4sqft. 250w cab design (looking for input)

[TheGreenBastard]

After a long absence I am finally building a new grow cab.

My dimensions will be 2'(l)x2'(w)x3'(h) using a single 4'x8' 1/8" hardboard and four 8' 2"x2".

I recently got a good deal on a 250w Sunburst that I will be mounting to the top of the cab with a adequately sized hole as to increase vertical space.

The cab will be exhausted with a single 120mm Delta fan rated at ~100cfm which will be attached to a DIY Carbon filter. The intake will be a 4"x8" vent opposite of the exhaust.

I plan on running four plants in 12" square pots using a mostly soil-less mix. It is likely that this will later be expanded into a scrog box.

I am currently acquiring all the need components and hope to have the cab running by the 1st. I will be stopping by HD at some point this week to gather the majority of my materials.

Basically I started this thread so I don't forget anything or overlook important details while building this cab (once I get all my materials I'll start uploading pictures and begin construction). Also, any specific advice regarding ventilation would be greatly appreciated. I am certain that this design will have enough ventilation, but I am worried about having too much. My biggest concern is that the CFM needed to cool my space will be to much for the plants themselves.

Any questions, concerns, and/or comments are welcome.

 

[pinecone]

I am certain that this design will have enough ventilation, but I am worried about having too much. My biggest concern is that the CFM needed to cool my space will be to much for the plants themselves.

You should not be worried about having too much ventilation because you don't have too much. I run a 400w and a 6" Vortex (440cfm) fan with no speed controller in cab that is just a bit bigger than yours - no problem.

Pine

 

[TheGreenBastard]

I'm worried about having too many air changes per minute. If my rough calculations are right I should have about 3ACpM with this design, which can be stressful for plants. I'd like to cut that to about 3 minutes per air change.

This is really all guess work until I build the cab, but if I do end up having to much airflow I probably just get a fan controller.

 

[gnaut]

are you exhausting all of the air through one scrubber or are you venting your light and plant area separately ?

 

[TheGreenBastard]

I'll be venting through the scrubber only, the ballast is self-contained in a non-vented hood without glass so I didn't have the option to separate them. I defiantly would have given the opportunity but I don't want to have to mod the hood.

Another concern I was having was if my WFB1212HE will have enough static pressure to push through a DIY scrubber, it is rated at 6.3 mmH20 and I don't plan on having a very big filter so I think I should be alright but I can't be sure; ventilation is defiantly not my strong suite.

 

[pinecone]

I'm worried about having too many air changes per minute. If my rough calculations are right I should have about 3ACpM with this design, which can be stressful for plants. I'd like to cut that to about 3 minutes per air change.

All I'm saying is that the amount of airflow that you are talking about is not excessive by any means, and may be short of what is ideal. One thing that you are dealing with in small cabinets that you may not be taking into account is air-resistance. Just because a fan is rated at 100cfm doesn't mean that it is going to move 100cf of air in a minute through light traps, carbon filters, and exhausting ducting.

I happen to have a cabinet that lots of people have put 400w and 250w lights in over the years with all sorts of different fans. The cabinet footprint is 25.5"X24.5" and is about 5.5' tall. The growers that were able to maintain the best environmental conditions in these cabinets were running 4" and, in some cases (mine), 6" centrifugal fans which rated at higher at substantial higher cfm rates than the fan you are proposing.

Pine

 

[thinkin]

ventilation

ventilation

100 cfm is not going to be enough.
especially after applying a scrubber.

My cabinet is under 2'x2'x2.5'
250 hps cooltube with 110 cfm pc fan.
2 pc fans for mixing internal air
2 Exhaust fans rated 80 cfm and 110 cfm into scrubbers

temps are +7 to +10 over ambient

I want to add more ventilation.
Want to run four (exhaust) 110 cfm pc fans on speed controller.

