lazypup:

Thanks for input.

Got any rational "as to reason why code requires" toilet must be at end?

My rational better to have lav or shower as will aid in flushing line.
Just curious to thinking behind this!

Actually connecting another drain line to the base of the closet bend will reduce the efficiency of the flush which is a watercloset is required to be on the end of a fixture arm and why low heel closet bends are not permitted under many codes.

The watercloset is also the singular exception that proves the rule prohibiting S-traps and limiting the length of a fixture arm.

When a liquid is discharged through an S-trap as soon as the top of the trapway is filled with liquid the velocity of flow and the surface tension of the liquid create a syphon action which continues until all the liquid in the trap is sucked up to the top of the S. When air finally reaches the top of the S and breaks the syphon the little bit of water remaining in the upstream part of the trapway will backflow back into the trap, but it is not enough to fill the trapway sufficient to create a seal. The end result is that the trap is left open allowing sewer gasses to enter the structure.

A watercloset relies upon that suction action to lift the liquids and solids from the bowl up, over and through the trapway. It further relies upon that liquid remaining in a large slow moving slug that pushes the air in the pipe ahead and creating a partial vacuum in the line behind the flow to continue the syphon action.

Under the UPC the maximum allowable length of a 3" line from trap weir to vent opening is 6' whereas under the IRC a 3" line may run 12' from trap weir to vent opening, except, if the only fixture served by that 3" line is a watercloset it may run an indefinite length from trap weir to vent opening.

(in my house I have an American Standard toilet that was $99 for the tank, lid, bowl, seat, wax ring and mount bolts set on a 19' run of 3" from closet bend to vent opening and I can assure you, if you happen to be seated when you flush this toilet you better hang on or your might go out with the waste.)

Now I know there are some ppl here who will argue the point, but the fact remains, if a vent is placed too close to the closet bend it will reduce the effect of the syphon action and the flush will not be nearly as powerful.

Connecting a vented lav and shower drain to a low heel inlet will have the same effect as putting the vent close to the closet bend.

This also explains why a watercloset is required to have the trap primer line connected to the fill valve and discharging into the standpipe. After the flush, as the tank is refilling the trap primer discharges a small amount of water into the bowl to insure the trap is refilled to assure a proper seal.

lasypup:

Very Very Interesting.

Regarding:

In my case I'm not planning on using a low heel connector.
Based upon your input I thought maybe this would work better?

1) Run the 3 inch branch line the entire distance then:
a) connect the toilet fixture arm (run from toilet to 3 inch branch is about 18 inches) using a long sweep Y (3 inch) and then upsize line to 4 inch for the toilet flange
b) connect the shower fixture arm using a long sweep Y (3 to 2 inch)-- (again the run from shower to 3 inch branch is about 18 inches) with an S trap between the shower and the branch line -- OR -- use a T and after the S trap come straight down from the S trap to the 3 inch branch
c) Use a reducer coupling to Downsize the 3 inch branch to a 2 inch at its end and run the lav 2 inch line (wet vent) into the end of the branch.
d) I have an existing 1 1/2 vent "down" stream from where the toilet "Y" (ties into) the branch about 18 inches from the center of the "Y".
e) The two vents on either end (1 1/2 and 2) are existing vents and this saves a lot of attic work.

Any recommendations to do otherwise?

I recommend you carefully print out that design then wad the paper up real tight and toss it in the trash can. Not one single point of that design would pass code.

First of all, there is absolutely NO guesswork in Plumbing. The code defines everything.

Now let us look at your design:

"1) Run the 3 inch branch line the entire distance then:"

A table in the code defines the drain & tailpiece diameter for each fixture. The associated trap is then to be the same size. (Code will allow increasing the trap and drain line by one nominal trade size)

Code mandates the lavatory to have an 1-1/4 drain & trap. Since the lav is the only drain in a residential structure that is permitted an 1-1/4" drain & trap, most plumbers now opt to increase it one nominal trade size to 1-1/2" to preclude the necessity of carrying an inventory of 1-1/4" pipe & fittings.

"a) connect the toilet fixture arm (run from toilet to 3 inch branch is about 18 inches) using a long sweep Y (3 inch) and then upsize line to 4 inch for the toilet flange"

You may use a wye & 1/8 bend or combo to make a horizontal fixture arm to the watercloset location, then install a closet bend on the end of the fixture arm to turn up to the closet flange. You MAY NOT increase to 4". First off, for WC that are 1.6gpf or less the code requires a 3" flange and fixture arm. Second, if you were to install a 4" flange that would then be reducing the line size in the direction of flow, which is again prohibited by code.

