Why is the maximum distance between a fixture outlet and a trap weir 24"?

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kendubya

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...long-time lurker. Thank you for all the knowledge. Why is the maximum distance between a fixture outlet and a trap weir 24"?
 

Jadnashua

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Things will accelerate because of gravity. Potentially, going fast enough so that it could clear the trap even if vented properly. IOW, to limit the amount of inertia of the falling water so it can fall back into the trap to keep it primed when the flow stops.
 

Jadnashua

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Gravity accelerates the water - the further it falls, the faster it goes - inertia keeps it moving. Momentum is the product of mass * velocity...doesn't really matter how much mass it has...get it going fast enough, it wants to keep going. There's a limit on how fast that can be, or it won't stop and keep the trap filled. A trap draining a big tub would have more momentum verses a vanity sink, but it's still the inertia or lack of it that allows a trap to function properly and limits how far it can drop.
 

SteveW

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Well, I learned something!

I had always thought of inertia as the energy required to initiate movement on a stationary object. I had to look up the actual definition and of course you are quite correct -- inertia is really more broadly the resistance of an object to any change in its state of movement. I tend to think of inertia as applying to nonmoving objects and momentum applying to moving objects.
 
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