Two tent ventilation - Will this work?

What's up Rollitup!?!? I am in the planning stages of a grow operation I would like to set up in the attic of a place I plan to live for the next several years. I want to do this right, and hope to set something up once that I can rely on for years to come. I have a little experience from college, but that was a different decade. I've got a pretty good idea of what I want to do, but would like some help getting through a few specifics.

First off, I have a family, so we want to keep it as legal as possible. My state allows any registered MMJ patient to grow 5 plants, so me and my partner together can grow 10. Secondly, I think I am going with HID lighting for the simple reason that I understand it. I did a lot of research back in the day and nothing seems to have changed except the lights got cheaper. I've looked into LED a little and it's a lot to unpack and the price tags get big fast. I'll stick to what I know works.

I still need to get up in the attic and see what I have to work with, but I plan to add an independent circuit up there. There is a window and I hope to run A/C in the summer and an electric radiator and humidifier in the winter.

I would like to run two 5'*5' tents. My research tells me it is best to have an air cooled light with its own ventilation SEPARATE from the ventilation that scrubs and exchanges the air in the tent. I have also seen that it is ideal to blow air over a light rather than suck it out. I think I can do all this for both tents with only two fans but this is where I need advice from someone with more experience.

I am attaching an early sketch that shows the gist of my idea. In it, the tents are facing each other, meaning I would stand between them and be at the front of each. For exhaust, I plan to put a 6" carbon filter out the top of each tent with an elbow, short duct run to a 6"-8" y adapter which will run to an 8" inline fan rated over 800 CFM with speed controller and out the window. The duct line from the filters to the y will have to be symmetrical, I have thought of using rigid exhaust pipe instead of duct here.
With the lights I plan to use an XL cool tube in each tent. I will run a fan before the tents, where the line reaches the first tent, I will add a "T" adapter to the 1st tent and continue the line to the second tent and tube. The 2 lines will meet at a second "T" after the second tent and light and run out the window. I believe this will create a closed loop with even airflow over both lights.

What I am unsure of here is what I can get away with. Originally, I was happy with 600W lights but I think I want to go up to 1K. But can I put a 1K in a 5'*5' tent? Will a 6" 400CFM fan be enough to cool both lights or should I get a bigger fan?Should I just stay with the 600s? 5 plants in a 5'*5' under 1K. Is that legit? I tried looking through grow journals but so many growers are unorganized and don't even post to the end so it didn't feel very productive.

Lastly, I plan to grow relatively large plants organically in 5,7, or 10 Gal fabric pots. I'd like to have the tents set for a staggered harvest. I will get a T5 for veg that I do in the open attic. I might be willing to occasionally have 15 plants as I get close to chopping five. Will use the tent to dry the harvest also unless someone has a good reason not to.

If you made it to the end of this post thank you for taking the time to read and any input is appreciated.

Another note, here's my ventilation math.
1 tent 5'*5'*6'8"~ 175'^3
2 tent ~350'^3
2 cooled lights (+5% each)
2 Carbon filters (+20% each)
350 + 50%
~525 CFM not accounting for duct length and turns, which are hopefully minimal. Not sure how the "Y" affects airflow but I know an 8" circle has nearly twice the area of a 6" so I don't see that being a problem.

Briefly , the idea of using one fan to vent two tents does work.
But your in an attic and it depends on the temps in that attic as to what cfm you need.
Calculating cfm on tent volume doesn't account for the temp of the ambient air.

I wouldn't be able to cool a tent with a 1000w light with a 6" fan most of the year so doing two with one 6" fan is unlikely to work unless you live in very cold climate.

As for the air cooling of the lights. You cant have a closed loop, the air needs to come from somewhere and exit the attic , the purpose is not to cool the bulb its to get the heat out of the environment otherwise the air will just get warmer and warmer and heat up the whole attic making it counter productive.

The first thing you need to do is go up in the attic and measure the temps as it is and do it often to get a night and day average , this will help you understand how many degrees c you can afford to rise by, though this should be calculated in sumer when its hottest and your ac is running so you know the warmest it gets up there.

Also you want to make sure the attic is fully insulated as best as possible.

Theres a calculation for working out cfm based on ambient air and allowable temp rise.

Calculating the airflow required to maintain a stable temperature, and so determine the size of fan needed, is surprisingly simple.

It boils down to CFM = 3.16 x Watts / DT(°F)

Watts is the lighting power (and any other 'hot' things in the grow space) in Watts.
DT is the allowable temperature rise within the enclosure (i.e. desired temperature minus ambient temperature) in °F.

So, to work out the size of fan required (in CFM) simply plug your lighting Wattage into the equation along with the DT value.

