milsurp

OK - I have everything set up. What I have is a lopsided affair I think and it may be my problem. I have a 180cfm (really 140 cfm with safety screen/4" adapter installed over intake) and an 87 cfm feeding a 2x3' by 4'10" tall closet. At the top of this 4'10" area, the area above it has been fashioned into a simple steeple whose apex is in one corner of the closet ceiling. IOW the back wall and one side wall start angleing in at about 60 deg and it basically makes a cone shape. At the top is a 6" inline duct fan rated at 250 cfm MAX - I think that has to be assisted in an already pressurized duct because it feels like about like th 65 cfm fan I have pulling air through my light. 6" fan vents to attic and terminates in a louvered door vent that only opens one way to prevent hot air from the attic getting into the house. Same type of vent as used for lots of 4" dryer vent applications on houses - only its 6".

Anyway - 87 and 140 cfm in (227 cfm in) - estimated 65 cfm out.

The closet is highly pressurized. Is that why no matter what I do, I can't get it to run cooler than 88.5 deg f in there?

Should I be working towards negative instead of positive pressure in the closet?

I mylared and its a bit too warm in there. RH hoveres around 30% fwiw.

My "steeple" idea might not be all bad, but currently, I think theres a little too much turbulence (rightly so!) in the closet for there to be a separation of hot/cold air and for the inline duct fan to be removing "hot" air. I's just venting "warm" air - and probably not enough of it. I is definitely positively pressured now - crack the door and a very noticeable breeze hits you.

Suggestions?

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Harv

very interesting....here is my thoughts...going why back to chemistry. Pressure will force ( I think its molocules, closer together ) which raises kinitic energy. which in turns raises heat. So less pressure does the opposite effect by lowering energy and lowering heat. Like the vacum of space is so cold. Thats my thoughts they maybe wrong or I may have called something the wrong thing. but none the less thats my

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Mendrel

Negative prossure will also help you direct any smell, if that's an issue, to where you want it. Another problem is that I believe you may not have enough intake\exhaust space. Just going with area and ignoring fan speed, you have two intake fans (one 6" and one 4"?) and a 6"(?) exhaust, I may not have the measurements right though. But you have a larger intake than you do exhaust. If you want them to equalize then you need equal power in an equal area (ie a 350 cfm, 6" fan intake and the same exhaust) Since you have more intake than exhaust, you get the positive pressure. If you reverse your fans, like I think you are suggesting, then you will get negative pressure as you have a bigger, more powerful exhaust. Another factor is your outside temp, where is the intake drawing from and how cool is that air? If the air outside is still 85F, that's as much as you will be able to cool the room to without an AC.

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Medicalman

Using powered fans for exhaust is usually far more effective than powering your intakes. Consider that very few grows are truly airtight. If you force air through intakes, you may well be forcing lower cooler air out through cracks as well as out the exhausts. Powering exhausts up high where the air is hotter allows you to draw out the hottest air, regardless of where the replacement air is coming in from.

If you use fans as exhausts, negative pressure will indicate that you are restricting airflow on the intake side, thus reducing cooling efficiency. The opposite is true if using powered intakes. Positive grow pressure in that case means that your exhaust is restricting your airflow, reducing cooling efficiency.

You also need more passive opening size than powered size, no matter which way you do things. If you have 2 4" powered exhausts, you would want 3 4" unpowered intakes to allow air to flow freely.

BEst
MM

milsurp

Thanks for all replies. More info:

I have built a closet inside another, larger closet. This is a fill closet with a door, trim, sheetrock, texture, matching housepaint, ect.

When you open the door to the grow closet, the top 1/4 of the door has the slanted ceiling going across it. You are looking at the edge of one of the3 "steeple" pieces. The back of the closet - its ceiling panel angles in towards you. At the upper left, near the top of the door, is the apex to the steeple and the 6" exhaust venting to the attic (one way airflow - attic is about 120 deg f during summer.

