Garden & Greenhouse


Ventilation System Automation for Indoor Gardens and Greenhouses

The atmospheric conditions in an indoor garden or greenhouse should be considered just as important as lighting or nutrition. In fact, efficiently maintaining the atmospheric conditions is essential for maximizing a garden’s overall performance. The atmospheric conditions for most indoor gardens and greenhouses are controlled or manipulated by the ventilation system. It is the ventilation system that brings fresh air into the garden space and evacuates the warm, stale air. In gardens that do not have CO2 supplementation, this exchange of air is more than just a way to maintain proper temperature and humidity levels. The fresh air brought in by the ventilation system contains CO2 which is used by plants in photosynthesis. Ventilation systems are multifaceted and can help provide plants with the atmospheric conditions needed to optimize growth.

Types of Ventilation Systems

Every garden is different so there is no “cookie-cutter” ventilation system that will work in every situation. As you can imagine, there are many different ways a grower can configure his or her ventilation system. The ventilation system for most indoor gardens and greenhouses will include an exhaust fan. An exhaust fan is used to remove heat or excess humidity from the garden space. For smaller gardens, the exhaust fan may be the only mechanical component of the ventilation system. In this case, the exhaust fan will remove hot or humid air and create a negative pressure in the room. Once a negative pressure is created, fresh air can be drawn

through a passive or non-powered intake placed on the opposite side of the room or greenhouse from the exhaust fan. Not all ventilations systems consist of only one mechanical fan. Larger greenhouses or indoor gardens may require multiple fans to adequately cycle the air through the garden space. Many larger indoor gardens will have more complex ventilation systems that not only have multiple fans but also utilize air conditioners and dehumidifiers. Regardless of how basic or complex the ventilation system, the horticulturist stands to gain a lot from automating his or her system. Automating a ventilation system not only gives the gardener peace of mind but it also creates a more consistent environment for the plants. Plants love consistency and will always perform best when the parameters of the garden are kept as consistent as possible.


Perhaps the simplest and most straightforward automation device for a ventilation system is a thermostat. A thermostat is a device that triggers a fan, heater, or air conditioner to turn on and off based on temperature. In other words, a thermostat lets the gardener decide at which temperature a device is turned on. Indoor horticulturists and greenhouse growers most commonly use the thermostat for the exhaust fan. With a thermostat, the gardener can decide at which temperature the fan should be turned on to cool the garden space. The type of thermostat used for this purpose is called a cooling thermostat. A cooling thermostat is triggered when the temperature exceeds the set point and will turn off when the temperature is cooled below the set point. For example, say a grower sets his or her thermostat to 75 degrees F. If the room’s temperature exceeds 75 degrees F, the thermostat will trigger the exhaust fan to come on. As the fan exhausts the warm air, the temperature in the room will decrease. Once the temperature is below 75 degrees F, the thermostat will turn off the fan. Another kind of thermostat is a heating thermostat. Heating thermostats are similar to a home’s heating system thermostat. If the temperature drops below the set point, the thermostat triggers the heater to turn on and operate until the temperature has risen above the set point. Both cooling and heating thermostats are regularly used by indoor horticulturists and greenhouse hobbyists. Thermostats are normally used by horticulturists to control fans, air conditioners and heaters.


Another atmospheric controller often used by horticulturists to automate the ventilation system is a humidistat. Humidistats are just like thermostats except, instead of triggering a device based on temperature, they trigger a device based on humidity. The two most common devices used in conjunction with a humidistat are humidifiers and dehumidifiers. Some growers will use humidistats with fans or air conditioners as well. Although there are some tropical plants which require high humidity, most gardeners use a humidistat to remove excess humidity from the garden space. Many plants can be adversely affected by high humidity conditions. Powdery mildew and other pathogenic fungi are notorious for showing up in environments with higher humidity. In many ways controlling a garden’s humidity is a preventative measure and helps to protect the plants from potential dangers. Large indoor gardens or greenhouses commonly have a humidistat connected to a large dehumidifier while smaller indoor gardens or greenhouses may have a humidistat connected to the exhaust fan. In either case, the purpose of the humidistat is the same: to automate a device in order to maintain a consistent humidity level below a set threshold. Since the optimal humidity level for plants can change from species to species or depending on the stage of growth the plant is in (vegetative or flowering), there is no standard for ideal humidity levels. In fact, many growers make adjustments to the humidistat’s set-point throughout the growing process to best optimize the humidity levels for their plants’ specific needs.


