Plumbing critique

Arky217 · September 28, 2012, 11:17:50 PM

This is my initial proposed DWV arrangement for my retirement house that I built.

The house is in a remote area that is not governed by plumbing codes, however, I want to make sure that I'm not about to make any major mistakes that would prevent the draining and venting from working properly.

The sketch shows a 4" main line, however, after reading many pros and cons of a 3" line vs a 4" line, I'm still not sure which size to go with.

The sketch is pretty much to scale. It's on a post and pier foundation with an open 3 foot
crawl space, hence the trap on the tub being above the floor to prevent freezing.

All the piping to be PVC; I was thinking of schedule 20 for the vents, 40 for the drains.
And, of course, all the elbows, tees, etc. will be standard plumbing fixtures, not with the sharp angles as shown in the sketch.

( I would have shown a 3" stack vent, however, the 6" wall happened to fall right under a truss at the top and right over a joist at the bottom, leaving slightly less room than needed for a 3" pipe. )

Appreciate any suggestions that point out any errors in this arrangement that would give me problems.

Thanks,
Arky

Rob_O · September 29, 2012, 12:26:18 PM

Don't have the book handy but I believe you will need to do something to vent the sink on the left of your image. You can run a second pipe for venting or use a "trailer vent"

September 29, 2012, 02:36:37 PM

One glaring flaw---your vent stack has to be at least as many square inches in cross section as your main drain pipe. I made this mistake in my current build----figured 4" drain was superior in every way since I could choose the fall and it was nearly the same cost. When it came time to run the vent stack, that's when I came across both the code and a few horror stories from people who didn't provide adequate cross section in their vent stack. (I just ran three vents through the roof to meet the code)
Your 4" drain has about 12.5 net square inches area in cross section. Your 3" vent stack only has about 7 inches. This is significant---you really want sufficient airflow to follow stuff down the drain. I had to add a 1.5" vent (1.76 square inches) and a 2" vent (3.14") to get the proper vent area. Or you could stick with a 3" main drain and you'll be fine.
I was curious about why having it choked back wouldn't just slow the flow in a 4" pipe to that of a 3" pipe. Talking to a lifelong plumber, he said you really don't care how fast stuff goes down the drain---but you do want it to all go together and not start spreading out as it goes. That will lead to slow flushes, especially if someone's doing dishes or showering at the same time. You want everything to shoot down the pipe. Adequate ventilation area is key to this end.
Really, you should look at more than just the vent stack's cross section area. More important is the narrowest point. Your toilet on your schematic goes right into 4" pipe. That's good. The vent reduces to 2". Not so good. Sure, there's a branch to another pipe, but those two 2" vents together only add up to 6.28"---not nearly enough airflow for your 12.5"flush.
So I'd beef up the vent stack or reduce the main and toilet drain. Also the sink to the far left is screamin' for an AAV---it's the perfect application and it'll save you that long run for that vent.

alex trent · October 01, 2012, 12:24:33 PM

Another plumbing myth...like the too much slope on the line will cause clogs. You do not need all that vent pipe unless you have a special situation..like a LOT of flushes and drains going off at the same time (think boarding house)

Look at the real numbers and you will see why.

If you have a 4" main drain (12 sq. in). A shower at full volume puts out maybe the same amount of water as 1/2 inch hose (0.75 sq. in). That gets in the main line and is just more than a trickle...1/2" in 4" or 1/15 the volume . Then Suzy washes the dishes at the same time ..same thing as far as volume. So the water rushing down the main pipe fills only a fraction of the pipe and the air in front of the water going down does not get pushed out (down) most of it it flows back over the top to fill the void behind the water and the vent supplies very little air. You do need some vent to let air in but not as big as the drain. You simply are not pushing out all that much air. The oly way that happens is if the main drain is full all the way down. In residential construction that is not likely.

Now assume a big load down the toilet Unless you have a mule in the house, it s pretty much the same thing...the load fills half the pipe and you get the same airflow over top to fill in behind.

Yeah, here a vent is a good thing even without a mule in the bathroom...but a 1" pipe is plenty. Has to do with the dynamics of air and water flow. Actually a 3' main drain warrents a bigger vent than a 4" one as more of the pipe is filled with waste or water and less abilityn to flow back over the top..

I have two houses with 3 inch and 1 inch vent and works just fine. Nary a suck in the traps. Of course, it is not written in the code, so it must be wrong

alex trent · October 01, 2012, 08:08:41 PM

More info on vents.

