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My basement apartment's hot water pipe runs starting with 3/4" from the heater to the first outlet, the bathtub and the 2nd one, the washer. From the washer, it goes to the bathroom sink (please don't question this design as I have structural reasons why it doesn't go from the bathtub directly and don't want to spend time explaining as it's not pertinent) and ends in the kitchen sink, which will also be supporting a dishwasher.

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My question is, can I run only 1/2" pipe for hot water starting from the washer, will it be enough bandwidth or should I go with 3/4" ? I know that some people have up to 3 outlets out of a 1/2" copper pipe but I'm not sure how to count the DW as it is a different appliance but using the same valve from the wall inside the counter. I have run 3/4 to the tub and to the washer and now I'm about to continue on from the washer towards the bath and kitchen sinks and need to make a design decision.

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    Does the length of the run have any bearing on this question? Does it matter if it's 3' or 30' from the Washer to the Bath Sink? – JPhi1618 Feb 8 '16 at 19:59
  • good question. my distances are all small, each pipe segment is between 6-10' not longer – amphibient Feb 8 '16 at 20:00
  • Not sure about the general rule, but as an example I have a 1/2" line feeding washer, kitchen sink with DW, and bath sink (via tees for the first two). It is fine; you notice a pressure drop on the kitchen sink if the washer is filling but it's not so bad that you would stop washing dishes. – Shimon Rura Feb 8 '16 at 20:14
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    Well, you should also note that the interior volume per foot of 3/4" pipe is more than twice that of 1/2" pipe (see engineeringtoolbox.com/…). The more wider-diameter pipe you use, the longer it will take for hot water to reach the fixture in the first place. If you're optimizing for getting lots of hot water to frequently used fixtures, use more 3/4"; if you want hot water quicker and your fixtures aren't heavily used, go for more 1/2". – Shimon Rura Feb 8 '16 at 20:18
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    Worse, your shower ends with that pipe full of hot water, and the heating of that water is wasted. Fatter the pipe, more the waste. And washing machines don't have the ability to "run the water til it warms up" so the fatter pipe means colder water for them. – Harper - Reinstate Monica Feb 8 '16 at 23:18
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According to Uniform Plumbing Code Table 6-5 (610.3), the Water Supply Fixture Units (wsfu) per fixture in a private residence are as follows:

  • Dishwasher = 1.5
  • Kitchen sink = 1.5
  • Washup sink = 0

So you're supplying 3 wsfu.

According to Table 6-6 (610.4), at 30-45 psi you can supply 3 wsfu with ½" pipe with a developed length of up to 100'. 46-60 psi you can have a developed length of up to 200', and greater that 60 psi can go up to 300'.

At 40' or less with 30 psi, ½" pipe can supply 6 wsfu. So with a short run, you should have no problem using ½" pipe.

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    Note that the Code's answer to this type of question always depends on three things: Distance, Pressure, and total Fixture Units required. In theory, an engineering analysis could go further, including other factors that affect flow rates (such as inline valves and pipe bends), water temp could even be a factor. But in actual practice the Code-based calculation is necessary, and because it is deliberately conservative, almost always sufficient. – jbbenni Feb 8 '16 at 23:38
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    I should have noted, the length from Table 6-6 (610.4), is the "developed length". Which means it should take into consideration any restrictions (bends, valves, etc.). – Tester101 Feb 8 '16 at 23:55
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Consider the advantages of 1/2" pipe over larger pipe, if it's close. The larger the pipe the more water it holds, the longer it takes for hot water to reach the fixture. This is extra true if it's a tankless heater, which does not pre-warm any of the nearby pipe water.

Your dishwasher in particular will use electric heat to make up for any shortfall in the pipe temperature.

Your alternative is to run two 1/2" pipes. In the above I might run 1/2" from heater to washer, then a separate 1/2" from heater to sink to sink to dishwasher. The 3/4" to the tub is fine if the distance is short.

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