An indoor garden’s ventilation system is just as important as its lighting system or nutrient regimen. The ventilation system is responsible for maintaining the atmospheric conditions (temperature, humidity, and CO2 levels) within an indoor growing space.

Temperature, humidity, and CO2 levels all directly affect photosynthesis and, therefore, the way a plant grows and develops. When these variables are kept in the desired range, a plant’s ability to photosynthesize will not be compromised.

Like other key contributors to a successful indoor garden, consistency of the ventilation system is important to providing the optimal conditions for accelerated plant growth.

In a hobby indoor garden, the ventilation system may consist of little more than intake fans, exhaust fans, and recirculating fans. For more advanced garden set-ups, the ventilation system could also include air conditioners, dehumidifiers, and CO2 enrichment equipment.

Regardless of whether a gardener implements a basic or more advanced ventilation system, the basic functions of the ventilation system remain the same: to provide uniform temperature and humidity levels within the desired range and to supply the plants with CO2 for photosynthesis.

Fans

The optimal temperature range for most indoor gardens is 72-80˚F. In grow rooms with enriched CO2 levels, the optimal temperature range is a little higher (80-85˚F) in grow rooms with enriched CO2 levels. The pieces of equipment used in the ventilation system will help a grower maintain temperatures in these ranges.

The most common piece of equipment in any ventilation system is a motorized fan. For a small indoor garden with a low heat signature, a single fan could make up the garden’s entire ventilation system. An exhaust fan could remove the excess heat from the growing area while drawing fresh air into the garden space.

The fresh, cooler air would help lower the temperature of the growing space and contain CO2 for the plants to “breathe.” Larger growing spaces or gardens with more grow lights would need multiple fans to regulate the temperature, humidity, and CO2 levels.

Using motorized fans for both the intake and the exhaust is common in medium-sized grow rooms. In this type of set-up, the fans work together to evacuate air within the grow space and replenish it with fresh air.

Read More: How to Tell a Quality Inline Grow Room Fan from a Cheap Knockoff

Atmospheric controllers with built-in thermostats and humidistats allow growers to automate motorized fans within the growroom and to provide more consistent atmospheric conditions.

Recirculating fans are another valuable tool for creating uniform atmospheric conditions within the growing space as they help to “mix” the temperature and humidity of the garden’s environment.

Air Conditioning

One of the most common devices used in a modern indoor garden is an air conditioner. More specifically, a mini-split air conditioning system. These types of air conditioners are very efficient at removing excess heat and maintaining optimal temperatures and humidity conditions.

Of all the devices used to control the atmospheric conditions in an indoor garden, an air conditioner gives growers the most control. There are many makes and models of air conditioners on the market, so it can be a little overwhelming for a new indoor horticulturist.

When shopping for an air conditioner for an indoor garden, a grower needs to pay close attention to the machine’s BTU rating. British thermal unit (BTU) is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

Don’t let the definition of BTU confuse you. Instead, just think of BTUs as a way to quantify heating or cooling equipment. In other words, a grower can use a BTU rating to find the appropriate size air conditioner for his or her garden.

Sizing Air Conditioning Equipment for an Indoor Garden

To adequately cool their indoor garden, a grower will need to consider the five major factors that influence the size of an air conditioner.

Those factors are the dimensions of the growing area, the lighting equipment, the ballasts of the lighting equipment, the CO2 equipment, and any remaining electrical accessories used in the garden.

Room Dimensions

When sizing an air conditioning system for an indoor garden, the first thing a grower should note is the size of the space that needs to be cooled.

Although the heat created by the lighting system and other garden equipment will need to be considered to determine the required cooling load, the size of the space in cubic feet will determine the minimum BTU requirements for the air conditioning equipment. This minimum size can be determined by using a general BTU chart for room size.

