Optimal Pipe Size for Bathroom Tub

[WorldPeace]

I've read conflicting information and I was wondering what the correct answer was. A pro here mentioned that they run as much 3/4'' pipe as possible, even running 3/4'' directly to bathroom tubs because code is just a minimum. They want to be safe.

However, wouldn't this have the opposite consequence?

I read that 1/2'' already has the capacity to supply more than 10 faucets running at the same time. But, by oversizing pipe, you will actually experience water pressure loss. If people happen to be using multiple fixtures at the same time, the increased pipe size will collectively lead to too much water loss so you experience the water pressure you were hoping to avoid.

I'm assuming that you need to make sure the pipe is not undersized so that it won't experience water pressure loss from a small decrease in pressure from the usage from other points of use. But, collectively, you need to make sure that all the pipes are not oversized so that there is not systematic loss in water pressure in the event that multiple people use it at once.

Is that correct?

 

[Breplum]

First off, even 1/2" copper is not adequate for 10 faucets. We have charts for sizing, but that statement is just WRONG.
Oversizing will not cause pressure drop, period. For hot water (without recirculation), oversizing will create a excess volume that needs to be dumped before hot water reaches the outlet.
Frankly, no new hot water piping should be done without a demand-initiated recirculation system. This is the most efficient resource management tool we have.

 

[Sponsor]

 

[WorldPeace]

I'm new and just learning but that's what I read elsewhere. That source may be wrong so I'm trying to figure it out. Here is the source that states 1/2'' in sufficient for more than 10 faucets.

https://diy.stackexchange.com/quest...gh-pressure-for-a-shower-head-and-body-spraye

Here is the source that states that oversizing can lead to water pressure loss.

https://diy.stackexchange.com/questions/108801/water-supply-line-size-3-8-inch-or-1-2-inch

By the way, I'm not advocating for reducing to 3/8'', but I'm wondering if 3/4'' to multiple fixtures would lead to water pressure in the event that multiple fixtures were in use at the same time. For example, if a family was watering their lawn, filling a tub, using their washer, using their dishwasher, and flushing the toilet, I can understand why in theory 3/4'' fixtures might lead to water pressure loss.

Here is another interesting situation. It's different than the one I am suggesting but it makes you think about the dynamics of water pressure:

https://www.quora.com/Will-bigger-pipes-increase-water-pressure

First off, even 1/2" copper is not adequate for 10 faucets. We have charts for sizing, but that statement is just WRONG.
Oversizing will not cause pressure drop, period. For hot water (without recirculation), oversizing will create a excess volume that needs to be dumped before hot water reaches the outlet.
Frankly, no new hot water piping should be done without a demand-initiated recirculation system. This is the most efficient resource management tool we have.

 

[wwhitney]

Here is the source that states that oversizing can lead to water pressure loss.

I didn't check your reference but that generally doesn't happen.

Let say you're on city water, so for residential flow rates, you can typically consider the water main in the street as a constant pressure source. Connected to the water main are your water lateral, your meter, your house piping, and your fixtures. When there's no flow, the pressure throughout the system is equal to the water main pressure, adjusted for elevation difference. That's the static pressure.

Now when you open an outlet, water starts to flow. Every component between the water main and the outlet, including the outlet, causes pressure drop depending on flow (with the pressure drop increasing with flow). The flow rises until all those pressure drops add up to the water mater main pressure; then there's no more pressure left to push additional flow.

So what happens when you upsize a pipe? It means that particular pipe will have less pressure drop for a given flow. So it will contribute less to the total pressure drop, and the system flow can go up (at least slightly) until the new total pressure drop again matches the water main pressure. If that pipe was the dominant effect limiting flow to the outlet (e.g. a long 3/8" PEX pipe going to a tub spout) it will have a significant effect. If that pipe was not dominant (e.g. a 1/2" PEX pipe dedicated to a single 0.5 gpm lav faucet), it will have little or no effect.

Often the outlet is flow limiting (faucets, showerheads, etc), in which case the outlet is dominant, it's designed to flow almost the same at 40 psi and 80 psi. In that case, as long your piping can get the desired flow rate to the outlet with at least the minimum pressure, you're going to see very little difference in outlet flow as you upsize pipes upstream of the outlet.

Now, when you have two outlets simultaneously flowing, can upsizing a pipe lead to less flow at one of the fixtures, and more flow at another of the fixtures? If the pipe is common to the flow path for both fixtures, then no, it can't. For a given flow rate on the common path, the upsized pipe will just give you a higher available pressure at the point the paths separate, and both outlets should get (at least slightly) more flow.

