Recycling Heat Between Dual Flowering Rooms

By Eric Hopper
Published: January 15, 2020 | Last updated: May 11, 2021 11:32:58
Key Takeaways

Indoor growers in colder climates can benefit from heat generated by grow lights and use the excess warmth to save money. Eric Hopper explains how to recycle the heat and reduce costs in the process.

All indoor gardens are unique. However, there is one thing most indoor gardens have in common: excess heat. The artificial lighting systems are notorious for emitting a lot of heat. To combat the excess heat, many growers employ mechanical devices, such as air conditioners or fans, to help keep temperature and humidity within the desired range. Even modern horticultural lighting systems, which are more efficient at converting electricity into usable light, emit enough warmth to create excess heat issues for growers.


Depending on the geographical location of the garden, some horticulturists get a break from battling the excess heat during the winter months. These growers can take advantage of the cold air outside and draw it into the grow space to effectively cool the environment. However, growers who live in cooler climates may experience a different form of inefficiency as they use electric or gas heaters to maintain the proper temperature in the garden when the lights are off. Though heat is generally viewed as waste by indoor horticulturists, there are times when that heat can be recycled to maximize the overall efficiency of the garden.

Read also: How to Deal with Humidity in the Growroom


Recycling “Waste” Heat

An indoor grower who lives in a climate where it is necessary to heat the “lights off” period of their flowering room stands to gain the most benefits from a heat recycling setup. To successfully implement this, a grower must have two flowering rooms operating on opposite 12-hour light cycles. The heat is transferred from the room with its lights on to the room with its lights off.

To ensure the heat transfer is in sync with the light cycle, the exhaust fan(s) should be programmed to correspond with the light timer. In other words, when the lighting equipment is on in flowering room #1, the heat that is being created by the lighting equipment is ventilated into room #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 providing heat to the room in its lights off cycle.

Without a heat recycling setup, an indoor horticulturist living in a colder region would have to rely on a heating device to maintain an ideal temperature within the flowering room during the lights off cycle. However, when using the heat recycling method, a heating device would only need to be used as a back-up system.


Most sophisticated indoor growing facilities are equipped with automated heating and cooling systems. During the winter months, these systems can alternate back and forth from air conditioning (during the lights on period) to heating (during the lights off period). Although growers in colder regions get a respite from excess heat during the 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.

Read also: How To Perfect Your Grow Room’s Ventilation System


Instead of alternating from cooling to heating equipment in one flowering room, a grower can opt for a dual flowering room setup. This allows them to divert the excess heat from flowering room #1 to maintain optimal temperatures in flowering room #2. Over the course of the entire season, a dual flower 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. However, some indoor gardens are more suitable for heat recycling than others.

The ideal 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 lighting fixture(s) and reduces the ambient temperature of the growroom. As the air passes through the reflectors, it becomes heated. Normally this heated air would be exhausted outside. In a heat recycling system, the heat is reused in the other flowering room during its lights off period.

Each closed ventilation system will need its own fan that is ducted directly to the adjoining flowering room. When employing a heat recycling system between flowering rooms, light leaks are a major concern. A flowering room’s dark cycle should always be kept as dark as possible and should never be interrupted by light. To avoid light leaks, a black interior ducting should be used, and the ducting should be bent into a U-shape between the rooms. This will stop any light from reflecting through the ducting.

Read also: The Effect of Light Leaks on a Sealed Growrooms

Ducting Length

Another 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. This resistance reduces the fan’s power and ability to move air. Because of this, the most efficient and effective way to recycle heat from one grow space to another is to have the two grow spaces adjacent or very close to each other.

If a longer duct run is required, it may be necessary to increase the fan size so it has enough power to adequately move the heated air. Assuming the grower has or can build two adjacent flowering rooms, an additional fan, ducting, and a relay or flip light controller are the only pieces of equipment needed to create a heat recycling system.

Relay or Flip-box

A relay or 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 output power is routed 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. In addition to the benefit of recycling heat from one growroom to another, flip-box light controllers allow an indoor horticulturist to operate two flowering rooms on half as many ballasts. Essentially, these devices allow two light bulbs, on opposite 12-hour light cycles, to be operated by one ballast.

Read also: How Slight Negative Pressure and Keeping a Light Tight Garden Can Help Maximize Yields

For example, a grower with a 20-light flip-box light controller could operate a maximum of 40 lamps from 20 ballasts. Just about all flip-box light controllers are integrated with, or directly connected to, a timer which ensures the two flowering rooms’ photoperiods will never overlap and remain on perfect 12-hour lights on and 12-hour lights off cycles.

Back-up Cooling Device

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 an additional exhaust fan, can be used as a fail-safe. A small exhaust fan equipped with a cooling thermostat is all most indoor gardens will need as a backup cooling device.

Recycling heat within a dual flowering room setup is one of the best ways an indoor horticulturist, who lives in a cooler climate, can improve his or her garden’s overall efficiency. Instead of redirecting the excess heat outside, a grower can take a few steps to implement a heat recycling system. Sometimes small growroom changes can compound to big savings. When applicable, heat recycling is one of those small changes that can result in immediate benefits.

Specialized equipment, such as a flip-box lighting controller, further increases the efficiency of heat recycling dual flowering room setups. Indoor growers are always looking for methods to increase their return on investment 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 cultivation during the winter months.


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Written by Eric Hopper | Writer, Consultant, Product Tester

Profile Picture of Eric Hopper

Eric Hopper’s past experiences within the indoor gardening industry include being a hydroponic retail store manager and owner. Currently, he works as a writer, consultant and product tester for various indoor horticulture companies. His inquisitive nature keeps him busy seeking new technologies and methods that could help maximize a garden’s performance.

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