This is CA code stuff so subtract 30% and divide by 1.36 for the real world. Even at this, 3 or 4 inch vents are not necessary.

Tomorrow I will do an empirical test on a house we are building up on the mountain. We have a place all plumbed in but the septic is not dug and the main sewer pipe not in. I'll attach a 3" pipe, run it 80 feet to here the Fosa will be and flush some macerated bananas and toilet paper down to see just how much of the pipe is taken up when they exit. My guess is 1/3 the diameter of the pipe. Grey water is on a different system so we have no way to check the trap gurgle phenomenon.

I'll let you know.

Here is the CA code....

With careful planning an entire bathroom group can be vented with one 1-1/2" vent.
Under the International Residential Code (IRC) all structures are required to have one "Main Vent" which must run undiminished in size from the bldg main drain through the roof. When additional auxillary vents are required they may be reduced to 1/2 the diameter of the line they serve but not less than 1-1/4".

By contrast, the California Code is primarily based upon the Uniform Plumbing Code (UPC) and the UPC takes a dramatically different approach to venting.

Under the UPC you are not required to have a "Main Vent" however the combined aggregate total of cross sectional area of all vents must be equal to or greater than the cross sectional area of the bldg main drain.

Under the UPC all vents must be computed by "Drainage Fixture Units"(DFU). A vent may be reduced to 1/2 the diameter of the line it serves providing that said vent meets the DFU restrictions of table UPC-T7-5.

To compute the vent size we must first determine the DFU load of the drain line or group of fixtures connected to the drain line.

Lavatory #1....................1DFU
Lavatory #2....................1DFU
Tub/Shower.....................2DFU
Toilet(= or less than 1.6gpf)..3DFU
Total load...... 7DFU

Code minimum drain line size for a toilet drain is 3".

A 3" horizontal branch drain with a 1/4" per foor pitch can handle up to 35DFU's so there is no problem connecting all the fixtures to the same branch line.

Under Table UPC T7-5 an 1-1/2" vent is rated for a maximum of 8 DFU for a maximum developed length of 50'.

A toilet is required to be on the end of the drain line. Under the UPC a 3" drain line may then run a maximum of 6' from the toilet to the vent. The unvented section of line upstream of a vent to a fixture is defined as a "Fixture Arm". The line downstream of the vent is then classified as a "Vented Branch" or "Vented Drain".

You could connect an 1-1/2" lavatory drain line to the 3" branch from the toilet within 6' of the toilet, then connect an 1-1/2" vent from the lavatory riser up to the roof. This will provide the code minimum 1-1/2" vent to the 3" branch.

The UPC allows an unvented 1-1/2" fixture arm to run 3' 6" from trap to vent therefore you could connect an 1-1/2" line from the 3" branch to the tub drain at any convenient point downstream of the lavatory line and the tub will wet vent from the 3" vented branch.

While the tub is required to have an 1-1/2" drain opening we are also permitted to increase the size of a trap or drain line by one nominal trade size so you could increase the size of the tub trap and drain line to 2" and it could run 5' from the trap to the point where it ties into the 3" vented branch without any additional venting.

Thus, we have effectively vented the entire bathroom group with one 1-1/2" vent.

October 01, 2012, 08:40:38 PM

Arky did say he was not in a code area, but he also stated "I want to make sure that I'm not about to make any major mistakes that would prevent the draining and venting from working properly."

flyingvan response is a valid one. It is a better response, IMO, that a statement to the effect that this is a plumbing myth. I am not a plumbing expert but I place more faith in the vent area = to sewer area than I believe this to be a myth. The plumbing code, as many other codes, has many exceptions to the rule. For example, I believe the IRC states that vent diamter is to be increased after a certain length is reached.

Stating that the vent area = sewer area is a myth is not fact, it is an opinion. There may be more than one absolute truth according to where one lives.

Because the responses here will be read and believed to be truth by many many more readers than the original poster, I strongly believe that information given here should be backed by facts.

alex trent · October 01, 2012, 09:15:47 PM

Read the extract from the CA code...I copied it in in red. I am not sure, but I think those qualify as facts. It is 1.5 inches vs. 4 inches. So, I think someone who ardently stresses that 4 inches is the minimum needs to be called on it...not to make a point with them but to keep the record on balance. You notice I said on balance, not to believe mine vs. the other. This site is not just about one person's opinion...it is that of many.

