Exhaust vent question - please and thank you

To keep it simple...

I am buying a new exhaust fan and need help choosing between vent options.

I want to vent outdoors but I have 2 options:

- Option#1: a 6" duct that I can tie into and dump to the roof. Total distance away 20ft then up 5 ft to roof
or
- Option #2: a 20 ft long 4 and 5 inch ducting in the ceiling running outdoors/ 20 ft away + length of duct - total 40 ft




I want to buy a 6" or 8" speed controlled fan and mount it in the ceiling of the work room to pull at about 5 minuted per air exchange (approx.200cfm)

can I use the open 5" or should I tie into the closer 6" duct? will the airflow velocity be to great in the smaller duct? will the airflow be too much force for the closer 6"? I don't want the roof cap coming off or have a noticeable storm rising from our roof...





stats:
- My room is a total of 775 cu ft
- room split into two: 395.5 cu ft work room + small veg tent, 380 cu ft Flowering room w/ 2x 600's
- The flowering room has a 6" max fan (on low 200cfm) pushing through a carbon filter into the work room
- already have a 6" fresh air intake in the grow room to a room with an open window and we have an extra 4" vent in ceiling



the extra 4" and 5" vents: 20 ft away and 20 ft long




The in use 6" vent to the roof. it maybe be bigger I didn't measure it.



I have more inof on my thread..... THANK YOU SO MUCH FOR YOUR HELP!!!!!!!!!


I am a non-profit care provider / horticulture student. much love

Smoke will this help?
http://www.freecalc.com/ductloss.htm

That was a lot of questions about running a duct! As a rule of thumb for me personally, I always run the shortest length. I would run a separate exhaust line for each room, but could tie in if the duct was bigger. I don't think the air pressure would be too much for the smaller line, it just wouldn't work as efficient. You might want to experiment, trial and error. All we can do on here is assume what MIGHT work the best.

Would it be possible to insulate that crawl space, dump the hot air into there, and run a much larger inline fan to exhaust it outside?

growshopfrank said:

Smoke will this help?
http://www.freecalc.com/ductloss.htm

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YES YES YES I knew if I kept trying I would get the help I needed.

growshop frank that link is PRICELESS.

yerboyBLue.. your name says it all. with helpful advice from you and franks chart I think I can figure this out.

ventin ain't easy, and either does your help.. thank you so much for your effort. I know noob questions can be a bit tardy

yerboyblue said:

Would it be possible to insulate that crawl space, dump the hot air into there, and run a much larger inline fan to exhaust it outside?

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no. I think that might be too costly. and I don't want to risk mold. etc

your idea about venting each room independantly is a great idea. I've heard the same advice from alienfarmer so I should listen to the pros.

In my first draft of calculations with using a aircooled system to vent outdoors using the 5" vent.
my fluid velocity is 1100 ft/min WHAT IS THE MAX #
pressure(inches water) in the final 20ft of g.steel duct: .076
- using the 6" max fan on Medium (264cfm)
ducting goes: Fresh Air>FAN>6ft flex>6ft cooltubes>10ft of flex 90 degree>90 degree>20ft of 6" insulated flex duct>20ft of 5" galvanized steel duct * i know I missing some info, but its late at night and I wrote and exam today.

I couldn't really say, not sure who could, there are a lot of variables in that setup. I can say that is a long run with a lot of bends for a 6" inline to push thru, let alone pull (my understanding is that these fans work more efficiently pulling air rather than pushing it).

In my setups I usually get the biggest fan that I can fit/afford. I have never had issues with too much air pressure going thru a duct, and I've ran a 10" elicent inline fan (about 800cfm) through 6" ducting. It seems like the duct size and how it is run is what ultimately will determine how efficiently the fan works.

As far as the 5 minute air exchange (the fan can clear the entire room in 5 minutes) suggestion, I don't think I've ever actually had success in cooling a room with a small fan like that, but then again I've always exhausted the room through my hoods, pushed through a carbon filter.

For your setup, with the long runs and turns, go with the bigger fan. It doesn't really draw much more energy, you can buy reducers so it will adapt to the 5" line, and if you ever wanted to expand you will have the bigger fan already.

P.S. I run 6-600w lights in a 10x10x10 room, and only 4 or them are on for veg. Even without a carbon filter on the end, I would have trouble cooling the room with an 8" elicent, unless it was cold outside. I have to run intake fans year round except winter, and add A/C in the summer.

excellent yerboyblue! your experience is priceless. I will upgarde to a 555cfm 8" S&P mixed flow fan and then tie into the 6" duct out. Since its closer and all that I have to do is buy a 6" y connector. and duct out. With some early calculations the 5" vent seems to have too high of flow velocity... I have been told elsewhere that an airflow rate of over 1800 ft/sec is too much but I still need the MAX airflow velocity #'s answered.

With growshopfranks handy calculator I have some things to figure out but I finally have a good idea what I'll do...


you guys have no idea how much I actually appreciate your help.

Glad I could help man, let us know how it all turns out.

yerboyblue said:

Glad I could help man, let us know how it all turns out.

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sure will...