Bottleneck at Dryer Exhaust Vent (8"->4" after a 20' run)

[LlamaSchool]

I'm helping a friend out with a basement grow in a Growlab grow tent (~10'x~10'x~8'). She is air-cooling two 1000's with an 8" Can filter, 8" hoods, 8" ducting, and an 8" ~800cfm fan. The only stealthy exhaust location in the basement is a 4" dryer exhaust that she was planning to use. The exhaust air has to run from the tent through about 20' of relatively straight ducting to reach this exhaust vent.

Is the 4" dryer exhaust vent going to be a serious bottleneck in an 8" setup? Is it worth running 8" ducting all the way to the vent to compensate for the long run? Will the air have lost enough speed after 20' of ducting that perhaps the downgrade from 8" to 4" in the final inches is not such a big deal?

 

[cracker420]

could you install another dryer vent? then you could just get a splitter and run it out the 2 vents. this would be way more stealthy than having one big 8in duct vent hanging off the side of your house.

 

[Care Free 1]

A bottleneck like that will not prevent you from cooling that setup, but obvoiusly you would want to avoid it. I would run it as is, and If you cant keep those ovens cool, then make the adjustment. A couple hour test run should reveal any issues.

 

[LlamaSchool]

Good ideas. I totally agree that adding another 4" vent would be optimum. The only trouble is the house is a rental and although the landlord is cool, it's never a great idea to start knocking out bricks to put in another vent at a house that is not owned..

We'll probably just test it out as it is. I'm guessing that the 8" will help with the long run leading up to the exhaust vent to the outside. I'm sure if I knew more about ventilation I could figure this out better.

 

[cracker420]

yeah prolly better not with it being a rental house. i think you will move enough air with the 4in. like care free said give it a go and see what your temps are at and go from there.

 

[BlindDate]

It would take 4 of the 4" vents to equal the same area of an 8". Do the math.

IMO that setup will not work. The restriction is far too great. Think of another way.

 

[JackTheGrower]

Is it possible to add a second fan and blow right at the 4, thus pressurizing the 4 and keeping the 8 moving??

I too think this is bad since it may be restricted with lint so my question is just academic.

Thanks for the Q:


jack

 

[LlamaSchool]

Well I do understand that 4" flows much less than 1/2 x 8" but I figured that not only was 8" overkill for cooling 2 lights with a 800 cfm fan, but that after 20' of ducting, that air was no longer flowing with enough velocity to make it even worth 8" by the end...Certainly if I needed 8" or even 1/2 x 8" then a 4" duct at the end would be no good. Problems will likely arise as I add a few more 1k lights of course.

Anyway, I'm going to test this out. Other options would be to expand the dryer venting, add another vent, or just vent into the sewer...easy enough...I'm always looking for simple though.

Jack - not sure if I understand how it would be clogged with lint...This is a straight exhaust with a louver that prevent backflow. No screens, etc, etc.

Thanks All!

 

[otherwhitemeat]

Curious how this worked out for you? I am looking to expand my little op also. I have two 4" Inline fans and concerned that these and my rollaway AC would be venting WAYY to much air.

My two options for venting filtered (but warm) air are:

-by hacking into the dryer vent
-knocking into the flue which already vents my HW heater and gas furnace

My friend is an HVAC guy and said the flue idea is OK as long as my grow air is vented ABOVE the HW and Furn, but he would prefer I used the dryer vent. Also advised me to install an array of CO detectors. Gas dryers can produce CO and backpressure could be dangerous if not monitored.

I don't have an 8", but two 4"s and another 4" from the rollaway AC might be quite a blast!

 

[JackTheGrower]

Curious how this worked out for you? I am looking to expand my little op also. I have two 4" Inline fans and concerned that these and my rollaway AC would be venting WAYY to much air.

My two options for venting filtered (but warm) air are:

-by hacking into the dryer vent
-knocking into the flue which already vents my HW heater and gas furnace

My friend is an HVAC guy and said the flue idea is OK as long as my grow air is vented ABOVE the HW and Furn, but he would prefer I used the dryer vent. Also advised me to install an array of CO detectors. Gas dryers can produce CO and backpressure could be dangerous if not monitored.

I don't have an 8", but two 4"s and another 4" from the rollaway AC might be quite a blast!

I Have had the CO detector go off.. They seem like a waste of money but that isn't true.

 

[otherwhitemeat]

I Have had the CO detector go off.. They seem like a waste of money but that isn't true.

If it DOESN'T go off, then your CO detector is a waste of money.