 

[señorsloth]

what do they say, your air should exchange like 4 times a mintue? don't quote me on that but if it were, you are working with 12 cubic feet...that means you would need to run at least 48 cfm to ventilate a space that big, if you have 100 cfm, a carbon filter could cut it up to half, so you should be just about right on that end, IF the number is 4, its documented somewhere on the sight...

anyway don't be worried about the wind and ventilation, i have a pretty directed stream of air going through my cab and a fan blowing directly on them to boot until the last month of flowering. i guess there is probably a point where it is harmful to plants but i don't think you are at it, after all, in the wild they are constantly getting blown with fresh air all day and they seem to like it.

another plus to strong ventilation is mold is never an issue, it keeps the humidity down...gotta be extra careful to not get pests in your home though cause if your ventilation is really high you will be sucking any stray mites and thrips in all the faster.

 

[TheGreenBastard]

what do they say, your air should exchange like 4 times a mintue? don't quote me on that but if it were, you are working with 12 cubic feet...that means you would need to run at least 48 cfm to ventilate a space that big, if you have 100 cfm, a carbon filter could cut it up to half, so you should be just about right on that end, IF the number is 4, its documented somewhere on the sight...

anyway don't be worried about the wind and ventilation, i have a pretty directed stream of air going through my cab and a fan blowing directly on them to boot until the last month of flowering. i guess there is probably a point where it is harmful to plants but i don't think you are at it, after all, in the wild they are constantly getting blown with fresh air all day and they seem to like it.

another plus to strong ventilation is mold is never an issue, it keeps the humidity down...gotta be extra careful to not get pests in your home though cause if your ventilation is really high you will be sucking any stray mites and thrips in all the faster.

The very reason it keeps humidity down is what can cause the stress, it literally drys the plants. Which can lead to higher water consumption and increased efforts to uptake and retain fluid.

It looks like our math for calculating CFM is similar. (BTW, the ideal number of minutes per air change is pretty widely accepted as 3-5)

All I'm saying is that the amount of airflow that you are talking about is not excessive by any means, and may be short of what is ideal. One thing that you are dealing with in small cabinets that you may not be taking into account is air-resistance. Just because a fan is rated at 100cfm doesn't mean that it is going to move 100cf of air in a minute through light traps, carbon filters, and exhausting ducting.

I happen to have a cabinet that lots of people have put 400w and 250w lights in over the years with all sorts of different fans. The cabinet footprint is 25.5"X24.5" and is about 5.5' tall. The growers that were able to maintain the best environmental conditions in these cabinets were running 4" and, in some cases (mine), 6" centrifugal fans which rated at higher at substantial higher cfm rates than the fan you are proposing.

Pine

I am aware of this and have adjusted accordingly, though perhaps not accurately. Even if resistance is as high as 75% I would still be moving ~25CFM through a 12cuft. space, still giving me ~2 air changes per minute. So theoretical I should be perfectly fine with one of these fans even with the added resistance, and in the chance that it is not enough, I still have two more identical to it .

 

[pinecone]

The very reason it keeps humidity down is what can cause the stress, it literally drys the plants. Which can lead to higher water consumption and increased efforts to uptake and retain fluid.

Your humidity will be unaffected by whatever fan you use. The plant mass is too small, and the air flow is too high, for humidity to pool in a cabinet. Although humidity is likely to be the same in and out of the cab, the temperature will not be. You should expect inside temperature to be at least a few degrees higher than outside temps. The more effective the your fan and ventilation system is at moving air, the smaller the temperature differential is going to be. More CFM in your fan buys you a smaller temp differential, all things equal.

Pine

 

[TheGreenBastard]

Your humidity will be unaffected by whatever fan you use. The plant mass is too small, and the air flow is too high, for humidity to pool in a cabinet. Although humidity is likely to be the same in and out of the cab, the temperature will not be. You should expect inside temperature to be at least a few degrees higher than outside temps. The more effective the your fan and ventilation system is at moving air, the smaller the temperature differential is going to be. More CFM in your fan buys you a smaller temp differential, all things equal.

Pine

I'm not expecting humidity to pool, but for moisture to be removed from the plants at an accelerated pace.

