Heat Management In Grow Rooms

gore December 31, 2011 19
Heat Management In Grow Rooms

For many indoor marijuana growers, managing heat is a constant uphill battle. Grow rooms use an enormous amount of equipment and electricity to maintain suitable growing conditions for marijuana plants. I have visited grow rooms as small as 12’x20′ that require over 15,000 watts to operate.  All this equipment generates heat. Without proper management, temperatures can become intolerably high. In order to maintain optimal growing temperatures,  you may need to make a systematic evaluation of each heat source.

When grow room temperatures become too high, plants begin to close their stomata and stop breathing. Without respiration, marijuana plants rapidly deplete stored carbon and cease photosynthetic CO2 assimilation. In a 1995 essay, Claudio Pastened and Peter Horton published their observation that bean plants ceased photosynthetic CO2 assimilation at 35ºC (95ºF). Another trial shows similar results at a temperature of 113ºF (P. Haldimann and U.feller, 2004).  To my knowledge, no such no studies on marijuana exist; but it is reasonable to expect that photosynthesis in marijuana is similar to beans, and other fruit-bearing annuals.  In my experience, marijuana grows best indoors at temperatures between 80º and 86º F (26°-30° C).

High Intensity Discharge Lamps

HID lighting is often a significant source of heat in your grow room. When purchasing lamp hoods, always opt for models designed for venting with a minimum ducting flange of six inches. Light hoods designed for eight inch ducting are also available and offer greater airflow. If five or more lamps are being run in series, use eight inch ducting. To maximize airflow, keep all ducting as straight as possible. Any turns should be gradual curves rather than right angles.

Even properly ducted hoods can heat the air in the grow room. Large in-line centrifugal fans are the perfect choice for drawing this heated air outside or into living quarters in the winter months. In-line fans are most efficient when pulling air rather than blowing air. So they should be positioned as close to the exit vent as possible.

For grow rooms using CO2 enrichment, a sealed light ventilation system is ideal. This design draws cool outside air through a vent at the beginning of the ducting. The air is sucked through the light hoods by an in-line fan positioned at the far end of the ducting near the exit vent. If your hoods are properly sealed, very little outside air will enter the grow room and very little air from the grow room will escape. Because this method does not directly exchange the air inside the room, it will not interfere with CO2 enrichment cycles.

The second option is best for grow rooms regularly drawing outside air to replenish CO2 levels. This method draws air from the grow room through the light ducting before expelling it out the exit vent. This creates a negative pressure in the room which pulls fresh air through a passive intake into the grow room. An in-line fan is located at the exhaust end of the ducting circuit to expel the heat. If odor is a concern, the air from the room can be pulled through charcoal filtration before it enters the exhaust duct. With this design, fresh air is being drawn into the room. So make sure you use a hepa filter to prevent pests and fungal spores from contaminating the room.

For either method, the lamp ducting fan can be controlled by a simple thermostat set a few degrees below your target temperature. When the room cools down past that trigger point, the thermostat will stop the blower and allow the room to heat up. For most applications, a thermostat will also prevent exhaust fans from running during the dark cycle. If your day and night heat differential is not great enough, and your fan runs during the dark period, consider a thermostat with a photo cell. Another option is to plug the thermostat into a cheap 12-hour timer synchronized to your lighting schedule.

Most lamp manufacturers offer water-cooled HID fixtures which greatly reduce heat emission. Some systems use a clear water jacket that brings water into direct contact with the bulb.  Water is pumped through the system, cooling the bulb before returning to a large reservoir. These systems are so efficient at removing lamp heat that your plants can actually be allowed to touch the water jacket.  Some red light above 680 nm is filtered out when light passes through water. Luckily, overall PAR lumens can be increased by moving the lamp closer to the plants.

Another method of removing lamp heat is through the use of water-cooled heat exchangers.  Radiator-like units are placed throughout the lamp duct system, using water to cool the air instead of the bulb itself.  This system still requires a reservoir and pump to circulate water through a network of small radiators. Adding a water chilling unit will make the system even more effective.  Water-cooled HID lighting holds great promise, but is not yet mainstream.


Many growers overlook the substantial heat generated by the ballasts running their HID lights. The older inductive ballasts produce the most heat, while newer digital ballasts produce less. If possible, ballasts should be located outside the grow room to prevent heat emissions from raising the inside temperature.  If this is not possible, ballasts can be placed in vented cabinets or housings that draw fresh air to keep the electronics cool. By integrating your ballast cooling system into your lamp ducts you can eliminate the need for a second fan. To help prevent the electronics from overheating, the air should flow through the ballast enclosure before entering the light ducts.