An example is:
Ambient temerature = 20 °C
Target temperature of the enclosure = 25 °C
DT in °C = 25 - 20 = 5
DT in °F = 5 x (9 / 5) = 9
Lighting Wattage = 250 Watts

Plugging these values into the equation gives:

CFM = 3.16 x 250 / 9 = 87.77

This represents the actual throughput required but it doesn't take account of the static pressure necessary to overcome the system impedance (how hard the fan has to suck or blow). But for a free-air system with no ducting or filters it should be fairly accurate.

Also if your stuck on 5x5s you defo need 1000w hps.
Even a lot of folks growing in 4x4 will opt for 1000w hps but plenty still use a 600 in that space

coreywebster said:

Also if your stuck on 5x5s you defo need 1000w hps.
Even a lot of folks growing in 4x4 will opt for 1000w hps but plenty still use a 600 in that space

Click to expand...

Hey man thanks for the reply and the formula. I misspoke when I said "closed loop". If you reread my description and/or look at the drawing, I am taking ambient attic air, pushing it with a 6" fan, splitting it over the two lights, rejoining it to one line, and then exhausting that outside. What I meant was the two lines split and come back together in a way that should make each line the same length and amount of turns. I meant the air flow should be even by closing the loop between the two separate lines. As for ambient air Temps I am hoping to be able to run a/c and heat during hot and cold seasons and keep ambient Temps between 65-75. So ideally, ambient Temps are fine and the air in each tent is being replaced about 1.5 times a minute by the 8" inline. So in this setup, does the 6" inline at 400cfm have the capability of cooling two 1Kw lights enough to keep them in the tents and close to the plants? That's the fans only job as air exchange in the tent is handled by the other fan and ambient Temps are controlled in the attic as a whole. I've seen people build manifolds and cool multiple lights with one fan, which is where I got this idea. I just don't know how hot a 1K really gets.

Ah yeah I get what you mean now, just didn't notice the bottom corner or the drawing where it goes off due to my screen been cracked.

So the amount of heat removed within the light cooling will depend on the air flow speed, obviously an 8" will move the air faster removing the heat quicker and thus less heat from the cool tubes will affect the tents.
Any size fan will remove some heat.

If I had 8" cool tubes I would use an 8" fan because it gives you more control and heat removal which means your ac will run less in summer and in winter you turn both extraction and air cooling down and save on heating bill.

But yeah you can use a 6" if you like, will just be less airflow so less cooling.

Not sure which cool tubes your planning on but they make them with full size hoods over them which will be far better than the usual tin hat cool tubes.

vs
The first gives far better spread of light.

idk how far those lines actually run but i believe if they are running more than 5 feet on each "side" I would either get two 6 inch or 1 8 inch. I've read that's it also easier for the fans to pull than push ‍ on that one but makes sense to me

coreywebster said:

Ah yeah I get what you mean now, just didn't notice the bottom corner or the drawing where it goes off due to my screen been cracked.

So the amount of heat removed within the light cooling will depend on the air flow speed, obviously an 8" will move the air faster removing the heat quicker and thus less heat from the cool tubes will affect the tents.
Any size fan will remove some heat.

If I had 8" cool tubes I would use an 8" fan because it gives you more control and heat removal which means your ac will run less in summer and in winter you turn both extraction and air cooling down and save on heating bill.

But yeah you can use a 6" if you like, will just be less airflow so less cooling.

Not sure which cool tubes your planning on but they make them with full size hoods over them which will be far better than the usual tin hat cool tubes.

View attachment 4800333 vs View attachment 4800334
The first gives far better spread of light.

Click to expand...

Yeah thanks man. I was looking at the xl and XXL reflectors for sure. I want to spend as little as possible without doing anything to negatively impact the setup. Obviously a big kahuna type closed glass reflector would be best but that adds another couple hundred dollars to the grow and I don't think it is essential.

I have been doing a lot of research on HVAC forums, learning about air pressure and flow in ducting. It seems like it might be less of a headache, and in the long run possibly more efficient, to have four 6" fans going. One for each light and one for air exchange in each tent. Idk y but this seems ridiculous to me. Having four duct runs out my attic window just seems Ludacris. I'm not even worried about stealth as this is a legal medical grow, but that will just look like I have something insane going on. I guess I could join the runs to a larger duct before the window.

Monster_of_Au said:

idk how far those lines actually run but i believe if they are running more than 5 feet on each "side" I would either get two 6 inch or 1 8 inch. I've read that's it also easier for the fans to pull than push ‍ on that one but makes sense to me

Click to expand...

Yeah for the light cooling design I have drawn up we're talking about 50' of duct plus two 90 elbows and two wyes or tees. I have read that sucking air is hard on the light, and that blowing cold air over the light is more effective than pulling hot air away from it. The air exchange fan will be sucking through the filter.

I would avoid the T and use a Y less reduction.