Double sink connection to cast iron DWV stack

[MrPither]

I am remodeling a second floor bathroom in a 2-story Dutch colonial (built 1928). The main DWV stack is 4" nominal (actual OD about 4.6") cast iron (CI). The 2nd floor bathroom has a total of three bells/drain inlets into the main CI DWV stack - a 5" horizontal bell (for the toilet drain - a 4" ABS pipe current goes into it, oakum/lead sealed), a 3" horizontal bell right next to it, at an angle to it (for the bathtub drain). Both of these bells are a few inches below floor level. Higher up on the CI stack (18" above rough floor) is a horizontal 1-1/4" NPT threaded opening, which was used for a single lavatory sink drain.

I am replacing the single sink with a double sink unit (one piece porcelain unit, with two bowls, with separate drains/p-traps and separate hot/cold supplies for each bowl). I removed the original, rusty 1-1/4" galvanized DWV pipe that was attached to the 1-1/4" NPT opening on the main DWV stack. I plan to connect the dual sink to the 4" main DWV stack using all PVC DWV pipe.

My question is, can I use the existing 1-1/4" NPT inlet on the main CI DWV pipe, or do I need a larger 1-1/2" opening because of the double sinks? If so, do I need to remove the section of CI with the 1-1/4" NPT inlet and replace that removed section with 4" PVC with a 1-1/2" inlet? Yikes, I hope not! but if necessary I will. Thanks!

Here's a photo of the CI DWV stack

[LazyPup]

First of all, allow me to comment that your description is excellant, I would give your paper a raw score of 90% for a B+.

A couple minor points.

1. "The main DWV stack is 4" nominal (actual OD about 4.6")" A minor correction here. This is not a nominal pipe, it is in fact 4"CIP (Cast Iron Pipe"

DEFINITIONS:
PIPE SIZE: is always referring to the Internal Diameter (ID)
TUBING SIZE: is expressed in Outside Diameter (OD).
PIPE SCHEDULE: The term schedule properly refers to the Shedule(table or chart) of size specifications as defined by the ANSI (American National Standards Institute), ASSE (American Society of Sanitary Engineers) & ASME (American Society of Mechanical Engineers) publications. The pipe schedules are listed as 10,20,40,80,120,200 and in some cases 300 + DWV grade. While we commonly think of the schedule rating as the wall thickness, it properly defines wall thichness, maximum working pressure, Maximum and minimum working temperature, type of coating or finish, and in the case of synthetic materials the schedule would define maximum sunlight exposure.

The Term "Nominal" is rarely used in the plumbing industry, but in general the term Nominal defines a piece of material that is slightly larger or smaller than the true dimension of the material it takes the place of, by example, consider a 1/2" PVC pipe. When making schedule 80 PVC the fitting sizes remain the same as schedule 40 fittings therefore the OD of the pipe must remain constant. While pipe is properly sized by its internal diameter, in order to keep the fitting sizes constant while making an increased wall thickness, the actual internal diameter opening is slightly smaller than 1/2". It is then referred to as 1/2" nominal and when sizing pipe runs by capacity a schedule 80 pipe may be used in the same place as its schedule 40 counterpart.

Now to your problem. A lavatory bowl is rated at 1DFU (drainage fixture unit) and an 1-1/4 pipe is rated at a maximum of 1 DFU therefore you cannot connect two sinks to an 1-1/4" line.

One solution would be to cut the stack and replace the 4 x 4 x 1-1/4 Sanitary Tee with a 4 x 4 x 1-1/2 Sanitary Tee, But, cutting cast iron pipe is a lot of work and you run a serious risk of having the remaining vertical portion of the stack crashing down on you.

A simpler solution would be to run an 1-1/2" line from the sink location to the stack location.

Where you have the 3" bell where you intend to connect the tub drain, come off that bell with a short lenght of 2" and tee the 1-1/2 lavatory drain into that line.

You could then either continue the 2" to the tub or you could reduce the 2" to 1-1/2" and run the 1-1/2" to the tub. ( 2" line is rated for a maximum of 6DFU and your load is 1 DFU for each lavatory (2 DFU) plus 2 DFU's for the tub/shower for a combined load of 4 DFU's. Well under the max rating of 6DFU's for the 2")

You could then install a plug or cap in the existing 1-1/4 opening on the stack.

You stated that your sinks will have separate traps. That may not be necessary. If the maximum horizont distance between the two drain openings is not greater than 30" you could combine both drains into one with an end waste or center waste kit such as used for a double kitchen sink, then both lavs can share a common 1-1/2 trap. This would require making the waste arm out of the wall a bit lower to compensate for the end waste kit, however if your local code is based upon the International Residential Code this could also work for you. You could then run a horizonatal 1-1/4 off the top of the waste arm riser back to the stack 1-1/4" opening as an auxillary vent. (If you are under the Unform Plumbing code you may not make a horizontal vent)

[MrPither]

Thanks LazyPup. Wow, I didn't even think of using the 3" bell to drain the double sink! It'll require some snaking around to get the double sink's 1-1/2" pipe in position to drain into that bell, but it certainly can be done. It'll be much simpler than removing the section of cast iron pipe (I had already installed riser clamps to do so - whew I'm glad I didn't have to go that route). You saved me a lot of work.

I thought about combining the two drains the way you suggested (the drain separation is 28"), but it's a console type sink and the plumbing underneath will be visible. So partly for asthetic reasons I think I will probably go with two separate chrome or polished brass p-traps/drains.

In case of the 3" cast iron bell, I plan to insert a Fernco donut (3"/2") and 2" PVC pipe instead of trying to use an oakum/lead seal. The original bathtub drain was a solid lead pipe (probably dating back to 1928), soldered/brazed to a short section of brass pipe that was oakum/lead sealed to that 3" bell. As you can see from the photo, I cleaned all of that out so it's ready for the Fernco and PVC. The other larger bell (5") has ABS pipe (I cut away the old toilet flange and 90 deg ABS elbow), and my plan is use what is there and hook up a new toilet flange to it rather than try to remove all that oakum/lead and start over.

Thanks again for all your help.