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Recycling Heat

  • Written by  Eric Hopper

Stop Wasting Winter Warmth

Even during cold winter months, most indoor growers still experience an abundance of heat that needs to be removed from the premises. The good news is that, unlike in summer, when you just have to fight that heat with sheer cooling power, winter gives some growers the option to utilize the heat generated by their indoor grow room.

Recycling Heat into Living Quarters

Heating bills can get outrageous during cold winter months, particularly for growers living in frozen places like Michigan and Canada. For growers whose operation is inside their home, it only makes sense to redirect the excess heat being exhausted from a garden to help warm the home.

Do I Have the Right Garden Set-Up For Heat Recycling?

Garden in a houseGarden in a houseWhen it comes down to it, some indoor gardens are just more compatible with heat recycling than others. Gardens using a closed ventilation systems for cooling the lights are ideal for a heat recycling application. This is because the air traveling over the lights is coming from a fresh air source. Air is drawn from outside the room, through the reflectors (which heat the air), and out of the room, without combining with the room’s atmosphere.

The air passing through the reflectors in a closed system is heated but not contaminated by odor, pesticides, or humidity. In most cases, the heat from a closed, air-cooled ventilation system can simply be redirected into the living space without any need for filtration.

Indoor gardens without closed, air-cooled ventilation systems, though less ideal, may still be used for heat recycling. However, further considerations will need to be made.

Pesticides and Chemical Fertilizers

If you plan to divert the atmosphere of the actual grow room into your living quarters, be sure that you are not using any pesticides or harsh chemical fertilizers. When you exhaust the heat from a garden’s atmosphere, you will be breathing in whatever is in your garden. This can be advantageous because your plants create fresh oxygen. However, a mite-infested garden, where the grower is continually spraying toxic chemicals, is probably not the best candidate for heat recycling into a living area.


Odor created by your plants or fertilizers is sure to end up in your living area if not treated properly. Growers looking to recycle heat into the home should purchase a carbon filter rated for a fan at least 150% larger than the size of their fan.

This is a bare minimum standard. More carbon is always better in this case, especially in a sealed environment like your home during winter. In other words, the biggest carbon filter you can get will do the best job. Without an oversized carbon filter, there is no way to safely ensure all the odor will be removed before entering the living space.

Heat recycling is just one of the many small changes a grower can make that, over time, compounds into huge savings.

Exhaust Distance

If any part of a functioning ventilation system is manipulated, it could change the garden’s overall performance dramatically. Make sure to take into account all the changes that you will be making to redirect the heat from the room. Longer duct runs equate to more resistance, which means the fan is performing at a lower CFM rating. In some cases, growers who want to recycle heat may need to purchase a fan specifically for that purpose, especially if the primary exhaust fan is being used to redirect the heat. Monitor all of the room’s parameters after applying a heat recycling method to make sure the room is still operating at optimal conditions.

Heat exchange between two flowering rooms on alternating 12/12 light cycles.

schematic of heat and energy savingschematic of heat and energy savingschematic-2schematic-2

Heating Another Indoor Garden

image showing how to heat your entire houseimage showing how to heat your entire houseIf you decide that pumping air from your grow room into the rest of your home is not a good idea, there’s still hope for that excess heat. Running a second flowering room on an opposite photoperiod is a great place to put that warm air.

In order for the heat recycling to work properly, the two flowering rooms must each have a fan dedicated to that purpose or a closed, air-cooled ventilation system. For growers without air-cooled systems, these fans should be mounted on the wall that separates the two flowering rooms.

Connect a length of ducting (black interior) to each fan. Bend the ducting just enough to block all light from polluting the dark cycle of the other flowering room. These fans can be set to operate on the same twelve-hour cycle as the lights.

For growers with closed, air-cooled ventilation systems, the ducting can be diverted to the neighboring room. The ducting must be bent or another specific light elimination method used, so that the opposite flowering room’s dark cycle is left undisturbed. The fans controlling the air-cooled system can be set on the same twelve-hour cycle as the lights.

Regardless of whether the grower has an air-cooled ventilation system or two fans mounted between the rooms, in order for both of the flowering rooms to function properly, they each need to have independent ventilation systems. This means a ventilation system in addition to the dedicated fans for heat recycling. Without an independent ventilation system in each room, heat recycling between the two rooms could be counterproductive.

The independent ventilation system for each of the flowering rooms should, at the very least, consists of an intake fan, an exhaust fan, and a thermostat controller. These fans should operate separately from the two fans designated for heat recycling between the rooms or the diverted, closed, air-cooled ventilation system. Furthermore, these fans should be set to operate on the thermostat controller.

This way the room’s ventilation system will operate as needed. For example, if the heat diverted into the room in its dark cycle builds up too much, that room’s thermostat controller will trigger its ventilation system to turn on and remove the excess heat. This failsafe negates any possibility of overheating or otherwise going outside the desired atmospheric boundaries of each room.

Whether it is to provide supplemental heat to your home, or to reduce or eliminate the need to heat the dark cycle of your flowering room, recycling heat is a relatively easy way for most growers to save a little bit of money. For growers in colder climates, heat recycling becomes an invaluable tool for creating the most efficient and productive indoor garden(s) possible. Heat recycling is just one of the many small changes a grower can make that, over time, compounds into huge savings.

With a few adjustments to a ventilation system or by adding a dedicated fan and some ducting, almost any indoor horticulturist can reap the rewards of heat recycling and increase the operating efficiency of the garden.

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Last modified on Monday, 24 March 2014 14:53

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