Need a Ventilation Guru

Hey ppl this is my first grow. Got most of my stuff today at local hydro store. Im doing a one site scrog in a small tent, GH Waterfarm, 400W CMH. The tent is 63"H X 36"W X 22"D. The tent is up and the light is in, now I'm trying to figure out what I want to do to cool the damn thing. I bought a honkin 4" inline fan with carbon filter to vent hot air out the 4" hole at the top rear of the tent. Im thinking its a little overkill for one plant and this size of tent, plus i'll have to mount it somewhere behind the tent and the whole rig sticks like 18" inches out the freakin back.

I did a trial run with the lamp on and the top 6" flap open to vent hot air (no fans). The bottom flap was also open. The ambient temp of my basement is around 58 degrees this time of year. Within an hour the temp was up to 94 in the tent. Seems to be stabilizing around 95, pretty toasty. So ya, I need some definite ventilation domination.

So here's where my options come in for you knowledgeable folks. I have have two 6" holes on the sides of the tent intended for cooling the lamp/reflector, there's another 6" on the top, and a 4" on the back wall. There's a large intake flap at the bottom with another 4" hole beside it. I'll post a picture of the tent below so you can see what I'm working with.

I'd really like to stick with the open air reflector if possible, that's why i tried to get around using the ducting and cooling fans. What about a couple intake fans? Do I need to exhaust the air at all?

How do you plan to situate the fan/filter? I'm thinking that if you have your intake (the place where air is drawn out of the tent toward your fan/filter) high up in the tent (above the lamp's level) you should do fine. From the tent's perspective, it'll be the exhaust, of course.
Another "trick" is to have a fan (household pedestal style, or a clamp-on) moving air up toward the lamp to cool it.

But you really need to have a trial run with your fan set up as you'd planned.
I cooled a 600W grow quite effectively with a 100cfm 4" fan drawing air through a cooled hood.
But you need air exchange. Wjhether you have a fan pushing air in, with passive exhaust ... or a fan pulling air out (recommended) with passive intake ... either way you are exchanging hot in-tent air for cool outside air. this is what you definitely need, and I believe 1 fan, and that 4" to boot, will be enough. cn

Is your filter located inside your tent sucking air in through it and exhausting it? What is the CFM rating on the fan?

[h=2]Ventilation Explained
Part 1 - Ventilating your growing area[/h]Correctly ventilating your growing area is a vital part of greenhouse/grow room design. Adequate ventilation in the grow room is important for a number of reasons – temperature, humidity, disease and odor control, and fresh/stale air exchange. The calculations that are used to correctly design a ventilation system can be quite complex and require numerous known variables. The purpose of this article is to simplify these calculations while helping you to better understand grow room ventilation.



How to size your exhaust fan

There are many calculations on the web for sizing a fan for ventilating grow rooms; however, what these calculations fail to take into consideration is the friction loss on carbon filters, increased temperatures from HID lights and CO2, etc. The following calculation can be used 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 required to ventilate the grow room):

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
The fan should be able to adequately exchange the air in a grow room once every three minutes. Therefore, 512 cubic feet/3 minutes = 171 cfm. This will be the absolute minimum cfm for exchanging the air in a grow room.

Step 3 – Additional factors
Unfortunately, the minimum cfm needed to ventilate a grow room is never quite that simple. Once the grower has calculated the minimum cfm required for their grow room the following additional factors need to be considered:

1. Number of HID lights – add 5% per air cooled light or 10-15% per non-air cooled light.
2. CO2 – add 5% for rooms with CO2 enrichment
3. Filters – if a carbon filter is to be used with the exhaust system then add 20%
4. Ambient temperature – for hot climates (such as Southern California) add 25%, for hot and humid climates (such as Florida) add up to 40%.

Example 1:
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, however, we will be using air from another room that is air-conditioned.

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)) + (Ambient Temperature 0 (Air coming 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.

The next step would be to match the closest fan to this CFM. 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. The filter that we will use is a Dutch Breeze Carbon Filter - DFS4. The DFS4 has a 6" flange. Our options for 6" fans are as follows:

- Eco Plus 6” – 440cfm
- Fantech 6” – 252cfm
- Fantech 6” XL – 392cfm
- Elicient 150B – 309cfm

The DFS4 filter has a cfm rating of 255-309, so the fan that we will choose for our room will be the Elicient 6” fan. This will be an ideal fan for this room and carbon filter, since the extra cfm will help compensate for the small amount of ducting we may have to run.

If only step one and two was used (the calculation that is most commonly displayed on garden websites) then the grower would have chosen a 4” fan and a 4” carbon filter; the grower would have soon found out that this fan and carbon filter would have been inadequate for their grow room.


Example 2:
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, thus the fan would be Fantech 8” XL, Elicient 8” orEcoPlus 8” and the DFS5 or DFS6 Dutch Breeze carbon filter.

The Really Simple Method:
If you want to keep things really simple, just remember that you want to replace the air in your grow room every one to three minutes. If you're in a hot area, exchange it every minute; if you're in a cooler area, you can take up to three minutes.

The Very Important and Often Overlooked Intake Port
It is very important to understand that you can't simply put an exhaust fan in a sealed room and expect to suck air out of it. You absolutely must have some kind of opening in the room to allow fresh air to replace the air you are exhausting. If you don't understand this concept, just put your finger over one end of a straw and try sucking out of the other end - you'll find that the walls of the straw collapse inward and that you can't suck any air out. If you were to poke a small hole somewhere in the straw, you will find that you can now suck air through it; the bigger the hole, the more air you can suck. Obviously, any fan you install in your grow room isn't going to be powerful enough to create enough negative pressure to suck the walls inward, but what will happen is that your fan will draw more energy, make more noise, run hotter, break down much sooner than it should, and finally, suck very little (if any) air out of the room. 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 aLouvered 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, odor, etc. from passing. You can get creative with these devices and use one fan to control two rooms, etc.

