There are two ways to calculate your fan requirements.

Calculating By Room Volume & Calculating By Wattage

1. Calculating By Room Volume 

You will find many calculation methods for calculating the size of indoor garden ventilation fan on the Internet; However, many of these calculations do not take into account the friction loss of carbon filter and the temperature rise of HID lamp. Therefore, this is my calculation method, which you can use as a guide for determining the size of the exhaust fan in the growth area (remember, this calculation will provide you with the minimum CFM (cubic feet of air per minute) required for indoor garden ventilation)

Step 1: room volume
First, you need to calculate the volume of the room. Calculate the product length x width x height of the growth area. For example, the volume of an 8 ‘x 8’ x 8 ‘room is 512 cubic feet.
Step 2: CFM required
Your exhaust fan should be able to fully exchange the air in the indoor garden every three minutes. Therefore, 512 cubic feet / 3 minutes = 171 CFM. This will be the absolute minimum CFM for indoor garden air exchange.
Step 3: other factors
Unfortunately, the minimum CFM required for indoor garden ventilation has never been so simple. After the grower has calculated the minimum CFM required for the indoor garden, the following additional factors need to be considered:
Number of HID lamps – 5% increase per air-cooled lamp or 10-15% increase per non air-cooled lamp.
CO2: for rooms rich in CO2, increase by 5%
Filter: if carbon filter is to be used in the exhaust system, add 20%
Ambient temperature: hot climate (such as Southern California) plus 25%, hot and humid climate (such as Florida) plus 40%.

The most important inflow!
The air inlet can be anything from a crack under a door to an open window – or even a hole in the wall. The best position of the air inlet is diagonally opposite to the exhaust fan; In this way, the air must pass through the whole room – very efficient. You can place a screen in the opening to prevent insects and animals from entering, an air conditioning filter to prevent dust from entering, or a shutter or backflow damper that opens when the fan is on and closes when the fan is off. You can also use electric dampers. This will be installed in your piping and plugged into any equipment that controls the exhaust fan. When your fan is on, it allows air to pass through. When your fan is turned off, it will be completely sealed to prevent the passage of carbon dioxide, air, etc. You can use these devices to be creative and use a fan to control two rooms, etc.
Another note about air intake – if you install a second fan to create an active (rather than passive) air intake system, you will see better results from the exhaust system. Usually, when your exhaust fan sucks air out of the room, the air will be passively sucked back into the room. By installing a second fan on the intake side, you will reduce the amount of negative pressure generated in the indoor garden, greatly reducing the workload of the exhaust fan and allowing more air to pass through. If you’re not sure or don’t want to spend money, start with the exhaust fan. If it doesn’t perform as well as you think, try adding an intake fan – you will smile when you see the difference!

2. Calculating By Wattage

This method of calculating fan requirements does not include active cooling for air conditioning system or cooling pipe design. What we are talking about here is the daily growth chamber, completely closed air flow control, and there is no large amount of radiant heat in and out of the box. For these reasons, your mileage may vary.

1) Start from scratch and design correctly! Before you buy or cut anything for a new project, please determine the maximum air inlet temperature when the lamp is running. Call it t (inlet).
2) Use these formulas to determine the temperature difference you can tolerate. 80 ° f (27 ° C) is almost the optimal temperature for most plant growth. If necessary, you can raise the temperature to 86 ° f (30 ° C), but the target temperature is 80 ° f (27 ° C).
Tdiff = 27 ° C – t (intake air temperature)
3) Add up the watts of all power supplies in the indoor garden. Lights, pumps, heaters, humidifiers, radios, coffee machines, etc! Add it all up and call it watts. If it turns on for more than three minutes and uses more than one watt, add it up. This will make your number the worst case, so it is a conservative value.
4) Use the following formula to calculate the absolute minimum fan power you need. Fan power is measured in cubic feet of air moving per minute. The following formula is the minimum fan rating required to achieve the temperature target. You will have to increase fan power to compensate for pipe shrinkage, small inlets, carbon scrubbers, screens, or other obstructions to the air flow.
Cfm = 1.75 x watts / tdiff (degrees Celsius)
If you prefer to work in Fahrenheit, try the following formula:
Cfm = 3 x watts / tdiff
5) At least get this fan power, otherwise don’t ask questions! If you will have multiple fans, if you want to add different CFM ratings, they should be installed side by side rather than in series. For series fans, use the lowest airflow rating of all fans in the path. The fan at the air inlet and the fan at the air outlet of the box are considered as in-line fans. Fans that circulate air only in the indoor garden should not be included in the airflow, but must be included in your initial power calculation.

You have several options:
1) stop growing for a while until everything gets cold, or try to run the growth lights at night when the entrance air is cool.
2) Reduce lighting to reduce heat load. If the incoming air has exceeded the critical value when it reaches the box, it is not good. This may be possible if the inlet air temperature is low, but you run too many lights to keep up with the cooling rate.
3) Use active air conditioning.

Well, you have it – two very different ways to calculate your extraction requirements. What do you think? Do you like one of these two methods or neither? Or maybe a combination of the two?