Water pipe size

I am installing copper pipe to a new bathroom. The pipe will be an extension from the existing galvinized pipe in the house. The distances are not great (10'-20' range)
The existing cold supply pipe is 3/4" everywhere but the hot water lines are only 1/2".
I want to install 3/4" CU pipe for both hot and cold (reducing to 1/2" just before the fixtures) in the new installation in anticipation of repiping the whole house in the future.
This plan requires that I transition the hot from 1/2" galv to 3/4" CU for the hot water. My question is "will this create a pressure or flow rate problem?" Normally you go from big to small pipe but in this case it is from small to big.
Should I keep to my current plan or change to 1/2" CU for the hot all the way through?
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PipeDown wrote:

<SNIP>
Unless you are planning a hot tub, there is no need for 3/4" on the Hot supply. There is a huge difference in the volume of water in the pipe (between 1/2" and 3/4") and all that water must be drawn before the tap gets hot flowing. Besides the waste, it can be a great annoyance.
I recommend 1/2" branch runs for each Hot supply.
Jim
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Knowing how iron/galv pipe ages, its effective size is probably considerably smaller than 1/2" CU.
Going from small to big won't cause much of a problem, aside from a "if you draw too much from one faucet, you might suck air in another" factor.
[For the most part assuming both faucets are on the same 3/4" CU leg.]
In other words, the upstream flow restriction will reduce the systems inherent ability to balance multiple demands on the downstream ends.
However, since your plan is ultimately to replace all of the old plumbing in the house, and your plan (3/4" to the last division before the fixtures) is the _right_ way to do things, you should be able to live with the potentially sucky bits ;-)
I'd recommend staying with your planned 3/4" CU.
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You would never suck air into a fixture unless for some very unusual reason you were pumping warter out of another fixture at a rate greater than the source can replenish it. Excessive flow at one fixture would cause a reduced flow at all other fixtures on the branch since all are passively powered by available water pressure. Only if a fixture were actively accelerating the water would a negative pressure develop in the pipe to cause air sucking.
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Heh, no, sorry.
Take a two story house with heavily restricted water feed. Turn on a faucet upstairs. Now, go turn on another faucet on downstairs.
Do you hear the faucet upstairs sucking air?
We do ;-)
--
Chris Lewis, Una confibula non set est
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Briefly said, gravity would be that additional accelerating factor.
Perhaps I was being narrow sighted since I am in a single story house but for that to happen the weight of the water column in vertical pipe leading upstairs would need to exceed the static water pressure minus that pressure relieved by the open downstairs fixture. In which case the 1/2" pipe would be advised due to the reduced weight of the water column. Such a situation should only occur in an installation flawed by inadequate water pressure or too large pipes for a given pressure/flow. I can also see that a reducing fitting an just the wrong spot on a vertical pipe could have unexpected consequences by changing the static water pressure at that point (a problem which should not happen in a horizontal run because there is no pressure gradient under static conditions)
Your reply may have been essentially correct but it sure was confusing. I'm not arguing, just trying to understand completely. In fact I am still looking for a way to calculate the head pressure for each of the two cases of 10' vertical pipe 1/2" and 3/4" . While it is obvious that water in a 3/4" pipe weighs more, the difference in area (of the pipe diameter) would yeild different PSI measurements which might be closer (i.e. mass is distributed over a larger area in the 3/4" pipe). Maybe I'll post later if I can puzzle it out to my satisfaction, dosen't seem hard just short on time and willpower now.
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PipeDown wrote:

No, a column of water 10 ft high will produce about 4.3 psi whether that column is 1/2" or 5'. Seems counter intuitive but them is the facts as I was taught in my last physics class ummmmm years ago ;). Total weight of water depends on column size but the pressure per sq inch doesn't change.
Harry K
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Thats what I thought but a reducing union at the bottom would increase the PSI by reducing the area while keeping the water column the same weight. A reducing Tee would be OK as long as the size does not change in the vertical direction
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No, you can't increase the pressure of a water column by reducing the pipe size anywhere in any direction. A 10 foot high cone with point at the bottom develops exactly the same pressure at the bottom as a 10 foot high cylinder. Also water pressure is exactly the same in a sideways direction as downward at any given point. Any increase in force (pounds) can only occur with a corresponding increase in area (square inches) so the pressure (pounds per square inch) is constant. I am not sure but I think that is known as "Pascal's Law". Don Young