Intakes are on the right side of closet towards the top to avoid having them crowd the plants when light is towards top of closet. Fans are mounted on the exterior of the right side closet wall and are inside the original closet (air is taken from inside my home's AC'ed living space air). This main closet has its own AC vent and I recently installed a bathroom type exhaust fan/light in the main closet to get rid of any warm air that might have accumulated from the two intake fan motors.

I keep the thermaostat for the house just above 75 deg. Actual temp in the main closet is usually 81-82 f with 37% rh. That is the exact condition of the air right above the intakes - that's what's being shot into the cab.



Done some experimenting. With the exhaust fan (6") off, the system doesn't really care too much - temp is 88.5 f. 87.1 is the lowest I can get it with the current setup of exhaust.

Not horrid, but I'd like to bump about 5 more degrees off of that.

Wil start planning to rearrange - more suck, less blow.

Thanks!

fyi, but unrelated: outside temp this afternoon is 100 with 28% rh

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suburbanfarmer

That's partially true, but at the pressure differentials you're dealing with, compression and rarefaction of air molecules is not going to have an effect on temperature. Medicalman's advice was right on- you should have exhaust fans at the top of your grow cab, and passive intakes at the bottom. This way, not only is the rising hot air forced out of the cab with no chance to circulate, but also the heat emitted by your lights is exhausted without a chance to circulate. If you have powered intakes, those fans may be circulating the air enough to actually move some of the hot air around the bulb downward, so if you have enough area for passive intakes (as stated by Mm) use all your fans for exhaust and you shouldn't really need any for intake.

If smell is an issue than doing this is even more important, as in a positive pressure closet, the air will escape through cracks and so will the smell. In a negative pressure situation, all the air that's leaving your cab has to go through the exhaust ducts and whatever odor elimination system you have installed in there.

naturall_mystic

You got it right milsurp, more suck less blow. for that matter if you have clear intak vents you could just use the fans you currently have blowing to the exhaust side of things in the upper half of the cabinet. That will give you negative pressure. It would be more efficent to have one large exhaust fan, which would also be helpful should you find you need to run that exhausted air through a carbon filter to keep odor down.

Good Luck

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milsurp

Many thanks for the replies - sorry it took so long to respond. Reason it took so long is that I had a lot of rebuilding to do....

I tried to really think this out and remember that the air doesn't care where I *want* it to go - its gonna go where physics directs it.

I wanted to work hard on cooling the source of heat directly - ie: "this air is hot - get it out of the closet right now - don't wait for flow or air rise - get it out now". For this reason, I decided to try to pull almost ALL of my exhaust air through or right past the bottom of my light.

All four fans have been moved to the top. They are mounted above the "steeple" outside the grow closet, but still inside the "normal" closet. They exhaust to the attic through one way dryer vents. I was trying to leave no stone unturned this time ant took every step to try to make this work. Through using reducers, I switched the 6" exhaust pipe from the anemic 6" inline fan that's at the apex of the steeple and put it on the 180 squirrel cage that's evecuating the light. The 6" inline barely had enough power to open the 6" exhaust flaps. The squirrel cage blasts the vents open like they're being hit with a hammer! I then put one of the 4" exhausts on the 6" inline fan using a reducer - not too restrictive with the low cfm of the inline and the inline can now efficiently open its exhaust vent.

The 180 has 4" semi rigid flex duct (all venting is now semi rigid flex duct) going down to a 4x4x4" T that's about 6" from the top of the light. The two "legs" of the T are ducted to BOTH sides of the Sun System I hood.

"If both vents are being pulled from , then pray tell, where is the air getting *into* the light from?". Well - that's a story in itself....

For anyone who hasn't tried it or is a doubter, let me assure you, you CANNOT alter tempered glass after it has been tempered!!! I read that this was the case, but "hmmmph - what do they know?". I cut off just the corners of the tempered glass and made two small cutouts on the sides of the glass. My purpose was to try to get more air flowing up through the light. I finished my shaping and polishing, washed the glass, dried it, and laid it on my bed til I was ready to install it. A few minutes later, I hear this "pop" in the bedroom.... I walk in to investigate and find that the glass has exploded! It broke just like tempered glass is supposed to do for safety purposes, that is, it broke into a million little fragments.