I agree that others will read these posts and that is exactly why they should get both (or all three or four ) sides of the story. If they do not and no one questions the one "answer"
the reader commonly takes as "this is it". On the other hand, if there is a disagreement, then the reader (either the original one or a later one) should ask again or do a bit of homework. to sort it out. If it is all that important, taking the answer of someone on a site like this is bit dangerous if that is all you do. That make sense?

Who on here signs off with "no one of us is as smart as all of us". That is a good quote...

No one here should to believe me or FlyingVan or anyone else in a vacuum. Isn't the idea to get input and decide for yourself? Or go someplace else to look it up, or ask another person? And then do it as you see fit.

Myth...here is the definition and I think not as "bad" as you think it connotes..."A widely held but often false idea".

John Raabe · October 01, 2012, 09:28:18 PM

I agree with Don on labeling our opinions as such. The strongest and most forcefully presented idea is not always going to be the best advice. It may however be the most remembered.

With specialty trades such as plumbing and wiring, I feel it is good advice to have a local tradesman review your work (or even just answer questions). It can keep you from doing all kinds of things you will later wish you had done differently.

But, I also appreciate the helpful ideas and suggestions that members present here even when they may not suit everybody or every situation. Our readers are big boys and girls, and we state this in the welcome message to the forum: "Please note: not everything shown or suggested on this forum is necessarily code compliant or good construction practice. The owner/builder should verify construction techniques for local conditions and building practices. If in doubt, get professional design and building advice. Check with your local building department to determine what codes apply to your project."

Nuff said.

MountainDon · October 01, 2012, 09:43:34 PM

We use the UPC in NM, though the IRC governs much of the construction industry here.

UPC 904.1, as used here, clearly states "The total aggregate cross-sectional of the venting areas shall not be less than the aggregate cross-sectional area of the building sewer..." As noted this can and does vary widely from state to state.... I take that to mean that if you add up the total number of vent pipe area and it comes to 11.75 sq. in. on a 4" main line you're not going to make code. The designer (in this case the DIY plumber) would have to add another vent pipe so that the square inches of all the vent pipes added together is over 12.56 sq.in... And sad as it is there are DIY'ers who mistakenly believe that two 2 inch pipes equal one 4 inch pipe.

I've run into that; maybe that is one reason I tend to simplify, try to get down to the simplest common denominator, so to speak.

October 01, 2012, 10:11:38 PM

Let me ask a equation...a real one not rhetorical.

How is the required cross section of the pipe in this case determined?

I can see beam spans, etc....you test till they break or something like that or nails and how much it takes to pull them out... But in this...is it calculated by physics guys or what....why not 70% or 120% of the stated value?

This is one thing that is hard to figure, but there are other things as well that I cannotsem to figure out how the number is arrived at.

MountainDon · October 01, 2012, 10:29:40 PM

I have to admit I do not know the methodology. Perhaps I shouldn't but in cases like this vent cross section area to sewer pipe cross section area relationship I defer to the published data whether or not I see or understand the theory. Or maybe that's I defer to the code book because I do not understand how it has been arrived at. There may be a lot of trial and error that goes back decades, and that the equal area thing is just the simplest thing that will take care of all eventualities. It does seem to me that some areas of plumbing have the appearance of being more of an art than a science, but that may just be me and the fact that I find it easier to go by the code book with some trades. Mechanical forces and electricity are subjects I can more readily get my teeth into.

Adherence to whatever the local code is will usually deliver a system that works. Deviation can deliver a system that works most of the time. If in doubt consider going by the book, whichever one it is. Maybe you get a toilet that gurgles if you deviate too much. Most of us could live with that. But if a trap got sucked clear of the trapped water and odorous air passed to the room, that would be tougher to bear. Those are just guessing, daydreaming, not real life examples. .... Although I do have a toilet that gurgles because I installed a fitting facing the wrong way. It seemed logical, but the inspector pointed it out and said it would likely gurgle when flushed. He passed it and left it up to me to correct or not. I did not and he was right, there's a gurgle as the flush completes.

flyingvan · October 02, 2012, 08:50:23 AM

Umm---Gringo, the code stuff you posted is backing what I'm saying---
'Under the International Residential Code (IRC) all structures are required to have one "Main Vent" which must run undiminished in size from the bldg main drain through the roof.'
'Under the UPC you are not required to have a "Main Vent" however the combined aggregate total of cross sectional area of all vents must be equal to or greater than the cross sectional area of the bldg main drain.'

