How many CFM for this space?

96"x96x78"
8x8x6.6"
No carbon filter



(3x cob bars with fans (240 watts at the wall each
(1x 600 watt hps open ended
(1x 6 cobs\blurple with 6 fans (630 watts at the wall

2100 watts at the wall of lights roughly. Not as hot as all hps but room but the fans from these cobs do put off alot of heat.

I will be using no carbon filters
Humidity is max 30% at all time.
Might jump up to 40% with plants in full bloom

Online says to figure out cfm for a room is
is W x H x L divided by 3 (exchange all.all.air every 3 minutes)

8x8x6.6=422
Divided by 3
=140cfm

Add 30% for 2000 watts of lights( being generous)

=42cfm
+
140cfm
=
182cfm

That cant be right.


I have tested a 600 open ballast metal halide bulb in a 3x3x6.6 foot tent and my fan ran non stop and the room leveled out at
29c (84f) with a carbon filter
Had it hooked up to environmental controll set to 23.5c
Without carbon filter room stayed at 24 to 25c but the fan still ran non stop.

So I'm having a hard time believing those cfm calculations.




I ordered two 6" vivoson 440cfm a peice. Total 880cfm for the room. They will be hooked up on an environmental control for the 8x8 tent NO carbon.

Think this will be enough?

Ya your math seems off there somewhere. You came to the right decision tho, so theres that lol

I think the 2 x 440s will be fine, they just may have to be maxed out to clear that entire space constantly, i own the same vivosun 440, decent fan, not too loud, been running non stop for months without issue in my 4x4. turned down on the lowest setting it still gives good negative pressure.

i just ordered one of these for my new room (7x14) 3x12 table/footprint
https://growershouse.com/vortex-inline-powerfans?367=8121&aid=6832&utm_source=google&utm_medium=cpc&adpos=1o1&scid=scplp6832&sc_intid=6832&gsacid=952889933&keyword=&gclid=EAIaIQobChMI7Jb65KGy4gIVgRh9Ch2IoQIbEAQYASABEgIAw_D_BwE

diggs99 said:

Ya your math seems off there somewhere. You came to the right decision tho, so theres that lol

I think the 2 x 440s will be fine, they just may have to be maxed out to clear that entire space constantly, i own the same vivosun 440, decent fan, not too loud, been running non stop for months without issue in my 4x4. turned down on the lowest setting it still gives good negative pressure.

i just ordered one of these for my new room (7x14) 3x12 table/footprint
https://growershouse.com/vortex-inline-powerfans?367=8121&aid=6832&utm_source=google&utm_medium=cpc&adpos=1o1&scid=scplp6832&sc_intid=6832&gsacid=952889933&keyword=&gclid=EAIaIQobChMI7Jb65KGy4gIVgRh9Ch2IoQIbEAQYASABEgIAw_D_BwE

Click to expand...

Haha. Theres alot of different cfm calculations online maybe I picked the wrong one.

Those fans from growers house seem like some good quality and the permanent lubricated ball bearings should keep it alive for a long time.

Since.the vivosuns were super cheap maybe I'll keep them for back ups for now and upgrade next run.
Cheers bro

Those fans should be more than adequate.....but intake air temps are key.

My problem was intake air temperature being too high.
No matter how much air I moved thru the space....too hot.

TintEastwood said:

Those fans should be more than adequate.....but intake air temps are key.

My problem was intake air temperature being too high.
No matter how much air I moved thru the space....too hot.

Click to expand...

That's a good point man. I got the tent in my basement which is about half the height underground so always a bit cooler.
21c
70f at the moment and we just started summer. Fans will probly run 24/7 when lights on.
No fans on at night tho other than oscillating.
Some people say they need oxygen at night with lights off others dont. I guess I'll see how they do in veg.

You want to exchange air 2.5 to 3x a minute. So 422.4 x 3 = 1267 CFM

Renfro said:

You want to exchange air 2.5 to 3x a minute. So 422.4 x 3 = 1267 CFM

Click to expand...