CO2 Generator

Burning propane or natural gas inside your grow room is the most efficient method to generate CO2. When propane (C3H8) burns in the presence of oxygen the result is:

C3H8 + 5 02 → 3 CO2 + H2O + heat
Propane + oxygen → carbon dioxide + water + heat

The byproducts of this process are huge amounts of heat and some water vapor. Carbon dioxide generators are most often rated by their output of CO2 in cubic feet per hour. Sometimes manufacturers will also print the heat output, measured in BTU. If heat generation is not indicated on a CO2 generator, use the following formula to determine the BTU output of propane-burning generators.

\frac{ CO_2/ hour\;in\;cubic\;feet\; \cdot\;(1000)}{ 1.18}=BTU\;output/hour

Some CO2 generators offer built-in heat exchangers and a flange for ducting the heat out of the room. Heat exchangers can be very effective, but must be kept free of dust and carbon build up for optimal heat reduction. These systems should never be incorporated into a light duct circuit, as they will add a tremendous amount of heat.

There are inexpensive DIY methods for keeping CO2 generator heat out of the room. if constructed incorrectly, however, they can burn down your grow room.

The best method of removing heat from a CO2 generator is through water cooling. By placing a water cooled heat exchanger inside the fire box, these units are able to extract most of the heat produced by the flame. Units like this forecast the future of CO2 generation for indoor growing and greenhouses.


A dehumidifier is a combination of an air conditioning compressor and a heater which always generates additional heat. At the time of this post, I  know of  no effective method for venting this heat. I have a few ideas; but they are just as dangerous as they are impractical. If I ever iron out these kinks, I will be sure to share them.

Really, the best way to limit the heat from a dehumidifier is to limit its use. The more you learn to properly manage your watering schedule and use the proper amount of growing medium, the less need you will have for a dehumidifier.

Air Conditioners

Ductless air conditioning units offer an additional option. These systems use a remote compressor, keeping the noise and heat outdoors. Coolant is piped to a wall-mounted heat exchanger and fan located inside your grow room. Units of this type are expensive, but use very little interior space and can be installed without damaging or modifying your room.

As a last resort, compressor-type air conditioning can be used to cool your grow room. Walls can be “customized” to accommodate window-mount models; but ducting-free, standing models offer a less destructive option.

Cool Tips

Different variables will cause the thermodynamics of each grow room to be unique. Here are some tips to help you keep your temperatures manageable.

  • Do not let your grow room exceed 86° F.  Keep it between 70°-86° (21°-30° C)
  • Heat rises. So place air intake vents as low as possible in the room and exhaust vents as high as possible.
  • Circulate the air in your grow room to keep temperatures even.
  • Grow rooms placed in the second story or in attics will naturally stay warmer than grow rooms on the first floor or in basements.
  • Digital ballasts run cooler than magnetic ballasts.
  • Always buy vented hoods for your lights–no exceptions.
  • Use insulated ducting when possible.
  • Keep ducting runs as straight as possible; avoid right angles.
  • In-line fans are more efficient when they pull air rather then push air.
  • Though not mentioned above, insulate the hell out of your grow room. The more outside heat you can keep out, the better. It will also help contain noise from grow room equipment.

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  1. Auldy July 8, 2012 at 9:26 am -

    Myself have ben interested in this topic for over 30 years. I like when I come across a new rule.


    Always buy vented hoods for your lights–no exceptions


    Like ;0)

  2. Kris February 6, 2013 at 7:45 am -

    Hi Gore, thats great advice! I’m struggling with keeping temps down in my grow cupboard which measures 36.75″x36.75″x70.5″.

    I’m running a 400W HPS with a magnetic ballast and using a 4″ temp/speed controlled inline fan to pull through a 4″ carbon filter before venting into the adjoining cupboard. I have 3×3.5″ passive intake holes on the bottom opposit side to the filter.

    I’m finding it hard to keep temps below 30 without leaving the door open slightly, which obviously defeats the point!

    I can’t add any more passive intake holes so i’m going to re-jig my layout and reposition my ballast on a seperate shelf above the grow area which will keep the heat seperate from the grow room. As well as that i’m going to change my reflector for a cooltube and use the fan to pull cool air through my filter, then cooltube, then vent out the cupboard. There will be at least half a meter of ducting between the filter and cooltube and between the cooltube and fan.

    Do you think this is a good idea and do you think my fan will be able to handle this?