This is pretty darned clear.

As far as figuring the math---(Like Don pointed out, 2+2 doesn't equal 4) Drain pipe is sized by inside diameter so divide by 2, square it, multiply by Pi. That gives each pipe's cross section area. (so a 4".../2=2, squared is 4, times 3.1415 is 12.564. A 2" comes to 3.141. you'd need 4 2" to meet the code)

Look, this site is my happy place and I don't need another place to argue, but a real tangible value of this site is encouraging people to build good structures. After rebuilding 3 homes and completely building 2 from the ground up, I've learned spending just a little more gets time and money gets a far superior product and shortcuts are for mass produced mcmansions...... I obviously don't have your experience in doing things the wrong way then seeing if it works. I have found building codes are NOT arbitrary. For your practical test to be valid, you have to add 40 years of buildup to everything, calcify your toilets, have frost build across your vents, and use all drains at the same time.
My approach is, comply with the codes, talk to life long experts in the field, ask 'why' a lot, don't try to save $20 now and be sorry later. By reducing the vent size, what have you gained? If your framing can't handle a 3" hole, maybe the whole structure needs to be looked at closer. Build for worst case scenario.
By undersizing your vents, you risk gurgling fixtures, creating vacuums that can pull the water out of your traps so sewer gasses get into your house, and frequent toilet clogs. A certain amount of aerobic air exchange is needed for septic systems to work properly and relies solely on turbulent flow when the house is vacant----almost zero in 2" pipes but significant in a 4".

alex trent · October 02, 2012, 08:54:10 AM

I certainly agree with you that if you can't figure out how its calculated or the "laws" behind it, you should go with the book.

I also believe in the power of empirical evidence.You have to be careful and not just take somebodies word. But in many cases it works just fine and make things doable or easier or more sustainable.

My drop from the house to the septic is 9 feet in about 100. The conventional wisdom is way to much and the solids will get left behind and clogs occur. But a few dissented from this and gave the logic. not wanting to just grasp as something hat fit my needs and then being sorry I did some tests. Mashed bannna, a bit of peel and some toilet paper flushed all came out the end like a shot.A couple of trials and seemed ok to hook up. i even did it with the low flush setting on the toilet. Seven months later all is well. This does not seem like something that will change over time...so I offer it as a way to deal with a situation like mine.

i would think that my description would be seen as "advice"..a bit more than an opinion, and not quite the rule of the code. As was stated...big boys and girls and before I did that based on one "test", I would surely do my own or some more looking.

I think my vent pipe reply was in the same vein...it talked about my theory of the air not being pushed out, etc. to anyone but the most simple minded, it is clearly 'thinking" not fact. but today i will test it in real life just to see what happens. I have experienced the gurgling and trap sucking actions in big houses...i htink they may, with all the connections and more complex piping be more prone to this than my straight shots, down and one turn...

alex trent · October 02, 2012, 09:16:25 AM

don't have your experience in doing things the wrong way

Maybe you should come down here and rewrite the codes for Nicaragua. To paraphrase one other member put it so nicely to me..."living in a dark place, you should bring some light"

My houses are not built the "wrong" way. They will stand up to anything you have overbuilt and wasted precious resources on. You could learn that codes are important and they are written to the lowest (or highest) common denominator and are for people like you who want to directly transfer what you read to what you build. That is fine for you and many others, but do not disparage others who do things differently. This site is also about opinions and practical experience and that is simply what mine are...clearly stated as such.

ummm..you get the point?

I have no frost, no calcium in the water, and I have used all my drains at the same time. Works fine. I did not do it to save money, I did it because for me it was the correct way. You see, we all are different and have different needs and opinions. and more than one thing works in life as in building. So, if you do not want to argue, don't...make your point and move on.

October 02, 2012, 09:32:27 AM

FlyingVan

Here is something you wrote a while back that i think sums my feelings as well...nice to finally agree..

While reading these posts it occurred to me that every owner/builder probably has their own set of priorities, and unique reasons for wanting to build a place. There's a balance between size, style, ease of construction, environmental concerns, expense, energy efficiency, logistics... I think it's more an art than a science. Just like art, there's science behind it all and you need some fundamentals.