General BTU Chart for Room Size

Cubic Feet = BTUs

  • 1 - 1,200 = 5,000
  • 1,200 - 1,600 = 6,000
  • 1,600 - 2,000 = 7,000
  • 2,000 - 2,400 = 8,000
  • 2,400 - 2,800 = 10,000
  • 2,800 - 3,200 12,000
  • 3,200 - 3,600 14,000
  • 3,600 - 4,000 15,000
  • 4,000 - 4,800 18,000
  • 4,800 - 5,600 20,000
  • 5,600 - 6,400 22,000
  • 6,400 - 7,200 25,000

Lighting System

In most indoor gardens, the lighting system creates a lot of excess heat that needs to be addressed. This is why it is so important to consider the lighting equipment when figuring out the required cooling load.

After a grower determines the required BTU based on the cubic feet of the garden space, he or she can determine the additional BTUs that will be needed to offset the heat created by the garden’s lighting equipment.

A good rule of thumb for finding this value is to multiply the total lighting wattage by 3.5. For example, if a grower has 10,000 watts of lighting equipment, he or she will need an additional 35,000 BTUs (10,000 x 3.5 = 35,000) on top of the BTUs required for the size of the garden space.

Ballasts

The ballasts used in the lighting system also need to be considered when sizing cooling equipment. If a grower uses a remote ballast, which operates at a distance from the lamp itself, and removes the ballasts from the actual growroom, no further cooling loads within the garden space will be required.

However, if the ballasts remain in the same space as the garden, they will need to be added to the calculation when determining the required BTUs for cooling. The best rule of thumb for ballasts is to multiply the total wattage by 2.5.

This number will be added to the required BTUs for the room size and the required BTUs for lighting equipment.

For example, if a grower has 10,000 watts of lighting equipment and is operating the ballast within the growing space, he or she will need an additional 25,000 BTUs of cooling (10,000 x 2.5 = 25,000).

CO2 Burners

Some indoor horticulturists utilize CO2 burners to enrich the environment with CO2. These burners add heat to the growing space and need to be considered when making calculations for cooling equipment. The BTU output of a CO2 burner will depend on the size of the unit and how many burners the unit has.

A very basic rule of thumb is to add 10,000 BTUs of cooling for every CO2 burner with two burners. However, CO2 burners sold for indoor horticultural use normally include a chart listing how many BTUs they put off. It is important to keep in mind that most CO2 burners do not operate continuously but, rather, sporadically.

To put it another way, the BTU rating on a CO2 burner is only applicable when the unit is operating. This time of operation may range from one hour to 12 hours per 24-hour cycle.

Other Electrical Equipment

Every piece of electrical equipment used in an indoor garden—air pumps, atmospheric controllers, fans, dehumidifiers, etc.—add a small amount of heat to the environment. In most cases, these pieces of equipment are negligible and will not have a dramatic effect over the required cooling loads.

However, when making calculations as to the required BTUs for cooling, it is always a good idea to go with a slightly higher BTU than a slightly lower one.

In other words, by choosing an air conditioner that has a BTU rating that is slightly higher than the requirements calculated from the room size, lighting system, ballasts, and CO2 burners, a grower can rest assured that any additional electrical equipment used in the garden will be covered. It is always best to error on the side of caution.

The ventilation system of an indoor garden is the heart and soul of atmospheric control. The temperature, humidity, and CO2 levels of an indoor garden directly influence the plants’ ability to photosynthesize.

When the atmospheric conditions of an indoor garden are kept consistently in the optimal range, and if lighting and nutrition are in check, the plants will have the ability to grow at an accelerated rate.

Motorized fans and air conditioners are the main tools used by indoor growers to achieve the atmospheric conditions most conducive to healthy growth.

Hobby growers can usually get away with a couple of fans to control heat and humidity, while commercial growers will most likely need to incorporate air conditioning systems.

One of the most difficult aspects of sizing an air conditioner for an indoor garden is calculating all the contributing factors that create additional heat.

By considering the room size, the wattage of the lighting system, and the heat created by CO2 burners, an indoor gardener can more accurately determine the amount of air conditioning he or she will need to provide the optimal atmospheric conditions.

Regardless of the size or type of indoor garden, the basics of a ventilation system will always remain the same: to control temperature and humidity and to provide the plants with CO2 for photosynthesis.

When these basic principles are met, a horticulturist will be rewarded with more consistent yields and a higher return on investment.