3-in. vs. 4-in. toilet drains – which?
I am about to pour a slab-on-grade foundation for a one-story house in central Texas, 3 bedroom, 2 bath on a septic tank and well. I understand plumbing codes allow either a 3-inch or 4-inch waste drain if there are 2 toilets or less. My question is, which is better? I will be installing 1.6 gallon/flush toilets. I will also be diverting the washing machine drain away from the septic tank. With the smaller amount of liquid, am I more or less likely to have blockages with a 3-inch drain? It seems that with the larger 4-inch pipe, there might not be enough liquid depth to flush the solids through the pipe. If 3-inch is best, should I run it all the way to the septic tank, or should I transition to a 4-inch where the two toilet drains merge?
I worry about this because I have a friend who replaced his old toilets with the low-flush variety, and he is now having trouble with occasional blockages. It occurred to me that maybe the pipe size or grade is based on experience with the old toilets, and with the low-flush variety and water conservation, different design assumptions should be made.
Any advice would be appreciated, with a slab-on-grade, I only get one chance to get this right!
Replies
There are arguments both ways, but I don't think I'd want the main line less than 4". (In fact, I was under the impression that code generally required the main to be one size larger than the largest fixture.)
obtain a stool sample from each of the prospective occupants of the new home. measure the diameter of each sample to determine the two largest. if the two diameters added together are less than 3", go with 3", between 3" and 4", go with 4", and above 4" recommend a kind, gentle proctologist with tiny fingers.
Joe,
Code allows 3 inch around here. We still install 4 to the fixture and still use a full size vent. We do downsize to a 3 inch vent if the plumbing wall size is tight.
In the case of drains I believe size is always better. Clogs are exactly that and if you have a larger diameter pipe it is simply more difficult to clog no matter what gallonage of toilet you use.
1.6 flush units are not all the same. I used to have (can't find it) a test result from an independent lab on the flush ability (movement of solids) on various models of new toilets. I install Gerber and the were the 4th best with Toto being number one. Interest enough American Standard and Kohler were like 22nd and 26th or something there abouts. The report I read is now probably 4 or 5 years old so some may have improved. But I have had no problems with Gerber. DanT
the sample runs on the bottom 1/3 of the pipe. most blockage is due to pitch not size. with a three inch pipe you will have more water in the bottom of the pipe flowing the sample. with a low volume toliet the three be better. it would be lie trying to flow a sample in a ten inch pipe with just one gallon of water. you need a 1/4 to 1/8 per foot thats either one inch in four feet or one inch in eight feet. thats the target for pitch. I think it would be more important to make sure you have every bit of the 1/4 to the tank and the least amout of turns.
On my house I had to raise the house two course of blocck to maintain flow and I also put a cleanout on every toliet and sink.
Slightly off subject I was googling looking for something about drains and vent and read a report indicating that many of the new low fush toliets put more of a strain in the drain vent systems.
They are designing them to dump the water so fast (even the non-pressurized tanks units) that water velocity in the drain is much higher than in the past. The person that wrote this indicated that they thought that there will be changes in the code concerning things like vents and back to back toliets.
Yeah the newer low-volume units dump the water pretty fast, and that means there's still a slug of water going through there. The danger, of course, is that the water will outrace the waste and leave it laying in the bottom of the pipe. This is a little less likely with 3" pipe, but a lot depends on slope, bends, etc.
The argument the other way is that some "waste" is darn near 3" in diameter, and it risks plugging a 3" pipe in a bend or where there's been some buildup.
At the very least, if you go with 3" be sure that sweep bends are used.
BB,
Ok, assuming your example is correct then why would water back up into the house during a blockage? After all it would only block the lower third or so so the water would continue right over the partial blockage correct?
I have plumbed/cleaned drains starting in 1978. Fall is certainly a major factor and I too like 1/4" per foot but that wasn't the question. Given all factors being equal I want more room.
The bulk of sewage water, 87% I believe is the figure used, is soapy water from all the other sources in the system. The toilet accounts for a minimum of sewage in terms of total volume. If in fact the "package" stops in a 4 inch pipe the water coming from the tub, sink etc. would/should finish the job by washing it down. And a 4" pipe would/should have less resistance as the side walls are further apart so the "package" would not be as apt to stick to the unlubricated (water) sides. DanT
Your friend's problem may not be pipe diameter, but instead lack of proper venting. Also, some PVC 3 and 4" pipe have tighter turns than standard, which reduce flow.
Edited 4/22/2004 8:36 am ET by JACKPLANE
A good argument for hand washing after you go. Helps wash it down the drain.