    • Rambo February 18, 2013 at 8:14 pm -

      Gore hasn’t had much time for this project lately so I will chime in. I think you are on the right track with removing the ballast from the grow area and trying a cool tube. You could also use insulated ducting which might help slightly. Question like this are challenging in the comments section because we can’t see pictures of your setup. If you still need help start a thread in our forum and post some pictures. That way more then one brain can try to provide solutions.

  3. Vereeniging April 1, 2013 at 12:05 am -

    I been intrested in growing indoor for a while but i finally have a couple of babies with i changed the hours to 12/12. They are about 20 in. In lengh and i have some one giving me advice who did this in mexico for many years but out side. I have 2 400 hps but i am only usiding 1. My hardest area is the watwring and nutrients wich i dont want to burn them or over wster them. Whay advice can you give me. I am using tigerbloom from focfarm and
    big bloom

    • Rambo April 9, 2013 at 9:56 pm -

      how often you water will depend on how quickly your soil drains and how much of it the plants are taking up. As for fertilizing… start off mild and build until the very tips of the leaves start to burn just a hair, then back it off a little.

  4. Pete May 2, 2013 at 1:18 pm -

    Great article! Many comments:

    You may or may not wish to insulate your grow room. Remember that insulation keeps outside heat out, but also keeps inside heat in (it’s a two way street). There are other factors at play here, such as humidity and molt, but lets keep it simple and just talk about heat here.

    So depending on where you live, and when in the season you grow indoors, it may or may not be a good idea to insulate your grow.

    For example, if you live in a very cold climate (say northern Maine, or northern Michigan, two states that allow legal cultivation) and you grow in a basement I would seriously consider NOT insulating and allowing the earth to help cool your grow 10 months out of the year.

    Again I’m talking about cooler areas, Vented lights may not be such a good idea depending on your setup. For example, If I was short on dough, living in the north, and still wanted a high performance grow I would hang my lights vertically (no hoods, preferably something to protect the light though) and depending on season just take filtered air from outside (late spring, all summer, early fall) and switch to inside air recirculation for the remainder of the year (I’m talking about getting your whole house involved here). That type of setup can save you big $ not just on A/C, but also your whole house heating costs! I would expel the air from my grow space to the outside / house only after being passed through a carbon filter.

    Three: Venting efficiency

    +Rigid ducting generally flows better than the flexible stuff at the same diameter, especially when you need to cover some distance. You only need a few tools and a few extra supplies to make this happen.
    +Vent insulation #1, the more ducting you have actually INSIDE the grow room the more benefit you’ll see from insulating the ducts.
    +Vent insulation #2, again in cold areas, say if you are using outside air to cool vented lights, you need to make sure you insulate the “cold” side of your venting run or you WILL have condensation problems. Otherwise what you’ll get is extremely cold ducts in an extremely warm and (relatively) humid room and the condensate is just physics kicking you in the ass.
    +Shorter duct runs are better than longer duct runs, plan your space accordingly when possible
    +When possible, NEVER run your light cooling ducts in a series configuration (i.e. hot air from light #1 is sent into light #2 and heated up some more). A more efficient arrangement is actually PARALLEL. The way this works is right from a large intake, say 12″, you split off into lets say 4 6″ vents run a separate line to each light and then a separate line back to a collector and the air goes out the room from a 12″ duct.

    CO2 tanks run much cooler than CO2 gens. Running CO2 tank ACTUALLY COOLS your grow area slightly when it’s running. CO2 gens run much cooler when they run LESS i.e. the space is nearly air tight.

    After you’ve taken venting into deep consideration and come up with the best possible plan for your sealed, air cooled hoods, CO2 enriched space, you may still want to bring in unenriched air (either from a space that has A/C or outside air depending on circumstances) for the plants night time.

    Also, only as necessary, you can time intake/exhaust cycles in opposition to your CO2 enrichment cycle _during_ lights on to keep your area cool.

    The only remaining way to combat heat in your hot grow space at this point is to add A/C. Full split, mini split, and window units can all do the trick when installed properly.

    Seriously consider vegging with T5HO lamps. 12W per square foot using normal 6500K daytime lamps has been shown to veg faster than mH can for the first 30 days of the vegetative cycle, and when used properly gives you nice short internode length. In addition to obvious power savings afforded, these fixtures put off significantly less heat. There are so many big commercial growers that veg with T5’s then flower with HPS I’d call that best practice. If you need to grow trees due to government regulations or are planning an advanced vertically lit RDWC or similar fast growing hydroponic grow then by all means rock an MH or HPS for veg. For most of us though, with smaller bushes (3-10 oz dry) using horz lighting, the T5 is the hands-down winner.

