Please check my numbers before i buy first fan!

Hello and thanks for reading!

Know ive already asked about fan brands but now i wana check my numbers!

Basically ive seen that you should get a fan n filter larger than needed but going by the numbers ill still only need a 4" fan n filter as far as i can tell?!?
So heres my numbers n thinking behind!

Tent size 80 cm X 80 cm X 160 cm @ 36 cubic feet / 1 cubic meter

So going by size of tent and adding 20% to account for filter im looking for a fan that can handle 45 cfm / 72 m3/h

Now the 4" fan ive been looking at (Envirovent Silent 4", in uk so cant get an apollo fan!) has an air flow of 140m3/hr or 180m3/h (2 speed motor)

As this is more than twice what ill need (72 m3/h) i take it that i can get this fan n run it at around 50% giving me the airflow i need plus the fan will be quieter due to not running at 100%?!?

Ive been reading up on ventilation and @Donaldj made the comment that too much air flow is just as bad as not enough so do i really need more than a 4" fan?!?

many thanks!

This looks like a nice exhaust fan bro
I would see if you can get a speed controller for it if the two speeds aren’t enough
So you can slow it done and dial it in to exactly where you need it


Here is a way to help judge if a fan is enough if you are not supplementing co2 you need an air exchange rate of once every 3 min optimum during flower. This is when your plants will be largest but if you can it is always best to be able to control exchange rates so you can adjust temps and humidity to some degree too slow room gets too warm too fast it gets too cold somewhere in the middle both temp and humidity are able to be managed.
Another big factor is Carbon filters which have max and min airflow ratings with a speed controller you can keep filter working effectively longer

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Listen to @Donaldj he knows his stuff

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thought of you other day

when you were having issues getting dimmer working I am just confirming I use same cheap arse things :wink: Also don’t see myself changing them out until I can afford fully auto speed and temp controller

@Fever I can run up to a 8" fan on my filter but the humidity would be nearly impossible and filter weighs 89lbs without fan lmao I run fan at bare minimum to scrub about 120cfm

thanks @Countryboyjvd1971!

yeah going by the stats the fan looks good enough
especially like the 21 db - 24 db sound level!

ive been looking at fans but not in hydro shops
The hydro shops all seem to do the same makes and are prob bumping up the price too!
Seem to have better choice just by going to a traditional electrical supply shop
Plus the Silent 4" is only £55!

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Other factors to consider hydro shops tend to sell to people who grow and order what they ask for and buy I am all for saving a buck but make sure you read up on it’s damp location compatiblity. Not too many fans like moving humid air for prolonged times. @Countryboyjvd1971 being an HVAC tech you would know the density calculations far better than I and how it would effect life span of fan. I buy cheap because I know if they are pulling air at 70% RH dust and rust starts to wear on them and they seem to last about half advertised life. It looks like a nice fan and quite is always nice

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Hi @Donaldj!
thanks for the reply!

so working the numbers out (i do like numbers!)
a tent that has a 45 cfm need (which is per 1 minute) then only really needs 15 cfm which would make 45 cubic feet every 3 mins?!?
which would give me the air exchange rate of once every 3 min you advised

So the fan im looking at with its max of 180 m3/h / 105 cfm (or even on its low setting of 140 m3/h / 82 cfm) will be MORE than enough for my tent!

Looks like a speed controller is next on the shopping list!

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Good point @Donaldj

They do state that they are made “to meet ventilation needs in domestic, commercial and industrial applications” so hopefully theyll have no probs being in my lil attic!
theyve also got a 5 year guarantee so if do break down im covered (as long as i remember to keep the receipt!)

Yeah your right with the humidity affecting life span I try to get Stuff made of plastic not metal myself for that reason but I do use some cheap in line room to room fans as intakes (don’t need to run them that much ) that have metal parts
Hey @Donaldj you inspired me to redo my exhaust ducting getting all materials together now
Picked up a 12" inline fan moves 400 cfm and a wall mounted speed controller will need to do some control wiring but that’s cool I pick up 208 volt model as well to be more efficient will need to run new power over but I’ll only have the one exhaust fan running instead of the 4 I have now
I’ll also put damper collars on each take off on main duct to have a little more volume control and plan on a second damper to control how much I exhaust out of house so I can recover some heat and humidity in the winter months :+1::grin:

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Lol mine will all be solid ducts when done only have one piece of flex rightnow because my filter is still working hard in my other space. Way quieter and more efficient with reduction in turbulance I may yet go to a 8" fan and 8" duct to my exhaust for cooling efficiency but for now can hardly exhaust without dropping temps too much. But heating space with single light most hours of the day ducting is easy upgrades as needed as you well know. My buddy is a 30yr HVAC tech so if I don’t do something right he harps on me about it and tells me about new products all the time.

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My fan is plastic and I got 50% rh so I should be good.

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glad you have it dialled in now

Nice it always good to have friends from other trades @Donaldj
@Fever any exhaust fan that’s labeled bath room exhaust in line or otherwise is good for humid conditions just a FYI


Hi @Donaldj!

Got a quick question for you (As Fever said “listen to Donaldj he knows his stuff”!)

Just wondering at what rate does air flow become too much?

As worked out i only need a rate of 15 cfm to replace the air every 3 mins as you advised (tent is 36 cubic feet plus 20 % for filter gives 45 cubic feet)

So the slowest the fan im looking at runs is 140 m3/h / 82.4 cfm meaning the air will be replaced nearly twice every minute or 5 times every 3 mins not once as you advised!?!

Is this a problem or am i worrying for nothing?!?

Ive emailed Rhino filters to ask what is the minimum airflow rate that their 4" fan needs to work so have that to take into consideration too

Have also called Envirovent to ask if running the fan that slow will cause any issues
Am waiting on them emailing me back!

Many thanks

The idea is to be able to have entire control of temps humidity while still getting fresh co2 rich air to your ladies with a speed controller you can slow fans right down most fans are adverised max rate of flow. I am running a 435cfm 6" to control a 380 cubic foot space it’s running at about 1/4 speed and passive intake which further reduces exchange rate. Fans are tested for max speed 0 resistance now factor in resistance of filter each bend in ducting size of intake vents you have so many ways to control airflow listing all would be a HVAC techs specialty. Also once every 3 min is a minimum not a maximum but as fan gets faster humidity is harder to keep there is a very fine line and the more you can control the easier it is to grow in

HI @Donaldj!

Thanks for the reply

That def helps especially knowing that the 3 min air refresh is a minimum NOT max!
Was worried id be taking the air away too quickly (?!?) but now i see thats not something i need to worry about!

Im also planning on passive intake

Ive got Thermopro in tent at mo to give me an idea of tents natural (lights off) temp n humidity
Gona run the light for 12 hours tomorrow so i know what ive got to deal with when the lights are running!
Then ill be in a better position to work out what im gona need in respect of heaters or cooling!

Theres so much more to this than i first thought!!!

Thanks again

Lol fan speed helps to regulate temps as well as provide fresh air too fast and humidity plummets too slow and temps raise you are doing right thing dry running space makes huge difference once plants are in adjusting things gets rushed and more complicated

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Yeah Dont think Ive spent so long thinking bout anything b4! N Ive still got loads 2 decide on! Oh well B worth it in the end!

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