If the pipe is specific to one outlet path and not the other, then it's possible. But it would only be significant if you had two non-limiting outlets flowing simultaneously. E.g. say you had two different bathtubs both supplied by the same water pipe, which splits and goes 10' to one tub, and 20' to the other tub. If the pipe sizes after the split are the same, and you foolishly try to fill both tubs at once, the tub that's only 10' from the split is going to fill a bit faster. Now if you upsize the 20' run by doubling the pipe size, it will be the faster tub to fill, and the tub 10' away would fill a little slower.

Cheers, Wayne

 

[Terry]

Here is the source that states that oversizing can lead to water pressure loss.

Wow! That page has some crazy stupid answers. Whoever wrote that has never done plumbing in their life. He would get laughed off any job he showed up to. I'm guessing his a keyboard junkie. Never left the bedroom to do a real job.

These charts are what we use to pass a plumbing inspection.

If I'm running pipe to a single bathroom, tub, lav and toilet, it's 3/4" for the cold and 1/2" for that hot.

 

[Jadnashua]

To fill a tub, a larger supply line has advantages, but to achieve it, it also calls for a valve capable of letting it pass without restrictions. If you're dealing with copper piping, the Copper Institute's recommendations to minimize problems is to limit the maximum velocity of the water. Their recommendations vary based on whether you're dealing with hot or cold water. Those values are 5fps (4gpm) on hot, or 8fps (8gpm) when using 1/2" piping. The maximum velocity does not change with pipe diameter, but the amount of water you can pass within range increases with pipe diameter. Bumping the 1/2" pipe on hot to 3/4" DOUBLES the safe volume to 8gpm,. For a really big soaking tub, that might still be a little slow, but you better have a huge water heater!

 

[WorldPeace]

Terry,

Thanks for the explanation. I appreciate it. I'm just starting to learn this stuff and it's tough.

I understand that 3/4'' is normal for branches but what if you ran 3/4'' to all the individual fixtures (all the twigs)? Would this have an effect on pressure?

I also found the below story from a plumber interesting:

Funny you ask because we tested this. We had a client that was a brilliant and wealthy engineer. He designed his entire house with oversized lines to every fixture 3/4″ which were fed from 1 1/4″ copper mains. He wanted 1/2″ NPT × 1/2″ compression stops so we could maintain 1/2″ as long as possible before we had to change to 3/8″ to hook up to his brizo and delta faucets. Which was a major pain if I might add.

Here is what and how we tested… the master bathroom had double vanities so this was the most controlled area for testing. Both fixtures had branch lines in 3/4″ copper off the 1 1/4″ mains. We plumbed one side how he wanted it done and one side with 1/2″ copper stub outs. From the test side a normal 1/2″ compression x 3/8″ compression stop. From the stop we ran the normal 3/8″ poly line to the faucet.

We hooked up our side how standard plumbing is installed. His side how he wanted. We did flow tests on both the 3 hole spread faucets. We timed and recorded the amount of water from both sides. Let me mention that we figured out the tests would be the same because lav faucets are regulated for flow rate. For some reason the extra volume decreased flow rate on the side we plumbed as the guy designed. It was surprising enough that we cleaned both faucets out and swapped the valves. This gave us the same results… Lower flow rate on his designed side. So for whatever reason larger pipes in this situation lowered flow rate on faucets designed for 2.2 gallons per minute.

He was on municipal water with a 1″ meter and we had almost 80psi before the meter.

The guy is truly brilliant but this may have been a bad example of his smarts. I loved chatting with him because of his mechanical mind. Anyways, more volume isn’t always good in residential plumbing.​

 

[John Gayewski]

Without knowing the faucet design that test means nothing. Especially since there were no specifics given. For instance how much less flow? How was this timed and measured? If you set a regulator on a pipe and then move it to a different pipe it'll need adjustment.

 

[Terry]

We use science for sizing lines.
There is a real thing called friction loss that happens on smaller pipes. If you look at the charts, you will notice that upsizing lines over a long distance makes a huge difference. A 1" poly line from the meter to the home 50 feet away has an amazing amount of water coming out. The same poly line with 350 feet barely comes out of the end. Just way too much friction loss.
You may not know this, but plumbers have been doing this for a long time. Under sizing pipes just makes things worse. Plumbing started with undersized piping, and we learned from that. Right now I'm beginning to think that WorldPeace might be trolling us. Pretty funny.

 

[wwhitney]

For some reason the extra volume decreased flow rate on the side we plumbed as the guy designed.

Nope, not plausible. Either made up entirely, or there was some other obstruction on the lower flow side.

Cheers, Wayne

 

[Jadnashua]

Friction losses are real...larger pipes have less friction, and a larger pipe can flow more water which is why they use a fire hose to fight a fire rather than a garden hose...the pressure in the system is the same, at least at the source...not so much by the time it gets to the end of the hose!