How To Heat and Cool a Grow Box

Rambo November 16, 2016 2
How To Heat and Cool a Grow Box


I built a grow box that measures 2 feet wide x 2 feet deep x 4 feet tall and I need to find a way to regulate the temperature inside. It was originally built for my house which has a pretty constant temperature but now I have to move it out to the garage to make space. I’m worried that the garage will get to cold in the winter and to hot in the summer and that this could harm or kill my cannabis plants. I’m looking for some kind of heating and cooling system that will regulate the temperature inside the grow box and ideally keep it at 75 degrees without requiring much of my attention.


Managing grow room  temperature is often a struggle for indoor growers. There are a lot of options for larger grow rooms but grow boxes present additional challenges. There just isn’t as much on the market to manage the temperature of small spaces and you have to keep the cost down. Usually grow boxes are kept inside so cooling is more of an issue than heating.

To manage air temperatures there are two models; Sealed Room and Vented Rooms

A sealed room is designed so no air flows in or out. As heat from the lights, dehumidifiers and Co2 generators begins to exceed the desired temperature for the room a dedicated air conditioning unit turns on to cool the room and keep the heat from stalling growth or damaging the health of the plant. The thermostat on the AC unit can be set to cool the room if the temperature exceeded your desired level so it’s mostly hands off. Most sealed rooms require additional Co2, because without the flow of fresh outside air, the plants will eventually use up all that is available and plants need Co2 for photosynthesis.

In sealed rooms the heat from the lights should be enough to keep your grow room heated to 75 while they are on. This means you really only need to worry about heating the grow room when the lights are off. If you schedule the light cycles so the lights are on at night when the outside temperatures are cooler, the AC unit won’t need to work as hard. While the lights are off during the day, the warmer daytime temperatures will help heat the room naturally.

This sealed room system is great for larger grow rooms but because your grow box is fairly small, it’s not practical at all. Sealed rooms are more expensive to set up and as far as I know,  the equipment needed isn’t really available for small spaces.

Your best option is a  vented room style grow box. This style uses a fan also commonly referred to as a blower to exhaust hot air from inside the grow box and bring in cooler outside air. Blowers generally come in two styles, centrifugal and “squirrel cage”.  Centrifugal blowers are better suited for pulling than pushing air and usually  use a lot less electricity for how much air they move.

Either style you choose there are only two ways to configure them; positive pressure and negative pressure. This sounds fancy but the only difference is whether you’re blowing air in, or sucking it out. Why would you not want to suck it in or blow it out? Because you want the fan/blower on the outside of the room or grow box. Fans run on a motor and motors get hot. Heat is what we are trying to get rid of so the fan goes on the outside of the room or grow box.

Your only option, vent your grow box. Here is what I would do.

Heat rises so cut an air intake in the lower portion of the grow box. Ideally you want to filter the air flowing in and you need to make sure no light comes in if the fan turns on when the lights are out. Attach a coil of rolled up ducting probably 4” to 6” in diameter. The rolled up coil should prevent light from getting in. Attach an air filter to the end so dust and bugs won’t get sucked in when the fan turns on. This is your air intake.

At the upper portion of the grow box cut another hole and attach another coil of ducting. To the far end of the ducting attached your fan and plug it into a thermostat with the thermometer on the inside of the grow box. Set this thermostat for a bit above your desired temperature, maybe 80 degrees.

When the heat rises inside the grow box, the fan on the top will turn on and suck the hot air out. This will create a vacuum inside the grow box which draws in cooler air from the lower vent. As it draws in the fresh air it also replenishes the Co2 in the grow box.

To keep your grow box warm in the winter, here are some ideas.

  • Run your lights during the coolest part of the day for free heat.
  • Insulate the outside of the grow box so it retains as much heat as possible when the lights are out.
  • Do what you can to keep the garage from getting to cold.
  • Water retains heat so keep your nutrient reservoir inside the grow box if this is practical.
  • Use as heating mat under the plants. The roots love to be warm and this will help heat the grow box as well

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  1. BigRedNuggz November 17, 2016 at 10:55 am -

    Thank you very much for your help. You had more info on that page then i could find anywhere. So thank you guys again and i will definitely be asking you guys for help with any other concerns, questions, or any inquiries i have.

    • Rambo November 17, 2016 at 11:37 pm -

      I’m glad to hear that this post was helpful. We are happy to help any time. Best of luck!