One last 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 grow room, thereby cutting down greatly on the amount of work the exhaust fan has to do and allowing much more air to pass. Ideally, the intake fan should be the same size as the exhaust fan, but it can be a little smaller if necessary. 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!

Fan and Carbon Filter Placement
If no carbon filter is being used then place the fan at the highest possible point in the room; the reason for this that hot air rises and it is the hot air that needs to be exhausted. Also, place the fan on the opposite side of your intake. The negative pressure that the fan creates in the room will pull air from the intake opening across the garden and towards the fan. This will bring the fresh air over the plants while displacing the old stale air.

If a carbon filter is being used then place the fan on top of the filter. There are two options that a grower can use with the carbon filter: 1) Suck through the filter or 2) Blow through the filter.

Sucking air through filter is the preferred method. Carbon filters should be placed on the floor standing upright or hung horizontally from the ceiling. Place the fan and filter in the room opposite the intake port. From the fan, attach ducting so as to exhaust the scrubbed air out the room. Keep the ducting as short as possible and avoid any bends in the ducting. Keeping the ducting as short and as straight as possible will reduce the friction loss and noise.

Blowing through the filter is a less commonly used method of exhausting. The benefit is that the fan can be placed at the highest point in the room and the hot air is sucked out of the room and forced through the filter. However, if you're using the fan/filter combination as part of your exhaust system (as opposed to just an odor scrubber), it will need to be placed outside of the growing area.

Before using a carbon filter for the first time always take the filter outside and then first suck, then blow through it before installing it in your growing area. This will blow out any carbon dust which is created in the filter during transport. During normal operation always keep the dust sock on the filter; this stops large dust particles from blocking the filter and will help prolong the life of the filter.

Air Circulation
Always provide your plants with adequate air movement and circulation in the grow room. Air circulation within the room eliminates stale spots and also reduces the formation of micro-climates within the leaf canopy. If air circulation in the grow room is poor then the micro-climate in between the plants increases in humidity and temperature. This can result in mildew and insect problems, as well as less deficiencies. For adequate air circulation use a wall mounted fan; 2-3 fans will provide sufficient air movement. Never switch these fans off unless you are spraying or fogging your room.

cannabineer said:

How do you plan to situate the fan/filter? I'm thinking that if you have your intake (the place where air is drawn out of the tent toward your fan/filter) high up in the tent (above the lamp's level) you should do fine. From the tent's perspective, it'll be the exhaust, of course.
Another "trick" is to have a fan (household pedestal style, or a clamp-on) moving air up toward the lamp to cool it.

But you really need to have a trial run with your fan set up as you'd planned.
I cooled a 600W grow quite effectively with a 100cfm 4" fan drawing air through a cooled hood.
But you need air exchange. Wjhether you have a fan pushing air in, with passive exhaust ... or a fan pulling air out (recommended) with passive intake ... either way you are exchanging hot in-tent air for cool outside air. this is what you definitely need, and I believe 1 fan, and that 4" to boot, will be enough. cn

Click to expand...

Thanks for the quick responses.

I have on open framed wall behind my tent, so i was just going to mount the fan on a stud, exhausting hot air out into my carbon filter. The flap at the bottom id leave open for negative pressure.

You should be pulling air through your filter not pushing out through it.

Catchin22 said:

Is your filter located inside your tent sucking air in through it and exhausting it? What is the CFM rating on the fan?

Click to expand...

The fan/filter isnt even hooked up yet, just preliminary planning, right now on what i want to do. The fan is 170 CFM.

If you have a inlet vent down low and your filter up high it should pull in cool air from below and the hot air out the top. It might still get higher temps up high but down around the plants should be good. You can run higher temps too, lots of guys have run 95 or higher even, more C02 is required with higher heat AFAIK.

Catchin22 said:

You should be pulling air through your filter not pushing out through it.

Click to expand...

so i should mount the filter on the intake side of my fan?

Yup, no doubt I just looked at a pic on the website they have it mounted that way.

You could keep you canopy temps around 80f (which I highly recommend for fast maturation and icy buds) using just the 170 cfm fan. Mount it up high inside the room and keep the flex tubing as short as possible. The only trouble might be if the intakes holes are too small in which case maybe you can modify them lol. Good luck!

The simplest way to do it is to place the filter inside the tent, then draw air through it by an exhausting fan. the trouble with this is that you're using valuable area, unless you suspend the filter at the top and back of the tent. However it will compete for space with the light hood.
If you are handy, consider placing the carbon filter in a sealed box/plenum. The fan is downstream, pulling air away. the duct from the top of the tent runs down into the plenum. This way you're using the filter correctly and not using tent space. cn

cannabineer said:

The simplest way to do it is to place the filter inside the tent, then draw air through it by an exhausting fan. the trouble with this is that you're using valuable area, unless you suspend the filter at the top and back of the tent. However it will compete for space with the light hood.
If you are handy, consider placing the carbon filter in a sealed box/plenum. The fan is downstream, pulling air away. the duct from the top of the tent runs down into the plenum. This way you're using the filter correctly and not using tent space. cn

Click to expand...

Sorry had to look plenum up. That is going to be my best bet Im thinking. Can I just tie string around the filter to suspend it where i want it sucking the air out, say around canopy height.

A rope maybe, they aren't light!

Ya this one's about 5 pounds.

Anyways I stayed up pretty late last night and got the fan mounted. There's going to be plenty of room in there to have the filter suspended by the side of the light. I'm going to do another trial run shortly to see how the fan does on keeping temps down.

Thanks again for the help.