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?

According to PipeDown :
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.

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

In article . net,
"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".

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.

--
Free men own guns, slaves don't
www.geocities.com/CapitolHill/5357/

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.

"Nick Hull" wrote in message
...
In article . net,
"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".

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.

--
Free men own guns, slaves don't
www.geocities.com/CapitolHill/5357/

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.

According to PipeDown :
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.

"Chris Lewis" wrote in message
...
According to PipeDown :
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.


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.

PipeDown wrote:
"Chris Lewis" wrote in message
...
According to PipeDown :
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.


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.

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

"Harry K" wrote in message
oups.com...

PipeDown wrote:
"Chris Lewis" wrote in message
...
According to PipeDown :
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.


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.

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


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

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

"PipeDown" wrote in message
ink.net...

"Harry K" wrote in message
oups.com...

PipeDown wrote:
"Chris Lewis" wrote in message
...
According to PipeDown :
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.


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.

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


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

Don Young wrote:
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


snip

Right. The reducing tee merely changes the size of the column to that
of the output of the tee.

Harry K

According to Harry K :

Don Young wrote:
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

Right. The reducing tee merely changes the size of the column to that
of the output of the tee.

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

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

In article . net,
PipeDown wrote:
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.



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".
--

Larry Wasserman Baltimore, Maryland