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Air Exchange with Led set up


GtotheA
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Hello, I'm thinking about setting up a growroom in which i will put 2 Led lamps which generate way less heat than the HPS do. What i know is that the air circulation in the box should be at the point of 20-30 times per hour. I have read some articles about the maths you need to do to calculate how much air flow from the vents you should have in your box but this calculations are specified for HPS lamps that generate more heat. If my temperature is around 26-28 celsius deegres should i still ventilate the box at least 20-30 times per hour or does that imply only when HPS lamps are used ? From what i read i can't define if the air exchanging is important only because of the heat or also the plants won't grow if not enough is provided. I'm using Co2 tablets in the room so even if the ventilation should be stronger to get more Co2 i think this is already covered.

Mostly i'm scared that if i invest in those expensive vents or put couple of those cheaper ones together and get them to work the temparature will drasticlly fall down and it will be hard to get it up. Does anybody here has a Led lamp set up and this kind of a problem ? The box's size is 2,4m x 1,2m x around 2,5m because the height is adjustable. This gives me 7,2 m3 and if i want to have the box propely ventilated my fan that takes the air out of the box should be around (7,2 x 30 = 216) so around 216m3/H ? 

I would really appreciate any help provided i got really confused with this led growroom ventilation. previously i had a 400W lamp with a cool tube in a smaller box so it was easier to make a working setup with the proper ventilation.  

With Regards Gouda. 

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Also i have just read about the Vapor pressure deficit and i was wondering if anyone has some tips how to controll it with Led lamps ? How does changing air in the box effect the VPS ? and if i am going to moisten the air with a humidifier should ventilataion be bigger so the water won't saturate the air ?

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Found this method on the internet somewhere, can't find the link but I saved the text so here you go :)

1m³ = 35,3ft³

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 (I use this to calculate LED) 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!

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Number of HID lights add 5% per air cooled light (I use this to calculate LED)

 

Thanks very much for your reply it really helped, i'v been reading about diffrent calculations, this seems to be pretty good with a diffrent approach - in the article i was reading they were multiplying the Watts of the lamp by 0,6 in gardens bigger than  5m3 and the result was just to high - I would have to spend 250euros on a vent :). Do you think this calculation for led as the same as HID is accurate ?  Are you maybe using Led lamps in your set up ? :) Thanks anyway very much for your time. 

 

Yesterday i was testing the lamps with no vemntilation in the box and the heat was building up but i think it's mainly because the lack of ventilation + high temperature of the air as it is summer. The amount of heat that the lamps produce is actually similar to HPS with a cool tube and a powerfull fan on it. So to sum up i think that the calculation for an HID air cooled lamp is going to be quite simmilar to the Led lamp and it will be optimal for such a grow space. Those are jus assumptions so if i will get going soon with my project I will post the results in temperature as I check it. 

 

Thanks again with regards Gouda.

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It's hard to calculate exactly and it's always better with too much than too little. I am right now on HPS, but the room I'm building now will mainly use LED's so I used the calculation as I said and it seemed to be alright. I am at 125 CFM but will have a system for 200 CFM so I can expand it or add extra light :)

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