    Thanks for reading

    -Pete from Southern Michigan (year round indoor grower in CO2 enriched space that doesn’t use A/C in flower)

    • Rambo May 2, 2013 at 10:27 pm -

      Pete, All very good points. Thank you for your contribution to our page.

    • Matt July 22, 2013 at 9:28 am -

      Pete, I’m also from Michigan. I have split my house AC to run insulated piping through my flowering and it works great! My only question is on the days when it’s 70 degrees or so… I’d like to turn off the house AC and use outside air to cool my room. I already have a hole i the house to do this. My question is how do I get a substantial SCRUB on outside air coming in? I have had a PM problem in the past and wouldn’t like to duplicate. Please advise. Thanks!

    • Victor May 14, 2014 at 9:55 pm -

      Whats up fellow grower ;)! I know this is a old post, but I just recently acquired my medical cannabis certificate and was looking online for information on how to set up my room. Your post, hands down…. Explained exactly what I was thinking but you had so many better ideas on how you recycled the heat into your home, Brilliant Bro! You seem to know what your doing, if it’s ok with you and if you had the time I’d like to keep in touch and I could learn from you? The names Victor, I’m from Boston Ma.
      Hope to hear from you!

    • Ronny May 27, 2014 at 12:43 pm -

      Can u help me out pete you know your stuff it could help

  5. crydog September 20, 2013 at 5:26 am -

    levelheaded advice

  6. crown heating and cooling March 24, 2014 at 8:04 am -

    Very good forum posts. Thanks.

  7. kevin stark April 18, 2014 at 12:36 pm -

    This technology will allow you to dump the hot air from the dehumidifier outside.
    It is perfect for large scale growers. You can also recycle the water back into the plants.
    It uses 50% less electric with no moving parts.

    Take a look.

  8. Anet May 3, 2014 at 5:17 pm -

    I have many questions and I will ask them in diff forums always staying on subject.
    Heat Management. My grow space is a cool large walk in closet in the back of the house.Growing small year round crops for personal production White matte walls and no heat production from lites keeps an average of 70 to 75 degrees. Am in 12/12 flowering status now. Other seedlings and veg growing plants on patio and will be moved indoors when I harvest the closet which seems to be working well.

    Can I increase quantity by changing temperature? And I so, when would I do this? With a constant temp of 70-75 degrees F and good air circulation, no heat from lites, can these beautiful girls remain in this environment/heat all the way through harvest? My plants seem to love everything they have, Fox Farm, Smart Pots, Dyna Gro (foliage and flower), rainwater (perfect PH), Loving conversation during the timers 12 lites on and misting nutrients one day and misting clear water the next to remove residue, constantly magnifying and looking for hermes or a male that may have slipped its way in the purchased WWF bags. Can heat fluctuation increase chances of hermes? And, how long after beginning 12/12 can I expect to prove my WWF are girls?

    Thanks for the help in advance and I adore your site.

  9. Ronny May 27, 2014 at 12:40 pm -

    Ok i dont have a grow room but i have two in my bedrroom at the window. Its very hot so i installed my air conditioner. But ill be using that at night only. Now if i move them away from the air conditioner at night will they sirve and flourish?

  10. dangermouse July 26, 2014 at 1:22 am -

    Hi all, this is my first time using a grow tent and could do with some help asmy plants r lovely in bud and doing quite well but im having to remove a plant out of tent to put in room to get more air. My set up is a secret Jordan 90x90x1.8 using rvk 5″ Extractor fan with carbon filter, cfl 300 dual spec standard shade and 4″intake fan even with allthis running im having to keep door about 8″ open 2 keep temp dowm. It is at 28.5 which I thought would b ok. Im finding that some days I come hpme and they are wilting so this is when I have to stick 1 of the 2 white widow autos in kitchen to get air to it which with in 20mins its perfect. I would be grateful for feed bk ideas and help please as done so much work and to loose them now would be terrible. I dont know if I can post pics on here as could show set up, but really everything is in correct place rvk and carbon at top of tent, intake at bottom light is on pullers but still getting probs with keeping temp decent. I have been thinking of a cool shade so can use 250wat or 400watt light to get some better weight. Hope info is enough 4 some ideas thanks and love site great.

  11. rand0 August 18, 2014 at 7:25 pm -

    i would take the exit duct and put it into a plastic storage container through a hole in the top a box cutter would be fine for that then cut a medium sized hole or rectangle fill it about a third with water and then use 2 or 3 litre soda bottles that have been frozen the warm air going in through the top will be blown out cool…. the hole for the air to exit should be cut on the side opposite of where the hole in the top is so the air has longer to cool