Venting

[PTkach]

We have a horizontal 1.5" vent. Because of an obstruction, we need to go upwards 6" and then downward to our new shower. Will this break vent?

[LazyPup]

It is difficult to provide a good answer without knowing the actual layout but suffice it to say that no matter how it is laid out, the answer to your question is "NO".

In the post you identified the line in question as a "horizontal vent". If this line is in fact a vent it would be a violation of code because the drain line runs horizontal from the shower trap to the branch or stack to which it terminates and the code prohibits any horizontal vents until the vent is a minimum of 6" above the flood level rim of the highest fixture served by that vent. This means that even if the shower were the only fixture served by the drain line a vent would have to rise vertically to a point 6" higher than the curb of the shower before it could turn horizontally.

If the line in question is in fact the drain line from the shower to a stack or horizontal branch line you now have a couple problems. First of all, the code minimum for a shower drain is a 2" diameter line, not an 1-1/2" line so the line is too small.

All drain lines 3" or less in diameter are required to maintain a 1/4" per foot drop in pitch from the trap to the point where they discharge. Under no circumstances may a drain line be offset upwards and drop down again. Not only would such and offset prevent the proper flow in that line it would also negate the required venting action for that line.

All horizontal vent lines are required to have a 1/4" per foot pitch dropping from the roof opening towards the drain line that they are connected to in order to insure any condensate moisture that developes in that line will ultimately discharge out the drain line. If you were to offest a vent line in the manner you describe it would result in trapping the condensate moisture and ultimatel obstructing the vent.

If you can provide a bit more information on your layout perhaps we can help you find a suitable method of connecting your system.

[PTkach]

Sorry, I guess I didn't phrase it correctly. Here's what I've got:

In the basement, we're installing a shower. We have a 1.5" dry horizontal vent pipe in the basement ceiling that services the laundry sink. Because of a heat duct in the way, we need to go upwards over the heat duct and then downward to tie into the 2" drainpipe for the shower. Is there a way to do this without breaking the vent?

[LazyPup]

First of all we must determine if the proposed shower will even requires a vent.

It would depend upon which National Model code your local code is based upon because the International Residential Code (IRC) takes some rather significantly differing views on venting from the Uniform Plumbing Code (UPC).

Under the IRC we may compute the maximum length of an unvented "waste arm" by dividing the diameter of the pipe by the required pitch. since all lines of 3" or less require a pitch of 1/4" per foot we could then compute the maximum length as 2"/ .25 = 8'. This means that you could run a drain line a maximum of 8' (measured on a line on the top of the pipe) from the trap weir, which is the point where the water spills out of the trap to the vented line or stack that this line is discharging into without requiring any additional venting.

The UPC uses basically the same formula but they then derate the length by approximately 40% so the maximum length of a 2" unvented waste arm is 5'.

Both codes require a shower to have a 2" trap and drain line, however they offer an exception that permits the trap and line to be increased by one nominal trade size. Since there is no 2.5" DWV pipe the next larger nominal trade size is 3". The IRC would permit the length of a 3" unvented waste arm to be 12' while the UPC limits it to 6'.

If the length of the shower drain is equal to or less than those lengths you would not require any additional venting. If the lenght exceeds those limits you would then be required to establish an auxillary vent within those limits.

In your post you state that the vent in question is serving both the shower and the laundry tub. In this case the vent must run vertical to a point at least 6" above the top rim of the laundry tub before it may turn horizontal. In this case you could make the primary section of the horizontal run at a level below the duct, then turn upwards after passing the duct.

In a situation where there is a conflict of location between an HVAC duct and a drain or vent line, it is the HVAC duct that must be offset.

You stated that this vent serves both the laundry tub and the proposed shower so I am assuming that the vent is okay for the laundry tub, it is only the shower connector that is presenting a problem, if this is the case their is one other alternative you could consider.

The International Residential Code requires all structures to have one main vent that must run undiminished in size from the building main drain through the roof. After that is in place you may termate auxillary vents in a number of ways:
1. Through the wall (subject to a number of conditions, most of which will not work for a basement).
2. In the attic space.
3. An unlimited use of Air Admittance Valves (AAV or Studor vents).

The Uniform Plumbing Code will permit a maximum of ONE Air Admittance Valve (AAV) per structure.

If you are under the IRC or if you are under the UPC and have no other AAV's in the structure you could consider using an AAV to create an auxillary vent for the proposed shower.