Plumbing Design Issues

So now that we've done the riser diagrams, now that gives us an opportunity we can see what the relationship is between all of the different fixtures and the main stacks. Do we have the stack way over to the left and we reach across a whole series of different fixtures at which point we have to think about how it builds up in terms of the size of the pipe. Maybe we do it a different way. Well, we've decided all that by how we drew it in the riser diagrams. So we're drawing those out in order to understand what the relationship is between all those different fixtures.

So once we have those, now we can size the different pipes. So if you have, for example, a waste pipe going along and it's got say four different WCs on it. And I have that on multiple floors.

Well, clearly this pipe can be a much smaller pipe than maybe this pipe. That once I have more of these things on that one line of pipe, I'm gonna need to have a bigger pipe, in order to make sure it can accommodate all these as it goes along. And the way that we do that is through the idea of fixture units. A little bit strangely, they use the same term, fixture units for both supply pipes and for waste pipes. So when we talk about them, we're using fixture units, we're counting up the fixture units, but the units themselves are actually different for waste than they are for supply.

Now, that sounds annoying and weird, like why would they make it more complicated, but if you think about it, it actually makes sense, because the nature of the relationship of a particular fixture to the supply water coming to it is possibly quite different than the nature of how the waste is going to be leaving.

The obvious example is toilets. When I have a water closet, I need enough water that's gonna fill the tank, or if it's flush valve, it's a little different, but let's just say a simple one is filling a tank, so it just used to have some water, enough water pressure that it'll fill a tank reasonably quickly, you know, it's a little over a gallon, so it takes a second, so you want to go reasonably fast, so I want a big enough and pressure enough pipe that it'll fill that tank up, but it doesn't have to be an enormous pipe, right, it's just filling a tank.

But the waste pipe is gonna be pretty big, because, like I said, people put all kinds of things down those toilets. So I'm gonna have at least a three-inch pipe, possibly a four-inch pipe every time I go out. Well, that's enormous, right? That's a big pipe. So, the reason that they have different fixture units for the supply versus the waste is because they're different situations. Now, if it was me, I would've called them something different than fixture units for one of them, just to keep it a little clearer.

So then all we have to do then is look up what each of the fixture units are for these particular fixtures and let's say it's three and we have three for each of these, and then we can say, well, okay, this pipe needs to be able to accommodate three fixture units. And this pipe needs to be able to accommodate six of them and this pipe would need to be able to accommodate nine, et cetera, et cetera, right. And so you can imagine if I have multiple floors of this we might be saying, well this one maybe needs to accommodate 30 fixture units, right.

So, each of these situations, we can use the diagrams to sort of plan out what the needs are for this particular situation. Once we have what all the fixture units are, we can then look up in these various tables that they have. And I say various, because there's a couple different types of situations, but it's pretty simple and straightforward. It'll be a fairly simple table.

And it'll say all right, for a situation where you have the three fixture units, well, then, you need a pipe that's this big. And then we'll have, well okay, what about the one where we need for nine fixture units? Well, it'll say, well, okay, for nine fixture units, you need one that's this big. And then for 30, well we need one that's this big, right. So, it's just a simple idea that we add up the fixture units and then we can look on a table and it'll tell us how big a pipe we need for that many fixture units. Like I said when you look at those riser diagrams, if I have a pretty big, complicated building with a lot of different pipes going, well, this could be sort of a complicated process.

You're sizing out a lot of different little pipes. But most of the time, once you start a pattern, it starts becoming pretty clear, once I've done one bathroom, for example, it's probably the same in all the other bathrooms. So it's not the hardest thing to do, but it is something that you have to make sure you're getting accurate.

So one kind of interesting aspect to this is as you are first doing this in the beginning, when you start thinking about, all right, I have maybe one toilet and one sink and now I have another toilet and another sink. Well, that first set of pipes, as you first start adding more elements to it, it's gonna enlarge relatively quickly. But then later, as I start adding a fifth, a sixth, and then maybe it enlarges at some point, and then I add a seventh, eighth and ninth and tenth, and then maybe it enlarges.

Like it won't keep enlarging at the same rate as it did early on. And the thinking for that is that if you have a situation where you have a couple of toilets, it's totally plausible that a couple of toilets might flush all at the same time. You know, you might have people using them simultaneously. But if I have 30 toilets, it's pretty rare that all 30 toilets are gonna flush all at the same time, in fact, it's probably gonna be sort of a general, across the board limit, and it'll probably be five or six might flush at any one time, something like that.

So you wanna build in a little bit of a factor of safety, but you wouldn't need to keep enlarging those pipes at that same level as you did in the beginning when you're first adding those additional fixtures. So, you'll see that the numbers get big very quickly, but then they stay for a long time, until it gets over some sort of threshold and then it'll enlarge to the next size. So that's partly because of this nature of how many things will flush at any one time, it's partly just the nature of the size, that the difference in the cross-sectional area of a three-inch pipe and one that's a four-inch pipe would be actually quite a lot of area.

And so as I start getting bigger and bigger, it's just so much area, it just doesn't need to get any bigger than that. So, we're figuring out the diameter of the pipe by looking up what the total number of fixture units are and then looking it up in that table. And then we're gonna take all that information and we're gonna put it back onto our riser diagram.

So we use the riser diagram to figure it all out, to get it laid out the way that we want it, and then we go to the tables and we figure out the sizes and then from there we put all that information back on those riser diagrams. This is why it's such a useful set of drawings.

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