Plumbing trap pipe connection only works for 1/8" over 12"but not for 1/4"

[HVAC-Novice]

I have the attached P-trap that is based on oob family. 

It is my understanding plumbing pipe fixtures have some tolerance to accommodate attaching the sloped pipe. So they don't have to be exactly aligned (correct angle of incidence?).

In my example the highlighted pipe is supposed to be 1/4" over 12". but when i do that, the Trim/Extend tool cannot make a connection (P-trap is selected in pipe routing). But when I change the slop of that little pipe to 1/8" over 12", it works. Actually adding the P-trap changes the slope of that little pipe to 13/64" over 12"

it probably is fine to have the model a bit incorrect for that short distance. But i wonder if the fitting family can be edited to make it accept pipes with steeper slopes? 

The MEP settings have a pipe connection tolerance that is set to 5°. This should be enough for 1/4" over 12". Even if is et it to 10° it doesn't work for 1/4" over 12". 

Is there a way to edit the family or change Revit settings to make this work for slopes steeper than 1/8" over 12"? or is that a general limitation i have to live with ? 

Or is there a better way to add a P-trap than to use trim/extend? 

i also tried to run the pipe by using a regular elbow and then swapping it for a p-trap. But that goes wrong:

[iainsavage]

Your family already has the "Allow slope adjustments" active (you can't see this when the connector is Fitting but you can see and set it if you temporarily change to Global, then change back to Fitting).

There's also an angle tolerance for the connector but to be honest I've never used that. The help topic says:

[HVAC-Novice]

Thanks. I didn't know connector has the hidden "slope adjustment" checkmark. but this didn't seem to be my problem. 

My angle tolerance was at 5° (default). but even setting it to 10° didn't help. but 1/4"over 12" is just a but over 1° anyway. 

maybe it is one of those Revit things. 

[iainsavage]

It might not be the trap that's the problem - it could be the elbow downstream?

I also wouldn't tend to use the trim/extend tool on sloped pipe, it is more reliable to draw the pipe with a predefined slope using the slope control

or to tab-select the run and apply the slope to the whole run using the Edit Slope tool.

[HVAC-Novice · Accepted solution]

I looked at the elbow and the slop adjustment was NOT enabled. So I fixed that for most my fittings. But that didn't help. I still only can do the trap if slope is 1/8"over 12". Maybe the distance to the elbow is too short. I'll look into that more when I have a bit more space between elbow and the trap. 

I think for now I just adjust the slope to 1/8".  I noticed some YT video about plumbing in Revit don't use slope at all for short distances. not sure I'm a friend of that, but it simplifies modeling 🙂 

Edit: I'm marking this a resolved since using less slope seems to be the best workaround at this point and will be OK for the short distances.

[robert2JCCH]

Guilty of zero-sloping short trap runs here 😃

It 'seems' like the issue has to do with P-traps being unable to maintain the correct pipe slope as you convert the fitting from a standard elbow to a trap, since the horizontal connection pushes significantly far away from the equivalent elbow connection. So you either wind up with weird fractional slopes (13/64 off an originally 1/8th connection), or an Unexpected Results error (any larger slope).

I suspect this might be why the original Trap P - PVC family was defined as a multi-port family, but considering that plumbing workflows slow to a crawl if the family isn't set up as an Elbow, it's a necessary evil. Thankfully code-based trap arm length limitations keep the inaccuracy from being too egregious, but I've yet to find a functional workflow that allows rapid modelling while also allowing for that 1/4" trap arm slope. Just be mindful of those 4" traps with 10 ft trap arms, I suppose.