Recycling Heat in Your Bloom Room

By Lee G Lyzit
Published: June 20, 2019 | Last updated: June 14, 2022 03:11:26
Key Takeaways

With the high utility costs associated with growing cannabis indoors, it’s a good idea to look at ways to save cash while helping your buds bloom at the same time. Lee G. Lyzit explains how recycling heat can have a positive impact on a grower’s bottom line.

In just about every indoor cannabis garden there is an excessive amount of heat. The artificial lighting systems used for indoor horticulture are notorious for emitting plenty of heat. Depending on a grower’s location, he or she may have to use a mechanical device, such as an air conditioner and/or fan, to keep the temperature and humidity within the desired ranges. Even horticultural lighting systems that have a low heat signature and are more efficient at converting electricity to usable light emit enough heat to warrant the use of an air conditioner and/or fan. For some growers, depending on their geographical locations, a break from this excess heat is during the winter months.


Due to the cold temperatures experienced during winter months, many indoor horticulturists can combat the heat within their indoor gardens more efficiently and effectively. Gardeners who live in areas where a cold winter may feel like they are chasing their own tails as they exhaust the heat from the garden when the lights are on, while paying to heat the garden when the lights are off. Even though heat is generally thought of as waste by indoor horticulturists, there are times when that heat could be recycled and utilized to maximize the overall efficiency of the garden.

Winter Flowering Room Dilemma

An indoor cannabis garden can create a heating and cooling dilemma for growers in cold-winter areas. There can be a large variance in temperatures from the lights-on period (where excess heat is created by the artificial light source) and the lights-off period (when cold outdoor temperatures drop the inside temperature of the garden space). Many growers prefer to draw fresh air from outdoors to cool their lights and/or atmosphere within the growing space. However, during the winter months, this can cause the garden’s temperature to get too cold, especially during the lights-off period. Most sophisticated cannabis growing facilities are equipped with automated heating and cooling systems. During the winter months, these systems can alternate back and forth from air conditioning to heating.


Although growers in colder regions get a respite from excess heat during winter months, they end up paying just as much to heat the growing space during the lights-off period as they would to cool it during the other seasons of the year. To increase the overall return on investment, there are some savvy cannabis growers who are recycling the excess heat from their grow lights during the winter months. The most efficient way to recycle heat is to do so between two adjacent blooming rooms that operate on opposite 12-hour cycles. The heat created by the artificial lights from the room during the lights-on period can be redirected to heat the other bloom room during its lights-off period.

Heat Recycling Compatibility

Some indoor cannabis gardens are more suitable for heat recycling than others. The best type of garden for heat recycling is an indoor garden with a closed air-cooled reflector ventilation system. A closed air-cooled system takes fresh (cool) air from outside the garden space and draws it through the lighting fixture(s), which, in turn, cools the fixture and the ambient temperature of the room. The air passing through the reflectors becomes heated by the lamps. Normally this heated air is then exhausted outside.

Since the heated air from a closed air-cooled reflector ventilation system never contacts the ambient air of the growing space, it is free of excess humidity (sometimes created by plants), any pesticide or fertilizer odors, or airborne pathogens that could be found within the indoor garden. In fact, this heated air can not only be used to heat another indoor garden but also a home. After all, heating a living space can be expensive and using heat that would otherwise go to waste is a great way to reduce costs.


Length of Ducting

One important consideration when setting up a heat recycling ventilation system is the overall length of the exhaust ducting. The longer a duct run, the more resistance, and, therefore, the more a fan’s power is reduced. This is why the most efficient and effective way to recycle heat from one grow space to another is to have the two grow spaces adjacent to each other. If a longer duct run is required, it may be necessary to increase the size of the fan so it has enough power to adequately move the heated air. In most cases, a larger fan may be the only piece of mechanical equipment required to convert a garden’s existing exhaust system into a heat recycling ventilation system.

