View Full Version : 2 Shower Drains

10-28-2009, 08:03 AM
Hi Everyone

My inspector has flagged my shower design in my permit application - he says based on his DFU claculation I need 2 shower drains

Design Plan
4x5 Shower - one 2 inch drain
Moen 3/4" thermostatic valve feeding 4 volume contols for:
Overhead shower - 2.5GPM
Fixed Shower - 2.5GPM
Slide Bar - 2.5GPM
3 Body Sprays - 3@1.8GPM ea - 5.4GPM total

The inspector is a good guy but his DFU explanation was a little unlclear - I was hoping someone could look at the above and see if I have any standing to debate - or at least clarifu DFU limits etc

Also - if I do have to add another drain in the shower - I assume that has to be a seperate trap as well with a wye tying into the drain line similar to the existing drain setup?

Many thx in adavance for any guidance - glenn

10-28-2009, 08:27 AM
The drain size is based on how much water COULD be flowing into the shower. In your case, since theoretically, there could be more water than a 2" drain could handle, I would opt for a 3" drain, rather than two 2" ones. The actual water flow will be determined by the capacity of the shower valve AND which flow controls are open at any one time.

Runs with bison
10-28-2009, 08:41 AM
I'm not familar with code on such things, but in designing process equipment for self venting flow (so that the drains don't back up) a 2" pipe is only good for about 8 gpm. Your design is for 12.9 gpm when all are running. So I agree with the inspector that a single 2" drain is insufficient.

10-28-2009, 09:07 AM
Thanks so much for the replies - appreciate the clarification

Going to a 3 inch drain option means running the 3 inch all the way back to the main drain - this would be a pain

Adding the second 2 inch drain - assume I would add a 2nd trap and tie into the exisiting drain line similar to the existing trap?

Many thx again - glenn

10-28-2009, 09:58 AM
If the two 2" drains run together as a single 2" line, then it doesn't improve the flow at all since functionally, all you have is a 2" drain. It only works if you run each 2" drain to a larger pipe separately. Otherwise, make it a single 3" drain.

If the valves are diverters and you can't run all of the heads at the same time, the story would be different. BUT, if they all CAN be on at the same time, you may have problems. Note, you'd need 3/4" valve minimum to achieve those flow rates, and you may still notice performance issues since the valve would be close to being maxed out.

Peter Griffin
10-28-2009, 04:15 PM
NO you have to go 3" back to the stack regardless.

10-29-2009, 09:29 AM
Two 2" drains into a 2" drain pipe does not make much sense if added flow is the reason for them, ESPECIALLY if there are ANY other drains connected to the 2". The flow capacity for a pipe is based on both the horizontal and vertical components, not just the pipe from the drain to the trap.

11-01-2009, 04:54 PM
With fixtures less than 2.5GPM each, and body sprays less than 1.8GPM each, you might save yourself a lot of work by using your existing pipes instead of redoing them. Make the total match a two inch drain. There is a maximum flow in any size of pipe.

What distance is between the shower drain p trap and the next bigger pipe?
What distance is there until venting?

Did you say in your first post that there is an overhead shower and a fixed shower, and a handshower? What is the difference between the first two?

Edited to add this: Also, behind the DFU numbers are arithmetic calculations that you can do, too. DFU tables are shortcuts that work well except in this kind of situation, which is more complex than was foreseen by the framers of the DFU tables.