I have a 6'' fan/carbon filter setup in my tent and am looking to vent it out the wall through a dryer vent. The dryer vent is a 4 inch. Can I put a reducer on the vent to fit my 6'' or is it going to be hard on my fan/restrict air flow?
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You can and it should work, , but avoid it if you can. It will make the fan work harder, which in turn makes it much noiser. And obviously will push less air. If your fan has a variable speed so you can throttle it back, that will help with the noise and the workload- just means it will be on more.
Maybe the noise isn’t an issue for you- but once I reworked my system and took away the reducer to make it full size duct all the way- my fan became super quiet.
I just did a very similar install two weeks ago. I did the reverse though. I run a 4" fan & filter combo with 4" flexible ducting. I plan on upgrading to a 6" fan & filter combo in the future. In preparation to the upgrade I purchased a 6" dryer exhaust vent for the outside of the house and a 6" to 4" tapered adapter so I could run my 4" exhaust through it until I upgrade to 6".
By restricting your 6" down to a 4" yes, you will most certainly restrict the air flow and this will put head pressure on your fan. Will it hurt your fan...? probably not. Considering we use our fans to suck out of our tents and cause a negative pressure that the fan has to fight against anyways... So one can not say these fans aren't designed to push or pull against some resistance.
In a nut shell... Yes it will work. Yes it will cut down on air flow. No it should not harm your fan in any drastic way.