One of the key features of a good growbox design is fitting a carbon scrubber to the ventilation system.

Let's take a look at CANFAN's stuff. They publish decent spec's with their equipment.

I'd like to fit a 6" CANFAN which is rated at 269 CFM with 1.5" WC pressure. This is a workhorse of a fan. https://www.canfilters.com/fan_metal_home.html

Now let's get a nice CANFAN 50 that's rated at 420CFM @ 0.75"WC.
https://www.canfilters.com/canfilters_50.html

Hey this sounds pretty good, I think I can get 270 CFM out of this combo, maybe? Well let's take a look.



The operating point for the 6" CANFAN/50 combo is around 200CFM. Canfan says 210 CFM so that's close with about a 25% loss due to the scrubber. So what gives, the filter is supposed to give 420CFM @ 0.75" and we have a 1.5" Fan, how did we end up running at 50% of the scrubber and 75% of the fan? This is where the fan curve is needed to see how both these components make a system where the actual airflow is the operating point where the two curves cross.

Let's hook this puppy and see what happens. Holy cow this thing is loud. No wonder, look at the noise spec. 6 Sonnes, that's like 70 dB. Look up 70db, its a vaccuum cleaner loud or the same as radio or TV music. And that's the noise of the fan, the fan spec's don't include the air rushing though the ducts either.

Am I happy. Let's see I just spent at least $300 on this stuff, my cabinet is 90* and I can here this thing on the street.

So what are my options. Add a speed controller, box runs hotter. Get a quieter fan, if it's less that 1.5"WC, the box runs hotter. Downsize the lights? Leave the door open?

Every growbox you see fitted with a scrubber/fan combination is using this brute force approach to the ventilation. This is caveman engineering by Fred and Barney.

An alternative is to run unrestricted quiet fans to cool the lights. A high airflow, low noise computer fan will run with 15-25 db which is whisper quiet compared to this hoover. Add a second small fan pushing through a small scrubber and you're set. Cool, quiet and smell free.

[FreezerBoy]

Originally Posted by redgreenry

A very common mistake is to cut four 1" inch holes for a 4" intake.

This example uses nine rather than four but, illustrates your point.

Originally Posted by redgreenry

I can't tell you at what point 10% to 100% oversize is best but the benefits are marginal beyond 100% or 2x duct size.

There was a guy here who swore by 4X. I've got double intakes and my fan still ramps up every time I close the door. The only reason I haven't gone to 3X is my cab is a giant foam sandwich with dual layers of steel for bread.

Love this thread!

Originally Posted by FreezerBoy

This example uses nine rather than four but, illustrates your point.





There was a guy here who swore by 4X. I've got double intakes and my fan still ramps up every time I close the door. The only reason I haven't gone to 3X is my cab is a giant foam sandwich with dual layers of steel for bread.

Love this thread!

Make a trumpet shape out of cardboard on the outside of your intake and it works the same as doubling the size of the intake.

[FreezerBoy]

Neat!

[puffmadizm]

Originally Posted by nag-champa

i cant afford for any smell to leak through any opening, so is it recommended that i run the exhaust 24/7? i have a room right next to the grow room, if i leave a window open in the next room, will the cracks in the door be enough for air exchange?

i dont know if its possible yet, but would it be safer if the room was sealed?

at home depot near me,they sell carbor filters that can be cut to any size,its $9 for a 20 by 25 sheet,you could cut them and cover your intakes as well,i actually use the carbon sheets to cover my outake for scrubbing,and they have 6inch by 6inch scented filters,i use the scented ones for my intake,just in case of leaks when my fan is not on,i also put a scented one in my central air vent in my room..they rule! the company name is 'web' $2.50 for the scented ones
i have a inline fan i got from homepot 80 cfm,i wrap the filter right over the 4 inch pipe and secure it with a rubberband,then 2 dryer sheets,then another layer of filter, works well

[puffmadizm]

here's a link to online,but they usually have them in stock
https://************/dk56mk

One of the challenges in building a good ventilation system is to get the right amount of airflow. If you're stuck and you need to add or replace a fan, many times it doesn't quite work out.

One of the reasons is that the Air Flow is proportional to the square of the Pressure. Adding a second fan will not double the flow but will increase it by square root of 2, (1.41), or by about 40 percent. To double the Air Flow requires four times the Pressure.

There are two ways to add a second fan and they have completely different effects on the system.

The first way is to add a second fan inline with the first. The fan curve is the single fan curve with the pressure doubled at the same flow rate value.



The second way is to add a parallel fan. This doesn't boost the pressure at all but doubles the air flow at the original fan curve pressure value.



If you like these fans check out the S&P Catalog. The fans have adapters that make it easy to stack and parallel them.

https://www.hvacquick.com/catalog_fil...vl79qmr7oqqii5




System A is very restricted. We can see that the air flow improves quite a bit with series compared to parallel.

System B and C are less restricted and the improvements in air flow are similar between series and paralllel.

System D is an unrestricted system with the parallel fans providing a significant improvement over the series fans.

****************************** ****************************** ****************************** *****

I did a little experiment a while ago with 4 computer fans. I connected them in series and boy did they ever make a good amount of pressure and they were pretty quiet too. Each computer fan can produce 0.2" static pressure. Four in series will produce the same pressure as a Dayton blower at 0.75". The noise of a computer fan is around 20 db with the noise doubling every 6 db so four fans would make about 38 db of noise which is way quieter than some of the noisy blowers that start at 65-70db.

I thought I'd have a bit more response on this thread.

I'd like to see us build up a database of fan curves.

The information that is really useful is the Flow vs Static Pressure curves with the operating noise level.

don't worry once you have all the data compiled this will be a very popular thread! A lil technical now for your average stoner...

yeah, I did that on purpose. See ventilation is a very technical and extremely subtle topic. I need to lay a technical foundation down so when people ask questions I have reference material to draw on and to be able to talk in a common "language" which is CFM, Static Pressure etc....

Or you can be a caveman and stick a furnace blower on your growcab and whine about the noise... LOL

Usually when I'm asked a technical question, I'll throw back a calculation, a graph or at least give a reason why the solution should work. There's lot's of misconceptions and misinformation out there.

I believe the future of our "OVERGROW" movement in every sense of the word will be won through small micro cabs that anyone can tend a few plants and be self sufficient. All you need is a wardrobe, tool cabinet, dresser or a nightstand, a couple of CFL's and a few computer fans. Quiet, smell free and efficient.