"b) connect the shower fixture arm using a long sweep Y (3 to 2 inch)-- (again the run from shower to 3 inch branch is about 18 inches) with an S trap between the shower and the branch line -- OR -- use a T and after the S trap come straight down from the S trap to the 3 inch branch "

The correct size for a shower drain is 2" and S-traps are expressly prohibited by code. Code also prohibits using a TEE on a horizontal line.

"c) Use a reducer coupling to Downsize the 3 inch branch to a 2 inch at its end and run the lav 2 inch line (wet vent) into the end of the branch."

The correct size for a lav drain is 1-1/4" and code will allow increasing one nominal trade size, which would then be 1-1/2". In this case we also need to use 1-1/2" because the line from the lav is the vent for the shower and WC. Code says a vent line my be reduced to 1/2 the diameter of the drain it is serving and given that the WC requires a 3" line, we then need an 1-1/2" vent.

Once this vent is established no other vents are required, but since you already have another 1-1/2" vent on the downstream side of the WC I would leave it there.

See attached illustration:

Appreciate your time and especially your feedback.

Not in a position to debate at this point (still learning and will review the IPC again to see what I missed) -- but -- I thought all IPC values were minimums -- as long as the minimum flow rate in the line is meet. My idea to use 3 inch the entire length was to make clean out easier.

Using smaller pipe would defintiely increase the flow rate (which is better IMHO) especially with the requirement of these new 1.6 low flow toilets.

Personally don't understand why the "Low Heel Inlet Closet Bend" gains anything other than the ability to save ($$$) by not needing the "2 to 3" reducer bushing

After review:
If you go to Table 709.1 and do a look up you get the fixture unit and trap size "minimums":

Lavatory.... 1......1 1/4
Shower.......2......2
Water Closet.6......integral to fixture

Then if you go to Table 710.2(2) it determines the maximum number of fixture units you can have on a particular line size. For example a 2 inch line can have a max of 6 FU while a 3 inch line can have a max 20 FU.

The way I read these tables is they set minimums. So you
can use a larger line size if desired.

No where could I find (still looking) limits on pipe sizing. For example say you had 10 showers in a bathroom.
The pipe size may need to be increased (didn't compute) if all 10 showers are used at the same time. Now if only one of those showers is being used it would still have a larger pipe size. The water volume would be comparable to a bathroom shower which may use a smaller pipe size. So as I see it (probably wrongly) the larger sizing makes no difference -- only the economics -- as it is cheaper to say run a 3 inch versus a 4 inch line.

What Have I missed??

I know that this is an old discussion, but does the position of the toilet in relation to where it links with the main soil line as important if you are dry venting?
So, each fixture has a trap arm that terminates in a sanitary tee (with a vent line that will tie to the main vent stack in the attic) and after going downard, the waste pipes meet up with the main soil pipe...

In this situation, does the order matter? Also, if I add in a washing machine to the mix, is there a maximum/minimum distance the standpipe drain must be from where the other fixtures tie in to the main soil pipe?

does the position of the toilet in relation to where it links with the main soil line as important if you are dry venting? Hard to understand what you are describing, a diagram would be helpful. Make sure you don't put a trap under the toilet, the toilet is the trap. Sanitary tees may only be used on drainage piping when transitioning from horizontal to vertical, you should use a wye or long bend 90 wye on horizontal drains (soil stack).

In this situation, does the order matter? Order of what dumps into the vertical stack matters, things further down the line are considered wet vented unless you dry vent them or unless the vertical stack is as large as the soil stack and wet venting is sufficient there.

Also, if I add in a washing machine to the mix, is there a maximum/minimum distance the standpipe drain must be from where the other fixtures tie in to the main soil pipe? Drain at washing machine should be 2", be dry vented, and have its own line to the main soil pipe. If it shares a line and the line is being used and the washer starts to pump, there wont be enough room for the pumped water and it end up all over your laundry room.

Here are some common DWV mistakes to avoid.

http://www.co.lincoln.wa.us/Assessor/Building/Top%2010%20plumbing%20mistakes%20handout-4-09.pdf