How to Size a Home Drainage System

hey this is gwen your favorite plumber here today to talk about how to size the drainage system of a private residence last time i was humiliated from my art and handwriting in the comments section so i tried even harder this time you won't believe the level of animation and art in this video so it's worth watching [Music] a dfu or drainage fixture unit is a numerical value that was calculated in some lab assigned to each fixture that basically determines how much load that fixture is putting on the drainage system so the higher the number the more load and that dfu is calculated based on frequency and amount of water discharge so frequency of use and how much it's dumping into the system a couple of caveats this video is based on the 2018 uniform plumbing code so you're going to want to check with your jurisdiction to see what code you're on prior to proceeding right now it is the year 2022 and the most current code is 2018 upc here in montana so that's what this video is based on if you're on international plumbing code you're going to want to reference probably a different video they are similar but there could be minor differences so the assumptions we're making for this drawing is it's an isometric view this is a private residence okay so all these water closets or toilets are 1.6 gallon gravity flush so your standard flush 1.6 gallon this is all one level so this is all crawl space piping in the horizontal position there's some clean outs being shown not all of them are there but we can do a future video on you know clean out locations and requirements first we're going to assign the dfus or drainage fixture units to each individual fixture trap arm and trap arm size the first table we're going to use in the code book is section 702.1 this table right here all the plumbing appliances all the fixtures their minimum trap and trap arm size their drainage fixture unit for private and for public but today we're going to be focusing on private so in table 702.1 we have all the drainage fixture units so we'll go down the list starting over here so our bathtub is two fixture units our toilet here is three the lav here is one moving to this next bathroom group the shower 2dfu toilet 3 lav 1 over here in this bathroom group lab 1 lab 1 toilet 3 bathtub 2 and then the shower here is two as well moving over to our kitchen we have a kitchen sink which is 2dfu we have a laundry sink which is bigger than a normal lab this one's two as well and then we have a closed washer over here which is three and then our last bathroom we have our lav as one and the toilet s3 so those are the fixture units of our system here okay if we look in table 702.1 in the column of minimum trap and trap arm size that's the next thing we have to do for each fixture so this tub is an inch and a half this toilet is three inches this lab is inch and a quarter the shower is two inches [Music] this toilet is three inches this lab is inch and a quarter this lab is inch and a quarter the slab is inch and a quarter this toilet is three inch this tub is inch and a half this shower is two inch this kitchen sink is inch and a half but this is where you have to read the sub notes you'll notice by kitchen domestic there's a little sub note 2 next to the word that means to reference these notes below and two says provide a two inch minimum drain and as i explained earlier the trap arm and trap this would be the trap trap arm is between the trap and the vent and so this is the trap arm but this section of the pipe is the drain so that sub note 2 says this has to be 2 inch so we won't get too in-depth in uh some of these sub-notes because we're trying to keep it pretty basic but for the diy-ers that section and reference is important there that that needs to be two-inch the laundry sink same concept minimum trap and trap arm is inch and a half but as you hit the drain this also is under sub note 2 which says provide a minimum of 2 inch drain so that section will be 2 inch close washer and stand pipe is 2 inch hopping down to our last bathroom group trap and trap arm for this lav is inch and a quarter and this toilet is three inch here next we're going to assign the proper fixture unit to each pipe section always start with the highest part of the system and work your way down the drainage system is flowing downhill this way so we're going to start at this upper left-hand corner here and i have conveniently labeled these pipe sections a b c d etc okay so we'll start with this group and this farthest upstream section pipe section a from here to here is picking up this tub which is two drainage fixture units so pipe section a is carrying two as we work our way down pipe section b is now picking up pipe section a and this new toilet so we have two coming from upstream and three more fixture units from the toilet so pipe section b is picking up five total drainage fixture units and continue to work our way down next pipe section c is picking up the five fixture units from above at pipe section b but then we're also picking up this lav which is one fixture unit so pipe section c is six total dfus demand on that section moving upstream to pick up this group as we're working our way this way we're going to pick up pipe section d which is carrying a shower the shower according to our table 702.1 is two drainage fixture units so that pipe section is two e is picking up this tub is the only thing on this tub so pipe section e is two as well uh pipe section f is picking up both upstream is e and d so it's picking up two and two totaling four so we're just adding the total load together as we work our way down [Music] so as we're working downstream here now j is picking up all of i and all of c so it's nine plus six so j has 15 fixture units on it [Music] now we are working downstream towards n so looking upstream n is