(Read also: Two Flowering Rooms and Others Ways You Can Adjust Your Indoor Grow for Winter)


Recycling Heat Between Two Flowering Rooms

Cannabis growers who live in colder climates can rely on heat recycling to make their indoor gardens as efficient as possible. As previously mentioned, this idea involves two indoor cannabis blooming rooms operating on opposite 12-hour light cycles. To ensure the heat transfer is in time with the light cycle, the exhaust fan should be programmed to correspond with the lighting system. When the lights are on in room No. 1, the heat that is being created by the lighting equipment is ventilated into room No. 2, where the lights are off. When the light cycle switches, the ventilation system switches as well so that the room in its lights-on cycle is always heating the room in its lights-off cycle. Otherwise, during the winter months, an indoor horticulturist living in a colder region would have to rely on a heating device to maintain an ideal temperature within the blooming room during its lights-off cycle. However, when using the heat recycling method, a heating device would only be used as a backup system, if necessary.

As mentioned previously, the most suitable set-up for heat recycling is two adjacent blooming rooms equipped with closed air-cooled reflector ventilation systems. Each closed ventilation system will need its own fan that is ducted directly to the adjoining floweringroom. A major consideration for heat recycling between flowering rooms is light leaks. A cannabis blooming room’s dark cycle should always be kept as dark as possible and never interrupted with light. To avoid light leaks in a heat recycling system, a black interior ducting should be used and the ducting should be bent into a U-shape to eliminate any light reflection within the ducting.

Additional Considerations

In the rare instance too much heat is exhausted into the lights-off garden space and the temperature exceeds the desired range, another cooling device, such as an air conditioner or additional exhaust fan, can be used as a fail-safe. Another valuable tool for any gardener looking to recycle heat from one indoor garden to another is a flip-box lighting controller box.

A flip-box lighting controller allows two lamps to be powered by a single ballast (on opposite 12-hour photoperiods, not simultaneously). The ballast connected to a flip-box light controller operates continuously. A 12-hour cycle timer triggers the relay within the flip-box and the ballast’s power is switched from one lamp to the other. Put another way, every 12 hours, the flip-box lighting controller’s relay switch engages and the light cycles of the two blooming rooms alternate. Aside from recycling heat from one grow room to another, flip-box light controllers allow a cannabis grower to have half as many ballasts as lamps. Essentially, these devices allow two light bulbs, on opposite 12-hour light cycles, to be operated by one ballast. For example, a grower with a 10-light flip-box light controller could operate a maximum of 20 lamps from ten ballasts. Another great thing about flip-box light controllers is they are integrated with, or directly connected to, a timer which ensures the two flowering rooms’ photoperiods will never overlap and will remain on perfect 12-hour lights-on and 12-hour lights-off cycles.

A cannabis grower living in a climate where it is necessary to heat the lights-off period in his or her bloom room stands to gain the most benefits from a heat recycling setup. Instead of alternating from cooling to heating equipment in one bloom room, a cannabis grower with two adjacent flowering rooms can divert the excess heat from bloom room No. 1 to maintain optimal temperatures in bloom room No. 2. Over the course of the growing season, a dual bloom room setup with heat recycling can reduce the cost of heating and/or cooling equipment and, in turn, increase the grower’s overall return on investment. Growrooms equipped with an air-cooled reflector ventilation system are the most suitable for heat recycling. Not only is it relatively easy to modify this type of ventilation system into a heat recycling system, but a closed air-cooled reflector system also reduces the likelihood of cross-contamination between the two flowering rooms. Specialized equipment for dual blooming rooms, such as flip-box lighting controllers, further increases the efficiency of heat recycling dual blooming room set-ups.

Cannabis growers are always looking for methods to save money through heightened efficiency. For many growers, a dual flowering room complete with heat recycling and a flip-box lighting controller is the most effective and efficient method for indoor cannabis cultivation during the winter months.


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Written by Lee G Lyzit | Grower, Writer

Profile Picture of Lee G Lyzit

Lee G. Lyzit has been involved in the cannabis industry for nearly 20 years. His passion for natural healing motivates him to learn as much as he can about the miraculous cannabis plant. Lee’s knowledge of cannabis gardening stems from his own extensive cultivation experiences and his past work as a hydroponic shop owner and manager.

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