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Plz help - 3/4 inch vs 1/2 inch

Posted by roey736 (My Page) on
Thu, Aug 7, 08 at 13:39

We are planning to run brand new 3/4" pipes to our brand new (not yet built) master bathroom. Will there be a big difference if we get fixtures that will take it down to 1/2" (thinking Hansgrohe Thermobalance trim and valve) verses getting separate on/off valves (Jaclo)for each function? (Functions: shower head, body sprays, hand shower, and possibly rain shower.) Contractor says it's not worth the extra money for the separate valves, but our fixture shop disagrees. Thanks for any thoughts!

Follow-Up Postings:

RE: Plz help - 3/4 inch vs 1/2 inch

How many fixtures will you have on at any one time? Remember that 3/4" pipe has double the capacity of 1/2" pipe. We have two fixtures on at once, and each one has a separate 1/2" line.

RE: Plz help - 3/4 inch vs 1/2 inch

We put 1/2" Kohler high-flow valves (K-300-K) on our whirlpool tub and they flow 18/20 GPM at our 75/80 PSI water pressure, which is just fine. I didn't think the 3/4" valves (K-301-K) were worth the extra $150 or so, and it turns out we don't need them.

Our single spray shower works fine with the standard Kohler K-304-K valve. There is a high-flow Kohler shower valve, but it is $250-$300 extra (K-306-KS). Their web site doesn't doesn't list the flow data on the standard or high-flow valves. You might call Kohler and ask.

Here is a link that might be useful: Kohler K-300-K high-flow valve

RE: Plz help - 3/4 inch vs 1/2 inch

Suero, I would guess that we would only have two on at once. I've never had a shower with any luxury options like this, so who knows?!?

Does that mean, if we do thermobalance, they don't run separate lines?

RE: Plz help - 3/4 inch vs 1/2 inch

Obviously none of the people who have responded have even a remote clue about how to properly size a line.

When designing a potable water distribution system you begin by selecting all the end use fixtures to determine what the end use load will be. In the case of the original post they want a shower head, body sprays, a hand shower and possibly a rain shower head.

Quote:"How many fixtures will you have on at any one time? Remember that 3/4" pipe has double the capacity of 1/2" pipe."

Code requires that we design a potable water system for a worst case scenario. The system must be capable of maintaining the code listed minimum fixture "Dynamic head pressure" even when every fixture in the house is turned on at the same time.

The statement that a 3/4" pipe has twice the capacity of a 1/2" pipe is not true. The capacity, or properly the volume of flow is determined by the size of the metering orifice in the end use fixture. It is true that a 3/4" line has double the cross sectional area of a 1/2" line however both will supply the same volume, the difference being that the velocity of flow will be twice as fast in the 1/2" pipe. The problem here is that as the velocity of flow increases the "fiction head pressure loss" increases dramatically. In addition to the loss of pressure due to friction head if the velocity of flow exceeds 8fps we start seeing sever pipe wall erosion, which leads to premature pipe failure. In fact, while acidic water is the leading cause of pin hole failure in copper pipe, excessive velocity in undersized pipe is the second leading cause of pinholes.

Preferably we should have the end use fixture data available when designing the system but in situations where that information is not yet available we use the gal/min flow rates that are published in the code.

Now getting back to the original question. All the end use fixtures in question are types of shower heads and per code shower heads are restricted to 2.5gpm so let us begin putting it all in perspective.

Using the code standard of 2.5gpm we get shower head (2.5gpm), hand shower (2.5gpm) and body sprays (2.5gpm)equals and a hand shower(2.5gpm) for a total load of 10gpm.

Consulting the "Type L Copper Friction Loss Table" we find that if we supply this shower with a 1/2" line the velocity of flow would be 13.80ft/sec and the pressure loss would be 67.72psi/100ft of pipe.

If we increase the pipe size to 3/4" the velocity of flow is 6.62ft/sec and a pressure loss of 11.17psi/100ft.

RE: Plz help - 3/4 inch vs 1/2 inch

Lazypup - apologies for the late response, but I was off having a baby! I am now quite sleep deprived so I am having trouble following, but I think from your answer that we are better off going with the 3/4" option. Is that correct?

RE: Plz help - 3/4 inch vs 1/2 inch

Just in case anyone finds this thread... What he is saying is that with the smaller pipe the velocity is too high. It's best to figure out what GPM your fixtures flow and then calculate from that. In general you don't want the GPM above 5 feet per second.


i.e. - a fixture such as this Shower Fixture that flows 6 GPM would be ok if supplied by a 3/4 inch pipe.

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