Why does plumbing code require supporting max water supply demand?

Question

So when sizing the supply lines for some plumbing, you have to calculate fixture units and then make sure the supply can support the max demand, but is that realistic? It seems pretty case by case, like a house with 4 bathrooms would be very unlikely to ever have all fixtures in use at the same time. The main problem I see with this rule is that it could be very energy inefficient, because it requires larger supply pipes, which means more time and energy to move hot water to fixtures? And what's the tradeoff? So that for the unlikely event you are using every fixture at the same time, you get good flow?

Answer 1

So when sizing the supply lines for some plumbing

Okay, so not all plumbing. Which specific plumbing situation are you asking about?

you have to calculate fixture units and then make sure the supply can support the max demand, but is that realistic?

If you're a plumber and the contract explicitly states this requirement then the client will pay for it.

It seems pretty case by case, like a house with 4 bathrooms would be very unlikely to ever have all fixtures in use at the same time.

Does your aforementioned requirement apply to residential installs?

The main problem I see with this rule is that it could be very energy inefficient, because it requires larger supply pipes, which means more time and energy to move hot water to fixtures?

You are describing a "trunk-and-branch" system.

I would like introduce you to "home-run plumbing" and "submanifold" designs.

And what's the tradeoff? So that for the unlikely event you are using every fixture at the same time, you get good flow?

If that functionality is in the design plans then you do what the customer wants.

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Consider the fact that most residences have a 3/4" or 1" supply line from the street to their home.

A 1" can handle breaking into two 3/4" lines. A 3/4" line can handle two 1/2" lines.

Consider the fact that most fixtures are reduced to 3/8" and have an aerator except for showers and spigots then you'll quickly see that a 1" main supply line is sufficient.

A 4 bathroom house would need larger than 1" if all 4 showers are running at once. Consider that a 4 bathroom house likely has an automatic lawn sprinkler system installed. If you don't have enough supply then avoid showering when the sprinkler is on.

Answer 2

Code-writers and code-adopting authorities want to ensure that plumbing systems will operate correctly even in worst-case situations.

In your example, flushing all four toilets at once and having enough water flow (i.e., large-enough pipes) to refill all four toilet tanks simultaneously serves that purpose. Doing so will cost a bit more than using smaller pipes, but the rule-makers think that's a desirable trade-off so that water piping will be able to handle worst-case situations, even if those worst-case situations are infrequent.

Answer 3

Keep in mind that you don't need the same size pipes everywhere. In many houses the supply splits in a few locations - e.g., one branch to serve the kitchen and another to serve the bathrooms - and each branch could, depending on a number of factors, be smaller than the initial entrance pipes. Using the bathroom hot water example, your hot water pipe coming straight out of the heater may not need to be as large as the cold water coming into the house, and where it splits to branches going to each bathroom, the pipes may be even smaller. There is no real energy issue with moving the water, the only energy concern is if you waste hot water, and there are a number of methods, such as point-of-use water heating and recirculating pumps to minimize hot water waste in a large building.

There are also newer technologies that can help. A PEX system with a manifold splits the supply to each room or fixture, so you can easily turn off sections for repairs and use larger inlet and smaller outlet pipes.

Answer 4

An important aspect seems to be missing in all answers and comments so far, even in the links to the side "finehomebuilding": The hygienic conditions. F.e. see the "3 liter rule" for the water content inside tubes.

Water in seldom used parts of the tubing net may become a breeding ground for dangerous bacterias, legionellas being only one example. That is why - depending on the jurisdiction - using manifolds could be sometimes against the rules and T-lines longer then a meter are to be avoided if not used frequently.

Especially since plastic is used instead of copper tubing. Copper has some anti- bacteriological effect (that is why door and cupboard handles were often made from brass/copper alloys in the past), while plastic is prone to build bio-films.

Those bio- films are not necessarily a problem (harmless bacterias), but could indicate that stagnant water gets spoiled faster.

Here are some information to lower the risk of contamination, but for European jurisdictions and conditions.

After loading or scrolling the page, wait some seconds for translation into English:

https://www-bosy--online-de.translate.goog/Stagnation-Trinkwasser.htm?_x_tr_sch=http&_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en-US

https://www-bosy--online-de.translate.goog/Trinkwasser-Leitungssysteme.htm?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en-US&_x_tr_sch=http#3-Liter-Regel