Cheers

dunno where you are at but growers house has a deal on 10 inch vortex blowers. I think it was 114 bucks for the 10 incher, cheaper than the 4 incher.

Renfro said:

dunno where you are at but growers house has a deal on 10 inch vortex blowers. I think it was 114 bucks for the 10 incher, cheaper than the 4 incher.

Click to expand...

I linked it earlier for him. Very good deal

A really big factor is ambient air temperatures. Pretty hard to cool a room with warm air. and if you have cool air you can get away with much less CFM.

Avoid recirculating the heat, exhaust it out of the building.

Many factors like static friction loss may need to be considered if you have long duct runs in your system. Basically CFM loss.

Calculating the airflow required to maintain a stable temperature, and so determine the size of fan needed, is surprisingly simple.

It boils down to CFM = 3.16 x Watts / DT(°F)

Watts is the lighting power (and any other 'hot' things in the grow space) in Watts.
DT is the allowable temperature rise within the enclosure (i.e. desired temperature minus ambient temperature) in °F.

So, to work out the size of fan required (in CFM) simply plug your lighting Wattage into the equation along with the DT value.

An example is:
Ambient temerature = 20 °C
Target temperature of the enclosure = 25 °C
DT in °C = 25 - 20 = 5
DT in °F = 5 x (9 / 5) = 9
Lighting Wattage = 250 Watts

Plugging these values into the equation gives:

CFM = 3.16 x 250 / 9 = 87.77

This represents the actual throughput required but it doesn't take account of the static pressure necessary to overcome the system impedance (how hard the fan has to suck or blow). But for a free-air system with no ducting or filters it should be fairly accurate.


This is taken from another site, credit goes to anonymiss.

That's what I got going on right now with a 4" fan and a 3x3 veg tent.

Just gotta make two 6" holes in a new piece.
This is temporary solution..

Going to get a friend to plumb some vents and hard line some ducting which will exhaust out under my backyard stairs. Next grow

For those who want a bit more info on how to work it out, here it is.

First, you'll need to know the amount of heat that needs to be dissipated.

The general equation for heat transfer is:

q = Cp x W x DT

where:
q = amount of heat transferred
Cp = specific heat of air
DT = allowable temperature rise within the enclosure
W = mass flow

Mass flow is defined as:

W = CFM x Density

DT is the difference between the ambient air (room) temperature and the target temperature inside the grow space in °F.

At sea level the density of air is 1.2041 kg/m3 (at 20°C) and the specific heat capacity (under typical room conditions) is 1.006 kJ/kgC. After doing some substitution and conversion this gives:

CFM = 3.16 x Watts / DT(°F)

coreywebster said:

For those who want a bit more info on how to work it out, here it is.

First, you'll need to know the amount of heat that needs to be dissipated.

The general equation for heat transfer is:

q = Cp x W x DT

where:
q = amount of heat transferred
Cp = specific heat of air
DT = allowable temperature rise within the enclosure
W = mass flow

Mass flow is defined as:

W = CFM x Density

DT is the difference between the ambient air (room) temperature and the target temperature inside the grow space in °F.

At sea level the density of air is 1.2041 kg/m3 (at 20°C) and the specific heat capacity (under typical room conditions) is 1.006 kJ/kgC. After doing some substitution and conversion this gives:

CFM = 3.16 x Watts / DT(°F)

Click to expand...

Wow cw. That's a whole new level of cfm

JungleSlut said:

Wow cw. That's a whole new level of cfm

Click to expand...

Probably more math than required for an 8x8 tent. lol Basic estimation will get the job done in most situations.

JungleSlut said:

Wow cw. That's a whole new level of cfm

Click to expand...

lol yeah, like I say its not my math, really the important bit is the large bold bit on the bottom of the second post.
But since airflow is greatly dependant on the rise in temps you can afford its pretty useful. Since we all live in different climates.

I used to have a great chart compiled by the same person which was super easy but the host site deleted it after several years.

I would use renfros and that formula and go with which ever gives the bigger figure.