I still wanted more air through the light and needed glass asap, so I had some non tempered glass cut for me. What I had was a 1/8 x 11 1/4 x 20 1/2" piece of solid glass (well, it *was* solid until I tried to alter it). What I have now is four panes of 1/8 x 11 1/4 x 5". This leaves me about a 1/8" gap at five places: both ends, and three gaps in the middle. These gaps, holes for mounting hangers, hole for light fixture, and other nooks and crannies are how air gets through.

At both ends of the hood, resting right up against the hood, are 4" exhausts sucking air across the bottom of the light. These are powered by 87 cfm computer fan on the left, and a 65 cfm comp fan on the right.

Apex still has the 6" inline.

For intakes, I tried to be creative there too. Only one wall can be used for intake - the wall on the right. How to get cold air distributed down low more evenly? I took 4" smoothwall duct and ran a 2.5' length of it along the floor behind the ebb & flow and 2' in front of the e&f. These ducts enter the closet from the absolute lowest and outermost corners of the right wall. They direct cool air towards the bottom left of the closet. Two other 4" vents are one on top of the other on the right wall, centered, and just above the top of the e&f. This will provide air from the right and give a little airflow through the foliage.

So far, so good, I think.

I wasn't able to take a full five degrees off of the 88.5 mark. Only able to get it down to 85.5, but when you consider that I'm starting off with 80-81 deg air in the normal closet, I don't think this is too bad. 4-5 deg increase with lights on using a 400 w in a closet; is this an average increase? Coments?

At least I don't have to worry about managing drastic changes in "day/night" temps - they're almost the same!

TIA

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grass_hopper

ditto naturall_mystic - more exhaust than intake will create a neg pressure and keep your temps in check. It will do nothing for your humidity, tho. If you're going to maintain neg. pressure there's not much you can do about RH - even if you ran a dehumidifier it's going to basically be on 24/7 and always struggling to keep up, esp. on muggy/ rainy days.

Important: the exhaust fan is going to be less efficient if it's got back pressure. The intake will work at slightly higher than it's rated CFM but the exhaust will run at less. It's hard to estimate, but the greater the delta between the exhaust and the intake, the lower the efficiency on the exhaust unit.

Dont' expect that the fans will do the job you want them to do right out of the box. My recom. is to get 2 fans of the same CFM, but both much larger than you think you need (like 2 or 3x). Then crank them back with fan speed controls or rheostats. The extra capacity will also give you more control on those peak days when the ambient temps are out of sight.

See if you can get the intake running just about 10% less than the exhaust to maintain the neg. You can fine tune the speed of the fan until your temps are stabilized. This will also reduce fan noise & rattling of the bearings. Get rhostats with some increments (or mark them yourself), so when you have to adjust for temps, you can adjust both proportionately.

If you set it up on a hot day and you won't have to go back and readjust (unless it gets even hotter) until the temps really drop. Let it run for a few minutes and then test it with smoke from a cig, joint (!), matches that you light and blow out, etc. Smoke should flow pretty much directly from one fan to the other. Once the neg pressure is working, it won't matter if the exhaust is on the top or bottom - the mech. air movement will overcome any natural thermal convection. Think about using one of your small fans for a cool tube or hood vent, ducted right to your exhaust.

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iwishicould

My thoughts on the situation exactly! A cool tube dramatically changes everything with regards to temp and allows you to bring your lights closer to the plants thus allowing you to drop your ceilling height as well (increase lumen) via a fake ceilling,heat not pulled out by the cooltube heat can be exhausted to the space above your false ceiling through a exhaust in your false ceiling (computer fan) and then vented out of the top of the box (another computer fan). If you exhaust above the false ceiling you also have less work when dealing with light leaks too.
You would require a really good exhaust fan for the cool tube I suggest inline and two computer fans,as for the intake I would situate it underneath the plants if possible as a passive in take covered by a grill
Best o Wishes!
iwishicould

P.S. grow cabs make things much easier to control!

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