Ventilation Question

[scurred]

Got a question on ventilation, I plan to take a 6" duct hose and use a Y adapter to split it into two 6" holes, which I will attach 2 6" duct blowers to. The duct blowers are those really basic duct fans you can use to boost airflow in home heating/cooling systems. Each fan is rated 250CFM but seems to do much less. Anyways, do you think it's ok to have them both drawing air from the same 6" duct host?

Second question, those 2 6" 250CFM duct blowers will be my area's intake. The exhaust will be a Dayton fan rated at 265CFM on a 4" duct host, no air cooled hood. Area is about 4ft wide, 6ft long, 4ft tall with a 400w CMH. Do you think this is enough ventilation?

 

[Cannabean]

I personally don't see the need for active intake, passive intake on 250cfm exhaust in what seems to be a small cab will do fine. My first concern was that the large surface area of the 2 6" ports on the y adapter will prohibit your exhaust fan from creating negative pressure.
I could be wrong about the negative pressure cus I don't know exactly how your room is set up, but imo active intakes are not needed, and those duct booster fans can't pull/push anything as far as airflow.

To answer your question, yes I think a 250 cfm fan in your area will adequetly cool a 400w cmh, however the intake fans are not necessary and I think you should determine the correct size passive intake you'll need instead of creating an active intake.

 

[scurred]

Yeah I understand about negative air pressure, only problem is I will be intaking from a room 5-10 feet away using duct hose. So I was worried passive wouldn't cut it. I guess I could always try passive, I will need to use that formula thing to figure out how big the intake will need to be.

Edit:

Looks like I'd need 126 CFM to cool the light, going by the formula 3.16 * 400 / 10, and around 30 CFM to exchange the air in the room. And if my exhaust is a 4" duct host, I'd need 8" passive intake?

What if I just used 1 duct blower fan for my intake, it is rated at 250 CFM but I know it's much less, it doesn't have the pressure the dayton does. So the Dayton 273 CFM with a scubber for exhaust, and a duct blower fan 250 CFM for intake. Reason I am so curious on active intake is I have always had trouble cooling areas with passive intake, but when I set one up with active intake it worked for me.

 

[touchofgrey]

Yeah I understand about negative air pressure, only problem is I will be intaking from a room 5-10 feet away using duct hose. So I was worried passive wouldn't cut it. I guess I could always try passive, I will need to use that formula thing to figure out how big the intake will need to be.

Edit:

Looks like I'd need 126 CFM to cool the light, going by the formula 3.16 * 400 / 10, and around 30 CFM to exchange the air in the room. And if my exhaust is a 4" duct host, I'd need 8" passive intake?

What if I just used 1 duct blower fan for my intake, it is rated at 250 CFM but I know it's much less, it doesn't have the pressure the dayton does. So the Dayton 273 CFM with a scubber for exhaust, and a duct blower fan 250 CFM for intake. Reason I am so curious on active intake is I have always had trouble cooling areas with passive intake, but when I set one up with active intake it worked for me.

Sorry but those duct boosters don't do anything when there is any static pressure involved. You've calculated how much air you need but you need a fan that can move that much air against all the losses in your duct system. If you use the corrugated flex duct that has a lot of friction loss compared to smooth metal duct.

You've got a decent chance at it working with just a passive inlet with the Dayton but according to it's spec sheet you're down to 150 cfm at .5" pressure and with a scrubber you can get that much pretty quick. Is your scubber a store bought unit with a pressure rating?

Read the Ventilation 101 thread to get a good understanding of static pressure. Total static pressure is adding up everything on both sides of the fan. Long runs with corrugated flex duct are a big consideration. Good ventilation is balancing your fan with your setup.

 

[Admiral Canna]

Best way to find out yer exhaust fan......

Room Dimensions..... L x W x H = cubic feet of yer grow room. Take that number and divide by 5.

Example: 10 x 10 x 10 = 1000 cu/ft...... 1000/5 = 200 CFM

So to move 200 CFM, get a 200 CFM fan. If you want to put a filter on it, get a bigger fan.

Also, yer intake needs to bring in less air than you bring out.

Definitely get a handle on how negative pressure works for you.

 

[scurred]

Sorry but those duct boosters don't do anything when there is any static pressure involved. You've calculated how much air you need but you need a fan that can move that much air against all the losses in your duct system. If you use the corrugated flex duct that has a lot of friction loss compared to smooth metal duct.

You've got a decent chance at it working with just a passive inlet with the Dayton but according to it's spec sheet you're down to 150 cfm at .5" pressure and with a scrubber you can get that much pretty quick. Is your scubber a store bought unit with a pressure rating?

Read the Ventilation 101 thread to get a good understanding of static pressure. Total static pressure is adding up everything on both sides of the fan. Long runs with corrugated flex duct are a big consideration. Good ventilation is balancing your fan with your setup.

Yeah, your right they definately don't seem to be 250 CFM. Scrubber will be store bought rated for 150-265 CFM I think. The dayton exhaust will not be on a long duct hose, only 12" or so with 1 bend.

@Admiral Canna
Yeah I understand it I think, you want equal exhaust and intake so all air leaving the room is through your scrubber. I just have never had good luck with passive intakes, hopefully it goes better this time.