# 4" to 2 1/2" reduction



## rcp612 (Mar 25, 2008)

Since the most common size for DC hoses is 4", and, the most common size for DC ports on machines is 2 1/2", I'm wondering just where the reduction should or can be placed? :icon_confused:
In other words, if I have a separator (such as a Thein) with a 4" connection to the collector, would it be alright to run a 2 1/2" hose to my table saw? Or do I have to run 4" to the saw and hook it up with a reducer? 
For the life of me, I can't see why there would be any adverse effects but, I also know that there is a lot of people smarter than me.


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## rrbrown (Feb 15, 2009)

Actually I believe the most common size port on machinery is 4". I say that because everything I own except the planer has a 4" port. This may be because of larger machinery or a move towards larger ports but either way your info follows.

The answer to you question is this. Reduce the size at the machine. No matter if its 6" to 4" or 4" to 2 1/2". Reason is DC systems move high volume lower velocity air flow. If you keep the larger duct size most of the way it creates a higher velocity at the machine to keep up with the higher volume needed for large duct.

I have 6" duct all the way through my Thien separator to every machine where it reduces down to 4" it actually increased the efficiency by 50% over having just 4" duct. Add a Canister filter and it increased another 50% because of greater air flow out means greater airflow in.

Smaller duct on longer runs reduces the volume which stalls the system.


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## Fred Hargis (Apr 28, 2012)

I would change all those puny 2 1/2" ports to at least 4". That may require some surgery, but generally it's as simple as a shop built hood fastened in place. You will be a lot happier in the long run.


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## rrbrown (Feb 15, 2009)

Fred Hargis said:


> I would change all those puny 2 1/2" ports to at least 4". That may require some surgery, but generally it's as simple as a shop built hood fastened in place. You will be a lot happier in the long run.


I agree depending on equipment. Some pieces won't be able to but reducing at the machine will help a lot.


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## rcp612 (Mar 25, 2008)

rrbrown said:


> Actually I believe the most common size port on machinery is 4". I say that because everything I own except the planer has a 4" port.


That's strange. The only 4" port I have IS on my planer. :icon_confused:
Anyways, I can't seem to wrap my head around WHY it would make a difference if the reduction comes before or after. Any restriction to the flow would still be the same wouldn't it?
This question may not be a good one, but it certainly is on my mind.


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## rrbrown (Feb 15, 2009)

rcp612 said:


> That's strange. The only 4" port I have IS on my planer. :icon_confused:
> Anyways, I can't seem to wrap my head around WHY it would make a difference if the reduction comes before or after. Any restriction to the flow would still be the same wouldn't it?
> This question may not be a good one, but it certainly is on my mind.


At the point of restriction the air velocity increases to keep up with the larger diameter high volume of air. The longer the run the slower the air gets and that's in any size duct. So if the restriction is to early it won't allow enough air to leave because the incoming volume of air is restricted. 

Remember a DC works on high volume of air that's why larger ducts work better. You may be confusing it with a shop vac hitch works on high velocity but low volume.

If you still don't understand it, you'll just have to trust me. :thumbsup::laughing:


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