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Posted by Chris Lewis on August 3, 2005, 2:47 pm
>
> 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.
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Posted by Lawrence Wasserman on August 3, 2005, 7:01 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.
>
>
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
lwasserm@charm.net
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Posted by Nick Hull on August 2, 2005, 9:18 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".
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/
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Posted by PipeDown on August 2, 2005, 10:43 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".
>
> 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.
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Posted by on August 3, 2005, 12:46 pm
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.
>
>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?
>
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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