# How can you measure cfm of your exhaust



## gottaloveplasma (Jun 1, 2015)

is there amy way to measure how many cfm are actually moving through your ducting?


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## The Hemp Goddess (Jun 1, 2015)

I'm sure there is a way, but not for the layman.

The fan will give you the cfms.  Just try and keep ducting runs as straight as possible and you won't loose much.


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## terky (Jun 1, 2015)

Tell me your fan cfm, duct size and length and fan model.


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## gottaloveplasma (Jun 1, 2015)

Predominantly straight about 15 ft run of 10 inch flex duct.  Presh hyper fan 10 inch.  950 cfm inline (inline) filter.  Filter is open on both ends.   Only got few 45 degree bends.  But you can't tell me what cfns I'm pulling out.  To make things worse when I turn everything on there's also an 8" intake.  If that's not complicated enough the passive vent is open still negative pressure I hope.  The inTake air is to displace the hot air.  Vent is for plants.  Didn't want to burn them from windburn.  Hope this keeps my tent cooler. &#55357;&#56840;


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## terky (Jun 2, 2015)

950 cfm carbon filter? What brand? Can filter?

I'll figure it out when I get home and get back to ya.I gotta look up all the specs and calculate it out.

So you have an intake fan too? Just an fyi unless the intake fan is the same size as the exhaust fan it's not doing anything. It's better to just have an exhaust fan pulling with a backdraft damper on the intake. If you put a small fan on the intake it just chokes the whole system down. The exhaust fan will pull whatever it's gonna pull.


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## umbra (Jun 2, 2015)

the way to measure air flow is with a hot wire anemometer. they are not cheap and honesty it will not tell you want you want to know


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## gottaloveplasma (Jun 2, 2015)

terky said:


> 950 cfm carbon filter? What brand? Can filter?
> 
> I'll figure it out when I get home and get back to ya.I gotta look up all the specs and calculate it out.
> 
> So you have an intake fan too? Just an fyi unless the intake fan is the same size as the exhaust fan it's not doing anything. It's better to just have an exhaust fan pulling with a backdraft damper on the intake. If you put a small fan on the intake it just chokes the whole system down. The exhaust fan will pull whatever it's gonna pull.


 
I'm trying to understand.. Fans have controls on them.  How is a smaller fan not helping.  It's lessoning the stattic pressure yes?  It's a special phresh inline filter. You dound like you know what your talking about.


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## The Hemp Goddess (Jun 2, 2015)

Most exhaust fans do not have controls on the, you have to use a speed controller to control the speed.  If the exhaust is significantly larger than the intake fan, it is making the exhaust fan work harder as the intake fan is actually impeding the inflow of air.


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## Dman1234 (Jun 2, 2015)

Passive intake is all you need. If your cooling your lights with the fan then your rooms temp is all you need to know if your cfm's are enough.


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## gottaloveplasma (Jun 2, 2015)

The Hemp Goddess said:


> Most exhaust fans do not have controls on the, you have to use a speed controller to control the speed. If the exhaust is significantly larger than the intake fan, it is making the exhaust fan work harder as the intake fan is actually impeding the inflow of air.


 
Yes THG, that is why I am also using passive intakes.  The intake is not meant for plants its being injected into the hot air mass right above canopy.  The passive intake and circulation fan below canopy is going to address plants needs.  

My problem was I needed so much airflow its bad for the plants.  Don't want them having 15 air exchanges every minute.


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## budz4me (Jun 2, 2015)

there is a device you can plug into your apple product (tablet/phone) that will tell you CFM.

This device from my experience works with supply (exhaust) registers best.

It does not read accurately for return air(intake). (In general, if your supply CFM is ok, so is your intake)

I use it in the field and i find it works ok.....but I use it as a general quick yes/no diagnosis...not looking for EXACT cfm numbers.

Cost was about 45$ US

I think with venting an indoor garden it should suffice.


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## budz4me (Jun 2, 2015)

gottaloveplasma said:


> I'm trying to understand.. Fans have controls on them.  How is a smaller fan not helping.  It's lessoning the stattic pressure yes?  It's a special phresh inline filter. You dound like you know what your talking about.




yes it would lower static pressure, but that doesnt help the main issue.  Static pressure is much like relative humidity, it is relative. Relative to the duct size and fan HP/CFM capabilities. 



Dman1234 said:


> Passive intake is all you need. If your cooling your lights with the fan then your rooms temp is all you need to know if your cfm's are enough.



Yeah I think this just about nailed it.


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## 000StankDank000 (Jun 2, 2015)

umbra said:


> the way to measure air flow is with a hot wire anemometer. they are not cheap and honesty it will not tell you want you want to know


 
Best most aqurate way is what umbra said and cheaper route is what budz4me suggested


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## gottaloveplasma (Jun 2, 2015)

Cool thank you guys.


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