Some of the greatest inventions in terms of automating a garden’s ventilation system are multi-controllers. Multi-controllers (sometimes called brains) are controllers that serve multiple automation functions in the garden. Most multi-controllers will have both thermostat and humidistat controllers along with light timers, nutrient monitors and CO2 controllers. Multi-controllers are great for larger gardens and greenhouses because they offer multiple solutions in terms of automation.

CO2 Automation

Another important component in the ventilation system is a CO2 injection system. Plants use CO2 during photosynthesis and we know that increasing the amount of CO2 can help speed up this process. There are a few different ways to supplement CO2 into an indoor garden or greenhouse. The two most common methods are using compressed CO2 tanks or CO2 burners. Compressed tanks of CO2 can be combined with a CO2 emitter (which regulates the amount of CO2 being released) to supplement CO2 into a garden space. CO2 burners are units that burn propane or natural gas and create CO2 as a by-product of that combustion. Regardless of which method a grower chooses to use, a CO2 controller should be used to ensure the proper levels of CO2 are maintained. These controllers will generally attach to both the CO2 equipment and the ventilation system (exhaust fan). The reason the CO2 controller is connected to the ventilation system is to maximize the efficiency of the CO2 system. If the CO2 system is producing CO2 at the same time that the exhaust fan is removing air from the room, the CO2 is wasted. To reduce this waste, the CO2 controller can automatically be disabled when the atmospheric controller (thermostat or humidistat) triggers the exhaust fan. Only when the temperature and/or humidity returns to the desired level will the CO2 controller turn on again. It should be noted that the ideal operating temperature of an indoor garden or greenhouse will increase when there is enrichment of CO2 in the atmosphere. In other words, in the presence of enriched CO2 levels, most plants will perform best at temperatures that are warmer than without increased CO2 levels.

Wireless Automation with Data Logging

The newest and most exciting additions to ventilation automation are wireless garden devices. These wireless devices serve the same purpose as traditional automation equipment but they provide a few distinct advantages. First, as the name suggests, these devices are wireless which makes them much easier to position in key places around the garden. The second advantage to wireless automation devices is that they can be monitored and controlled remotely. In fact, many of the new wireless automation devices can be monitored from a computer or cell phone. This means that much of an indoor garden’s or greenhouse’s ventilation system can be controlled from anywhere a gardener can get an internet connection. Perhaps the most distinct advantage of a wireless automation system is that it allows the grower to log significant amounts of data that can later be reviewed or compared. Comparing data from previous gardens gives vital information to the gardener and allows him or her to experiment with various techniques. All in all, the ability to log and compare data over a long period of time gives horticulturists the tools needed to continually improve their craft. It will not be long until wireless automation devices are the norm for virtually all indoor horticulturists and greenhouse hobbyists.

A garden’s ventilation system is the keystone to controlling a garden’s atmospheric conditions. When a garden’s atmospheric conditions are in check and maintained the plants will make the most efficient use of light and nutrients. The easiest way a gardener can gain control over his or her atmospheric conditions is to implement automation devices within the ventilation system. Whether it is a basic thermostatic controller or an advanced wireless automation system, automating the ventilation system will help to maintain consistent atmospheric conditions conducive to vibrant growth. There are many variables that make up a flourishing garden but few are as important as a well-functioning ventilation system. Gardeners who take the time to properly set up and automate the ventilation system will be rewarded by vigorous growth and bountiful yields.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at

Want more information? Read these articles:

The Basics of Greenhouse Ventilation

The Importance of Ventilation in a Greenhouse

The Importance of Ventilation in Hydroponics

Why Greenhouse Ventilation is Important

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