How do you intend on keeping you room odourless? I mean do the carbon filters work 100%?
Once the room are complete they will be virtually airtight. Ill have an air inlet fan which forces 160m3/hr of fresh air into the rooms and an extraction fan which will remove 350m3/hr of used air out of the rooms. (the formula i used for this calculation is underneath) It's important to ensure the extraction fan is always more powerful than your inlet fan. This is to avoid Negative pressure which basically means instead of all the used smelly air being removed by your extraction fan, some of the air is forced out of gaps within the grow room construction itself. if this happens then you'll be bypassing your carbon filters and releasing odour into areas where you don't want it to be!
2 x carbon filters will be used in my new grow rooms and from tests which Ive carried out already in my old grow room the extracted air is virtually odourless!
just to be extra safe though, ive vented the extracted air into the soil vent pipe of the property the grow rooms are located at. This vent pipe travels up though the loft and out of the roof at an actual higher point than the property itself... no one at all will be able to smell anything!
Dan's Method
Calculating By Room Volume
You will find many calculations on the web for sizing a fan for ventilating indoor gardens; however, what many of these calculations fail to take into consideration is the friction loss on carbon filters and increased temperatures from HID lights. So here's my calculation method which you can use as a guide for sizing an exhaust fan for a growing area (keep in mind that this calculation will give you the lowest required CFM (Cubic feet of air per minute) required to ventilate the indoor garden.)
Step 1: Room Volume
First the volume of the room needs to be calculated. To calculate multiply length x width x height of growing area e.g. A room that is 8' x 8' x 8' will have a volume of 512 cubic feet.
Step 2: CFM Required
Your extraction fan should be able to adequately exchange the air in an indoor garden once every three minutes. Therefore, 512 cubic feet / 3 minutes = 171 CFM. This will be the absolute minimum CFM for exchanging the air in an indoor garden.
Step 3: Additional factors
Unfortunately, the minimum CFM needed to ventilate a indoor garden is never quite that simple. Once the grower has calculated the minimum CFM required for their indoor garden the following additional factors need to be considered:
Number of HID lights add 5% per air cooled light or 10-15% per non-air cooled light.
CO2: add 5% for rooms with CO2 enrichment
Filters: if a carbon filter is to be used with the exhaust system then add 20%
Ambient temperature: for hot climates (such as Southern California) add 25%, for hot and humid climates (such as Florida) add up to 40%.
An Example
In our 8' x 8' room we have 2 x 1000w air cooled lights, and we plan to use a carbon filter. We also plan to use CO2 in this room. The ambient temperature is 90 °F (32°C), however, we will be using air from another room that is air-conditioned. Here's the minimum required CFM to ventilate room:
1) Calculate the CFM required for room (see above.)
2) Add 10% (for 2 air cooled lights.)
3) Add 5% of original CFM calculation (For CO2.)
4) Add 20% of original CFM calculation for Carbon Filter.
5) Air is coming from air-conditioned room so no need to add any other percentages.
6) CFM = (171 CFM) + (171CFM x 10%) + (171 CFM x 5%) + (171CFM x 20%) + ( 0 )= 231 CFM.
This is the absolute minimum CFM required to ventilate your room.
The next step might seem to match the closest fan to this CFM. However, for this example I'd choose a six inch fan with a CFM of around 400 or more, and a 6 inch carbon filter to match. The extra CFMs may seem a bit excessive (calculations on most indoor gardening websites would recommend a 4" fan and a 4" carbon filter) but it's always better to over-spec since we need to compensate for air resistance in ducting too.
Also, as we are using a carbon filter we will need to match the fan with the filter so that the fan that will neatly fit onto the filter.
If all the variables are kept the same and we changed the room size from 8' x 8' to a 12' x 12' then the minimum required CFM would be 519 CFM.
The All-Important Inflow!
An intake port can be anything from a gap under the door to an open window - even a hole in the wall. The best place for an intake port is diagonally opposite from your exhaust fan; that way, air has to pass across the entire room - very efficient. You can put a piece of screen over the opening to keep insects and animals out, a piece of A/C filter to keep dust out, or a louvered shutter or backdraft damper that opens when the fan turns on and closes when it turns off. You can also use a motorized damper. This gets installed in-line with your ducting and is plugged into whatever device controls your exhaust fan. When your fan turns on, it allows air to pass. When your fan shuts off, it seals completely, preventing CO2, air, etc. from passing. You can get creative with these devices and use one fan to control two rooms, etc.
One additional note about intake ports - you will see much better results from your exhaust system if you install a second fan to create an active (as opposed to passive) intake system. Normally, when your exhaust fan sucks air out of your room, air is passively going to get sucked back into the room. By installing a second fan on the intake side, you will reduce the amount of negative pressure created in the indoor garden, thereby cutting down greatly on the amount of work the exhaust fan has to do and allowing much more air to pass through. If you're not sure or you don't want to spend the money, start out with just an exhaust fan. If it's not performing as well as you thought it would, try adding an intake fan - you'll smile when you see the difference!
