New Shower

[LazyPup]

Kactuskid,
Although the differing codes take some radically differing views on venting there is one concept that they all seem to share. "The Main Vent shall run undiminished in size from the main drain through the roof." The only notable exception to that rule is that when the total length of the main vent is equal to or greater than 40' the main vent must be increased one nominal pipe size.

After the main vent has been established, additional auxillary vents may be downsized to 1/2 the diameter of the line they serve once they are 6" above the flood level rim of the highest fixture served, but in no case may an auxillary vent be less than 1 1/4".

In geographic areas subject to freezing temperatures (outside air 32deg or less) all vents must be increased to 3" at least 1' inside the attic space before passing through the roof. (Some local codes require 4")

[kactuskid]

Thanks for the reply LazyPup, I'm still a bit confused though. How is the size of the main vent established? Is the rule of thumb that it should be the same size as the main drain line? I know local codes vary but what does the IRC say about this? I assumed from the logic being these main vents basically serve dual purpose, for venting the structure and the municiple sewer, that the diameter needs to be equivalent in size to the main drain line.

You say that in freezing climates that all vents should be increased to 3" size one foot before going through the roof. Does this then assmue that the pipe diameter was less than 3". I don't understand this, I thought the drain piping needed to be at minimum 3", this would require a min. 3" vent stack then wouldn't it. I'm in Wisconsin and mainly interested in the cold climate rules.

Boy, this is so confusing, I'm basically just trying to understand a few rules of thumbs to go by. I really appreciate your input on all this.

[LazyPup]

The smallest diameter trap that is allowed on a Drain, Waste and Vent system is an 1 1/4" diameter trap, which may only be used on a bathroom lavatory sink.

By physical experimentation it was determined that an 1 1/4 trap could pass 7.5gal/min at standard gravity pressure, therefore 7.5gal/min was assigned as one Drainage Fixture Unit. Once the DFU was defined they they created a table of DFU values for each type of fixture that may be found in the structure.

The codes have tables that list each type of fixture, the minimum trap diameter required and the number of DFU's assigned to that fixture.
(From the International Residential Code)
Bathroom Lavatory 1DFU 1 1/4" trap
Bathtub/shower 2DFU 1 1/2" Trap
Shower Stall 2DFU 2" trap
Water Closet > 1.6gal/flush 4DFU N/A
Water Closet < 1.6gal/flush 3DFU N/A
Kitchen sink 2DFU 1 1/2"trap
Kitchen sink w/dishwasher 2DFU 1 1/2"trap
Kitchen sink w/disposal 2DFU 1 1/2"trap
Clothes washer standpipe 2DFU 2" trap
Laundry tub 2DFU 1 1/2" trap
laundry Group (washer & tub) 3DFU 1 1/2"
(Note that the table does not list a trap size for the water closet. Water closets-or toilets if you prefer, have a built in trap, therefore the line does not get a trap, however, the drain line must be equal to or greater than the trap diameter, therefore if the water closet has a 3" discharge port from its built in trap, the watercloset flange and drain must then be a 3" minimum.)

The table continues on and lists many other fixtures such as water coolers, Bidets, bar sinks, etc etc.

To layout the drain we first start with the "floor Plan" that shows the location of each fixture in the structure. Considering one fixture at a time we continue to add the DFU values until we know the total combined DFU load for the whole structure.

Consider a small house with two bathrooms, kitchen and laundry.

Bathroom #1
Water closet < 1.6gal/fl 3DFU
Tub/shower 2DFU
Lavatory 1 DFU
total 6DFU

Bathroom # 2 (master bath)
Water closet < 1.6gal/fl 3DFU
Tub 2DFU
Shower stall 2DFU
1st Lavatory bowl 1DFU
2nd Lavatory bowl 1DFU
total 9DFU

Kitchen
Kitchen sink/disposal 2DFU

Laundry
Washing machine standpipe 2DFU
Laundry tub 2DFU
4DFU (Laundry group May be derated to 3DFU)

Therefore the Total DFU load for the whole house is 20DFU's.

We then consult a table that lists the maximum DFU load permissible by pipe diamter.

1-1/4" pipe 1 DFU (Horizontal) 1 DFU (Vertical)
1-1/2" pipe 3DFU (H) 4 DFU (V)
2" pipe 6 DFU (H) 10DFU (V)
2-1/2" pipe 12DFU (H) 20DFU (V)
3" 20DFU (H) 48DFU (V)
4" pipe 160DFU (H) 240DFU (V)

First we must consider that the minimum line diameter that may serve a water closet is a 3" diameter line. From the table we can see that a 3" diameter line will carry a total combined load of 20DFU, therefore in this instance the proper size for the main drain, and house sewer line would be 3"

Depending upon the physical layout of the floor plan, it may be necessary to install a separate vent for each bathroom as well as the kitchen and laundry. At least one vent directly attached to the main drain (3") must be designated as the house "main vent" and must run from the main drain up and out through the roof, undiminished in size, meaning it must be 3" (Equal to the main drain)

The additioinal vents (Auxillary vents) may then be reduced to 1/2 the diameter of the line they serve once they are 6" above the flood level rim of the highest fixture they serve, but that line may be no less than 1- 1/4".

In a climate that is subject to freezing temps (northern climate) the auxillary vent must then be enlarged back to 3" diameter at least 1' inside the attic space before passing through the roof.

It is very important that we use these table to determine the proper line size.

When properly sized a should only be 1/2 full at full load. That allows the lower half of the pipe for waste flow while allowing the upper half to permit vent air to move.

If a line is too small, the load then fills the pipe and restricts the movement of vent air resulting in the water seal being sucked out of the traps.

If a line is too large the level of liquid in the line is not sufficiently deep enough to suspend and carry any solid wastes. Therefore, the solids will sit idle in the line while the liquids flow around the solids, then end result is a line that continually clogs up.

[kactuskid]

Wow, thanks a ton for all the great info. Sure clears up alot of questions, though I still have one more. Around here there are alot of 4" drain lines. My own homes main drain is a 4 incher. Then the 4" main vent pipe stubs up outta the basement floor for about 1 foot. Then the piping is transitioned to a 3" pipe that continues all the way through the roof line. My home does not exceed the allowed 48 DFU's for a 3" pipe. So, my question is, is it ok that a 4" drain was installed. I'm guessing as long as the proper sloping for a 4" drain line is followed that it's within code to install this larger diameter pipe and possibly prefferred over a 3" drain pipe.

[LazyPup]

You will note in the example i cited that the total combined DFU load for the structure was 20DFU which is the maximum allowable DFU load for a 3" horizontal line. If we were to also add a floor drain in the laundry area it would then put the total over the 20DFU and require the main drain be enlarged up to the next size (4").

Many local codes also establish a minimum diameter line size that may be run under a slab. It may be that the actual DFU load would call for a 3" line and main vent, but your local code requires a minimum of 4" underslab, in that case it would be legal to run the 4" underslab with a 3" main vent, but it would required a written approval of the local code inspection authority.

[kactuskid]

Ok, that clears things up tremendously for me. I can't thank you enough for all your time, it's really appreciated. Let me know whenever I can lend a hand your way.