picking up all of m and all of j so it's 15 plus six there's 21 fixture unit load on n [Music] okay now moving down to r we're picking up all of this q plus all of this n so 21 plus 7 is 28 total drainage fixture units in this section r and lastly picking up this guest bathroom pipe section s just has a toilet on it for three picking up one lav here so pipe section t has the lav and the toilet together one plus three equals four here okay and then very lastly u is picking up everything above it here at r so 28 plus this bathroom group of four is now a total of 32 dfus for a total fixture unit on you the next table we need to reference is table 703.2 and this is what pipe size we need for each section okay to explain 703.2 it's pretty easy to read on the top here is the size of pipes between inch and a quarter and the table goes all the way up to 12. in my little cliff notes here i'm referencing inch and a quarter to four because that's the most common in residential so we have the pipe size here in this first row and then below that is how many dfus or drainage fixture units that pipe can support both in the vertical and horizontal position the difference in maximum fixture unit values between vertical and horizontal drainage piping is because of the two differing flow characteristics of vertical versus horizontal what that means is a vertical pipe can hold more flow in this model most of our stuff is on the horizontal plane so we're going to focus in this horizontal column so let's go back to pipe section a okay pipe section a has a total of two drainage fixture units and it's in the horizontal so we cannot run inch and a quarter we cannot run an inch and a half we have to run two inch pipe there because this is the maximum fixture value that it can hold so two is obviously more than one so we gotta size it to two inch so that's going to be a two inch pipe moving up we're picking up pipe section b this has five fixture units which could be carried by a two inch pipe but wait the toilet minimum pipe size is three inch as we saw before and we can never run a larger trap into a smaller branch so pipe section b even though it's five fixture units still has to be three inch okay moving down to this lab we're not really focusing on the vertical section but we will quickly uh this vertical lab is one fixture unit this pipe section section's in the vertical it can hold inch and a quarter can hold one so that pipe is okay to be inch and a quarter in real life most supply houses are only going to carry inch and a half so realistically in real life this is usually inch and a half but if we're sizing to the minimum size because it saves money that can be inch and a quarter moving down we have c here which is six fixture units according to our table it can be two inch butt weight remember we have the three inch toilet upstream so that still has to remain three inch so c is still three inch okay working towards the shower we have two fixture units here this minimum trap and trap arm is two inch for the shower so this pipe is two inch d in the horizontal here is two fixture units if we go back to our chart two fixture units and the horizontal has to be two inch so d is two inch he is picking up this tub which is two inch e in the horizontal picking up two fixture units is once again two inch [Music] and we're working our way down it's getting exciting pipe section j total of 15 dfu so if you look according to our table that has to be three inch [Music] moving down we're picking up our clothes washer we set it before that's a two inch trap and trap arm size it is three fixture units so it has to be two inch anyway so section o is two inch moving down to p picking up a laundry sink with its minimum trap and trap arm size of inch and a half but when we hit the drain that's a minimum 2 inch pipe as i explained earlier which then flows into p which is a total fixture unit of 5 so that's 2 inches well lastly we're picking up this guest bathroom this water closet is three inch as we've said before pipe section s is three inch picking up this lav with four fixture units is still three inch because it has a toilet upstream and now we're connecting into u so u has a total drainage fixture unit load of 32. 32 is less than 35 so that's a three inch pipe but wait there's a subsection note that'll get you confused here notes from table 703.2 note number four only four water closets or six unit traps allowed on a vertical pipe or stack and not to exceed three water closets or six unit traps on a horizontal branch or drain so remember this system is mainly in the horizontal and so if you count these toilets section u has one two three four toilets and as we remember from that note the horizontal is only allowed to pick up three so this actually has to up size to a four inch pipe as i mentioned earlier another key component to a drainage system is where to locate cleanouts cleanouts are so if there's ever a blockage you can snake the system and get it cleared this video does not cover those we'll cover that in a future video so more to come on that alrighty i hope everyone learned something today on sizing drainage for a private residence we covered most of the basics there are some advanced sizing with batteries of multiple fixtures and commercial settings that we could get into more in depth in a future video if you have any questions ask them in the comments and we'll try to get them answered [Music]

Channel: Williams Plumbing & Heating

Views: 40,772

Keywords: drainage fixture units explained, drainage fixture units, DFU, drainage system, residential drainage systems, plumbing sizing, drainage plumbing system, plumbing dfu calculation, residential plumbing basics, residential plumbing code, 2018 uniform plumbing code, plumbing system animation

Id: 1nLTn260OD4

Length: 14min 2sec (842 seconds)

Published: Tue Mar 08 2022