-2010, 02:45 AM

Join Date: 12-11-2009
Posts: 55
Great work! Thanks for informing me on cooling systems! Very impressed your writing style, and how you were able to explain things in a very concise fashion.
Old 05-25-2010, 05:14 AM

Join Date: 08-07-2009
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Awesome job Lazy!
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Old 05-26-2010, 10:22 PM
. . dynamite . .

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. . hi, i'm having terrible problems at the minute with heat, your article is very good, thanks a lot . .
. . Creme De La Creme . .
Old 05-27-2010, 02:47 AM
New Member

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nice read LM

Old 05-28-2010, 12:37 AM
Bush Doctor

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Good read Lazyman.
Now Prop 215/SB420 compliant.

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Old 05-28-2010, 01:46 AM

Join Date: 09-04-2009
Posts: 95
great article, but the best way to keep it cool is to grow through the night... i have my lights at 5til 5
Old 05-28-2010, 08:19 PM

Join Date: 07-12-2006
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I want information on the coolerado units. Though they have been mentioned here, no one seems to have used one.
Old 05-30-2010, 05:33 PM
Feeling good is good enough.

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Some strains raised in the tropics (third generation and so on) bring specimens that DO well in 80's sans Co2.

They "shed" the winter coat... ;-)

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