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Don Young wrote:

<snip>
Right. The reducing tee merely changes the size of the column to that of the output of the tee.
Harry K
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Right. One must factor in the flow rates. The reduction doesn't have to be in the vertical bit. For "suck" to occur, you merely need a flow restriction upstream of _both_ faucets that reduces the pressure (given the flow rate thru the lower faucet) to less than required to force the water up the water column to the other faucet.
This is the typical situation on older homes fed with 1/2 pipe, especially iron.
Given that the OP's upstream is 1/2 galvanized, and given that galvanized pipe _usually_ eventually suffers from deposition/rust reducing the effective pipe size (and at the same time increasing wall friction), the effect might be quite drastic.
[I've seen iron pipe effectively reduced to less than 1/4" ID.]
I would say, however, that given the probable condition of the galvanized, chances are even 1/2" copper would be ALMOST as likely to suck as 3/4".
Plumbing with 3/4" is the right way to go (even in a two storey) as long as the plan is to replace the rest of the galvanized.
The only drawback, as mentioned elsewhere, is increased time to deliver hot water. Which can be reduced by pipe insulation, and completely eliminated by adding some sort of recirc system.
My ideal plumbing system is 1" supply, then 3/4" sub-branches up to the last point before splitting to individual fixtures. Maximum water flow, least amount of pressure imbalance due to multiple demands.
In our previous house (2 storey, municipal supply), I replaced < 20' of 1/2" in the basement with 3/4", and went from one shower + filling a glass of water -> the showerer getting scalded to two simultaneous showers and being able to run the sprinklers without temperature change in the shower. (without balancing valves!)
In our current house (on a well) we can run multiple legs of our inground irrigation system, have a shower and run the clothes and dishwasher simultaneously without noticable volume change in the shower. [Mind you, this time we have a pressure balance faucet in the shower ;-)]. The two storey house was built for and plumbed by a plumber, and almost everything is 3/4".
[Mind you, the fact the well can deliver 30+ GPM helps ;-)]
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What you suggest sound intuitively correct but in fact it CAN happen. Look up venturi effect, or google on "fire hydrant" and "water supply contamination".
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Larry Wasserman Baltimore, Maryland
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The reason the hot water is 1/2" is so the hot water moves faster and you have to drain less cold water from the hot water line to get hot water at the faucet. There is more than twice as much water in a 3/4" line as a 1/2" line.
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wrote:

Thanks Jim and Nick. That was the critical observation I was missing (that reduced water volume in the 1/2" hot line would reduce wasted water when priming the line with hot water). I was overemphasizing the pressure/volume considerations in a non critical application like a bathroom.
I am installing a spa tub in the bathroom and considering an outdoor hottub in the future but unless I want to put in a hot water priming (recalculating) line then even then the 1/2" would probably be adequate since I can wait a few minutes longer if need be but the potential saved wasted water could be significant. On the other hand, the shower has multiple heads and would benefit from larger volume.
I think I will stay with plan A (3/4" pipes) but stub in a spot for a hot water recirculator when I do the repipe. Most of the hot run is 1/2" for the time being and the additional volume of the addition should be managable.
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wrote:

No problem, just do it. You already have an inbalanced system so it wont get worse. I suggest you put a union right before the transition from 1/2 to 3/4". It will be easier to change later. Your other option would be to change all the hot water pipes NOW. I did that in my own home but for different reason. I had clogging hot pipes but the cold were fine. I changed all the hot pipes but left the cold ones until several years later.
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Normally 1/2 copper is adequate for residential branch water lines going to a single fixture. If there are a lot of fixtures teed off a line that are likely to be used at the same time, you may want to use 3/4 copper up to that point.
--

Larry Wasserman Baltimore, Maryland
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