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During some home-improvement, we need put in some new dryer venting. The most aesthetically pleasing option is to have a 90deg turn out of the dryer, then into a 3" pipe that goes for 9 feet. There is another option that would have a 90 degree turn, vertical for 4 feet, another 90 degree turn and then 9 feet out through a 4" pipe.

We have received different advice from different technicians, some who say that the 3" pipe isn't a problem, and that it is preferable since it avoids the second right angle and the vertical distance. A different technician told us that going through a 3" pipe is definitely not good, and even though it is more corners and distance, the second option is preferable (still under 25 feet of piping, even taking corners into account).

The main question is on the impact of a 3" pipe. Should this be avoided at all costs, and if put in there is a good chance that it will lead to dryer damage/reduced lifespan? Or is it manageable, and worth the tradeoff for shorter overall exhaust pipes?

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  • 4" diameter = 2" radius = ~ 12.57" area
  • 3" diameter = 1.5" radius = ~ 7.07" area

which makes the 3" diameter 3^2/4^2 = 9/16 = 56% the size the 4" diameter.

This causes two problems:

  • The dryer has to work harder to push that air through. That is a general problem affecting any fan or ventilation system. Getting enough air through determines specifications for duct size as well as the number of elbows and length of ductwork.
  • Specific to dryers: Lint. No matter how much you try to prevent it, there will be lint collecting over time in the ductwork. The smaller the ductwork, the worse the problem for a number of reasons. This is not just a comfort or efficiency problem like ductwork for HVAC systems, it is an actual fire hazard.

Think about it like hardening of the arteries - you want to prevent cholesterol, but you can only do so much with diet & medicine. If you have smaller arteries to start with, then any buildup of cholesterol would be even worse. (Not a doctor, just making an analogy here.)

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3" and 9' with an early elbow is a mild disaster. 3" ducting gives just over half the cross sectional area of 4".

You're constricting and complicating airflow, which can lead to clogging.

4" would be preferable, even with the extra turn.

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