Transpiration is the process of water loss through stomata in plants, if airflow is too high more water could potentially be lost through this process. My concerns are that the combination of the intensity of the light (which will promote the stomata to open), the low humidity (~20% here), and excess air flow (which can physically remove water from the under side of the leaf, were most stomata are present) will increase my overall water use as well as potentially stress the plant.

BTW, I am fully aware that their will be a temperature difference and I have taken this into account. Although, you have aroused my curiosity, how many cubic feet of air do you move through your room in a minute? If you have a thermometer you can use this equation which had been uploaded by RedGreenery years ago:

How to Check Your Cabinet for Proper Ventilation

For they guy wanting to know how well his cabinet is working, you can get the EFFECTIVE AIR FLOW rate from this chart. Here's how to do it. Measure the temperature of your exhaust air and subtract the ambient temperature to get the temperature rise number. Use the bulb size line and temperature rise to get the EFFECTIVE AIR FLOW RATE.





For example, a 250 W HPS lamp in a growbox with 90*F exhaust in a 70*F room is operating with 40 CMF of effective airflow.

Otherwise you'll need an anemometer.

 

[indianajones]

100 cfm is not going to be enough.
especially after applying a scrubber.

My cabinet is under 2'x2'x2.5'
250 hps cooltube with 110 cfm pc fan.
2 pc fans for mixing internal air
2 Exhaust fans rated 80 cfm and 110 cfm into scrubbers

temps are +7 to +10 over ambient

I want to add more ventilation.
Want to run four (exhaust) 110 cfm pc fans on speed controller.

+1
you aren't taking the static pressure from a carbon filter into account
static pressure is the pressure that a filter needs to get airflow, like if you put your hand over your mouth and blow through your fingers, it takes a lil effort to break the seal of your fingers.

i use a 100 CFM fan (without a scrubber) for my 4 sq foot cab that is 44 inches high inside with a 250 and it works alright. temps get up to 90 quick though if its not cold outside during the day.

you should relax about the issue that yer stressin on

the transpiration rate isn't gonna make your plants stress to where you can tell it unless they are actually windburned. you will know when you're actually windburning a plant, because when its blowing around youre just thinkin to yourself "theres no way thats gonna be good for that plant".

sounds silly but when you see a plant moving as hard as it takes to actually windburn you will understand what i mean.

 

[TheGreenBastard]

I am taking into account static pressure, and I believe 6.3mm will be enough for a small DIY CF in my space. Though, like I had said before, this is really all speculation until I get it built. If it happens that I don't have enough air flow or static pressure is too great I have two more identical Delta fans as well as several other 80mm axial fans (which will probably be used for circulation).

As far as stress, I'm actually more concerned about water usage then windburn. I live in vegas now and the water quality here is terrible. Until I get a RO filter I'm going to be using primarily distilled water, which isn't cheap when used in these amounts.

Also, I was expecting about a 10 degree rise in ambient which will keep me at about 80-82. I might need to drop a few degrees but we won't know until its done. I might go to Home Depot today, I was putting it off because I worked 12 hours last night and this was my day off.

 

[TheGreenBastard]

Just got back from HD and picked up everything I need save for scrubber materials, which can wait a while.

 

[thinkin]

.02

I am interested to hear the cabinet temperature over ambient.

Towards end of flower here.
Due to buds bulking up and running out of space, ventilation is getting restrictive.
I HAVE to add another Fan+Scrubber.
On paper looks like overkill but its not.

Plants seem to like the fresh breeze.
(air is FREE USE IT!)

then, again my bulb is vertical.
Maybe horizontal mounted bulb/hood would be easier to vent.

 

[TheGreenBastard]

I built my first scrubber last night and have decided that another one wouldn't be a bad idea. If I run into issues with excessive airflow I'll just make my own "fan controller" by simply using a less powerful PSU (I have several old wall worts lying around that can help me achieve this). I originally wanted two exhausts opposite of each other for more uniform airflow to begin with but then I had the above mentioned concerns.

Construction on this cab has been delayed, I've been very busy lately. The plan was to get it running by the first but it looks like that will be postponed for the time being. I have all my materials, its just a matter of finding the time to put it together.