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.
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.
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.
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.
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.
Heat Management In Grow Rooms ,