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Water pipe size PipeDown 08-02-2005
`--> Re: Water pipe size Lawrence Wasser...08-03-2005
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Posted by PipeDown on August 2, 2005, 8:29 pm


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?




Posted by Speedy Jim on August 2, 2005, 5:15 pm


PipeDown wrote:

> 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.

<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


Posted by Chris Lewis on August 2, 2005, 8:48 pm


> 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.

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.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.


Posted by PipeDown on August 2, 2005, 10:31 pm


> 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.


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.




Posted by Chris Lewis on August 2, 2005, 10:59 pm


> > 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.

> 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.

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
It's not just anyone who gets a Starship Cruiser class named after them.


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