Ventilating your growing area is an essential constituent of the grow room. Proper ventilation helps control the humidity, temperature, odor, moulds and diseases risks in the grow area, as well as the elimination of the stale air. The information below includes the description of the ventilation systems parts and equipment, as well as their functioning and the benefits they offer. This data is intended to help in the planning and designing the ventilation system in your grow area. Nonetheless, it is up to a grower what components to choose and what design to prefer. 

How to size your exhaust fan

There are many resources, which offer a number of recommendations on how to choose the fan size for ventilating your grow room. Nonetheless, some important details, as excessive heat from the HID lights, or friction loss on carbon filters, may be omitted here and there, while they are also important when ventilating the grow area. The information provided below can be treated as a guide for determining the size of the exhaust fan for a grow room; however, the resulting figures listed here will depict the lowest requirements, necessary to ventilate the grow area.

Step 1 – Room Volume

It is important to calculate the room volume. To do that multiply the height x length x width of the grow area. For example, a place of 8” x 8” x 8” has 512 cubic feet in total.

Step 2 – CFM Required

The fan should be capable of properly exchanging the grow room air every three minutes. With our example it will be 171 cfm (512 cubic feet divided by three minutes). This figure is the minimal necessary cfm for the ventilating air in the grow room.

Step 3 – Additional factors

It should be noted that the minimum cfm works for the ideal conditions. In the actual grow room such circumstances should also be taken into account:

1. HID lights available and their number – add 10-15% per non-air cooled light, or 5% per air cooled light.

2. CO2 utilization – add 5% for areas with CO2 enrichment.

3. Filters presence – add 20% if carbon filter is available in the exhaust system.

4. Environmental temperature – add up to 40% for hot and humid climates (e.g. QLD), or add 25% for hot climates (e.g. Perth).

Example 1:

If a gardener prefers to use a carbon filter in an 8” x 8” room, he will have to utilize 2 x 1000w air cooled lights; thus, it is also necessary to use CO2. The environmental temperature is 90°F, but a gardener will use air from the air-conditioned room.

Minimum Required CFM to ventilate room:

(CFM required for room – step 2) + (CFM required for room – step 2 x 10% (2 air cooled lights)) + (CFM required for room – step 2 x 5% (CO2)) + (CFM required for room – step 2 x 20% (Carbon Filter)) + (Environmental Temperature 0 (Air comes from air-conditioned room)).

= (171cfm) + (171cfm x 10%) +
(171cfm x 5%) + (171cfm x 20%) + ( 0 )

= 231cfm – this is the absolute minimum cfm required to ventilate your room.

Having done the calculations, a gardener has to find the most appropriate fan to this CFM. Note that using a carbon filter it is necessary to match the fan with the filter in order for the fan to fit onto it.

As a result, ignoring the additional circumstances mentioned in the step 3 and using only steps 1 and 2 for the calculations, a gardener would use a 4” fan and a 4” carbon filter. With the lapse of time, however, he would most likely find out such fan and filter inappropriate for his grow room.

Example 2:

Let us keep all the variables the same except for the room size, which is changed to 12” x 12”. In this case, the minimum required CFM would be 519 cfm, making a gardener to choose from the 8” carbon filter range.

The Really Simple Method:

The ventilation can also be as simple as that: exchange air every minute. Gardening in cooler climates may allow carrying out this procedure once every three minutes.

The Very Important and Often Overlooked Intake Port

For the exhaust fan to be really effective it should not be placed in a sealed room. Instead, a room should constantly receive fresh air, whether it is a door, an open window or any kind of opening or hole in the wall. Otherwise, the fan will work ineffectively, make more noise, use more energy, become hotter and even break down, and still will not do its job.

Thus, the intake port should not be underestimated. It is best to place the exhaust fan opposite the intake port for the fresh air to move across the entire room. However, it is necessary to protect the grow room from the insects and dust that may enter through the intake port. For this reason you can use a piece of A/C filter or a screen over the opening.  Motorized Damper, Back draft Damper, or a Louvered Shutter are also very convenient to use.

Motorized Damper sould be installed in-line along with the ducting and plugged into the device that switches on and off the ventilation fan. It will let the air in when the fan is on, and it will be tightly closed when the fan is off, not allowing the air, odor, CO2 or anything else to pass through. It is even possible to install the system so that one fan could control two rooms at the same time.  

Some growers prefer to use two fans: an exhaust fan to suck the stale air out of the grow room and the other fan to increase the intake of the fresh air. The latter should be either the same size as the exhaust fan or a little smaller. Such system will help increase the amount of the fresh air in the grow room and reduce sucking back of the stale air. In addition, the working load on the exhaust fan will also be reduced. Consequently, if you are not satisfied with the functioning of the exhaust fan, maybe it is time to try installing one more fan to improve the intake system? 

Fan and Carbon Filter Placement

It is recommended to place the fan at the highest point of the grow room if you do not use the carbon filter. It is a common knowledge that hot air rises up and it is it that needs to be exchanged. So the fan will suck the fresh air, and it will move across the room and the plants to the fan, removing at the same time stale air.

If you use a carbon filter, the fan can be installed on its top. However, you may arrange the system so that the fan will blow through the filter, or suck through it.

Most gardeners prefer to put the fans so that they would suck through the carbon filters. The latter may either hang horizontally from the ceiling, or stand upright on the floor. It is best to place them opposite the intake port. Attaching the ducting to eliminate stale air try to make the dutcs as short as possible and without bends to avoid excessive noise and friction of the fan during its work.

The method of blowing through the filter is not very common among the gardeners. If a filter and a fan are used to neutralize the odor, they may need to be placed outside the grow room.

If you are going to use a carbon filter for the first time, suck it and blow through it outside prior to placing it at the working position. These simple procedures will help eliminate carbon dust, which forms in the filter during its transportation. The dust sock should always be kept in the filter during its use. The sock will catch the larger particles of the dust and make the filter last longer.   

Air Circulation

Always provide air circulation in the grow area. This helps avoid stale spots in the room and the development of micro-climates in the leaf canopy. Reduced circulation may cause the increased humidity and higher temperature between the plants. Such micro-climates are favorable for the formation of mildew, multiplying of the pests, and the development of various plants diseases. To ensure adequate air circulation wall mounted fans or several fans in one grow room can be used. It is not recommended to switch them off, unless you plan spraying the plants or fogging in your garden.