Changing diameter of trap arm to gain distance?

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KSdoit

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The distance from my sink's trap weir to its connection to the drain/vent fitting is 9 feet, so using 2" diameter at 1/4 slope exceeds the physics and code.

However ... is it legal under IPC to start with a 2" diameter trap arm (starting from the trap weir at 1/4" slope) and then after 6 feet increase the diameter of the trap arm to 3" diameter for a total of 9 feet of trap arm (all at 1/4" slope). This increase from 2" to 3" diameter after 6 feet would allow the physics (allow air) to work properly but is this legal and/or has anyone seen something like this done?
 

hj

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Unless you were to use an "eccentric reducer/increaser" the change in size would introduce a "step" in the flow line. And all codes prohibit increasing the pipe size in drain lines in the upstream direction.
 

wwhitney

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I agree the basic anti-siphon physics would work for the arrangement proposed in the OP. But the codes may impose additional requirements.

For the UPC, 1003.3 says that "The trap shall be the same size as the trap arm to which it is connected." If we interpret that as "a pipe segment that is part of a trap arm shall be the same size as the trap served" then the arrangement in the OP is prohibited.

For the IPC, the arrangement may not be prohibited, as I do not find any equivalent language. However, IPC 909.1 "Distance from Trap to Vent" has a table based on trap size, not trap arm size. So increasing the trap arm size does not provide any additional distance allowance, as the trap size remains unchanged.


For the OP, what sort of sink is this? The proper solution is most likely to pull a dry vent off the fixture drain before you reach the stack; that dry vent needs to rise to at least 6" above the fixture flood level. Then the dry vent can turn horizontally to follow the path of the fixture drain to the stack, just at the higher elevation. At the stack assuming nothing is draining in from above, the dry vent can join the stack.

Cheers, Wayne
 

Jeff H Young

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sorry for being too blunt but trap arms cant change sizes basicaly to protect the trap seal and to eliminate siphon or clogging .
when a code is written there is often no reason given. its the code. you might be able to run a 2 inch x10 ft trap arm and not have a problem. or an s trap
 

KSdoit

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I agree the basic anti-siphon physics would work for the arrangement proposed in the OP. But the codes may impose additional requirements.

For the UPC, 1003.3 says that "The trap shall be the same size as the trap arm to which it is connected." If we interpret that as "a pipe segment that is part of a trap arm shall be the same size as the trap served" then the arrangement in the OP is prohibited.

For the IPC, the arrangement may not be prohibited, as I do not find any equivalent language. However, IPC 909.1 "Distance from Trap to Vent" has a table based on trap size, not trap arm size. So increasing the trap arm size does not provide any additional distance allowance, as the trap size remains unchanged.


For the OP, what sort of sink is this? The proper solution is most likely to pull a dry vent off the fixture drain before you reach the stack; that dry vent needs to rise to at least 6" above the fixture flood level. Then the dry vent can turn horizontally to follow the path of the fixture drain to the stack, just at the higher elevation. At the stack assuming nothing is draining in from above, the dry vent can join the stack.

Cheers, Wayne
 

KSdoit

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The sink is a bar sink under a staircase. I have no room vertically to do a closer-to-the-sink dry vent because I cannot offset horizontally above 42 inches. So a proper (non AAV) vent closer to the sink itself is a non-starter due to my under-the-staircase vertical limitations. I'll do a 3 inch trap and 3 inch trap arm before I put in a AAV if I have to, but I am first wanting to explore my question. As you have pointed out, in the IPC, I cannot find any specific language prohibiting increasing the trap arm diameter and as you have pointed out the basic anti-syphon physics would indeed work which is the purpose of the code. The code lengths (e.g. 8 foot for 2 inch) are just basic guidelines/shortcuts to the real meaning of the rule. For example, if you slope an 8 foot by 2 inch diameter trap arm more than 1/4 per foot, you have broken the code in spite of the act you have not surpassed the 8 foot guideline. Saying you are code compliant with your 2" seven-foot trap arm is meaningless if you've sloped it too much. So in my situation, I am starting with a 2 inch trap and a 2 inch trap arm AND I am not decreasing to a smaller diameter pipe, i am increasing pipe diameter (which is allowed elsewhere) and I am doing so well before the 2" diameter 1/4 slope cuts off the air by increasing to a 3" pipe. As you pointed out, this would keep the anti-siphon physics still working correctly which isn't that the real purpose of the codebook rule? I understand what I am asking about is unconventional. I would like to know for certain whether or not what I am proposing is spelled out as illegal in the IPC code book.
 

wwhitney

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What you propose clearly violates IPC 909.1. Whether the table 909.1 was originally derived as a short cut to the physics or not, it was codified as written.

Even under a staircase, I don't see why you couldn't use a 1-1/2" trap arm and after at most 6' of run at exactly 1/4" per foot fall, take off a vent that rises to 6" above the fixture flood rim level, i.e. 42" to 48" above finish floor. Seems like 6' of run should be enough to get to a wall with sufficient headroom.

If you really can't do that, use an AAV. A 3" trap on a bar sink is ridiculous, and probably violates IPC 1002.2, which requires traps to be self scouring.

Cheers, Wayne
 

John Gayewski

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I thought Kansas was upc. Maybe I'm thinking Minnesota, either way a dry vent is almost certainly possible it probably depends how far you want to take this project i.e. tearing out drywall or building stone kind of soffit to run the pipe.

I agree a 3" trap arm for this is worse than an aav.
 
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