effect of pipe size on pressure/flow

[RBKM]

We are installing a new bathroom and new hot water heater. Our plumber was planning to run a 3/4" line from the new hot water heater up to the shower. He'd also like to run 3/4 to the hot water heater (we currently have 1/2" pipe running to and from our hot water heater). Our supply line comes into the house at 3/4" but somewhere between where we can see the 3/4" pipe and the hot water heater is a 3/4 to 1/2 reduction--he doesn't know exactly where it is yet since it seems to be in the ceiling of the basement. Woud this short stretch of 1/2" pipe have a big effect on the pressure (and flow) if most of the line is plumbed with 3/4"?

 

[Plumber1]

pipes

It makes that 3/4" line 1/2"

 

[Sponsor]

 

[Terry]

Any decrease in size would be a restriction

Here is a pipe sizing chart

 

[RBKM]

It makes that 3/4" line 1/2"

Does that mean these two situations would yield the same flow out (for a given pressure head, with flow left to right)?

 

[Plumber1]

water volume

either way you essentially have a 1/2" line. with a little less drag.
No more pressure and less volume.
I'm not technical, you could split hairs, but that's my best thought.
you could go from 11/2 '' pipe down to 1/2" , It's still a 1/2" line.
A bigger volume will flow through a 10" city main than your 3/4" water service..

 

[RBKM]

So I guess the real question is: was it foolish for the plumber to run all this 3/4" pipe from the HW heater to the shower if the supply coming to the HW heater is only 1/2"?

Another question: will I be able to get 6 gallons/min to my rain-type shower head with that stretch of 1/2" pipe in there. The thermostatic valve for the shower is 3/4" and will accomodate at least 14 GPM at 40 PSI. So is the issue what the pressure is just after that valve (ie. how much pressure loss is there from the supply to the house to the shower?)?

 

[Plumber1]

water line

No, it's not a waste because there will come a time when you will want to change that 1/2" to 3/4"
You don't loose pressure.
You lose volume.
I've never had an occasion to do those calculations.
I think, if you wait a little longer, there may be someone here that can help you.
Don't think you will get 6 gal per min...........

 

[Gary Swart]

Here's a point you may wish to consider. If you have a long distance from the water heater to the end of that 3/4" pipe, it will take much longer to purge the cool water from the line than it would with 1/2". A 3/4" pipe holds over 8 times as much water as a 1/2" pipe if the same length. (A=DxDx.7854) A recirculation pump/line would remedy this if you want to stay with 3/4".

 

[Rshackleford]

Your drawing will not yield the same amount of water. The losses associated with the first drawing will be less then those with the second drawing. Using a Hazen Williams C=120 would represent a very old piece of copper. At 5 gpm the loss in 100' of 1/2" would be 37 psi and 3/4" would be 5.2 psi. This is a substantial difference and gets worse the higher the flow. A 6" piece of 1/2" would lower pressure by .2 psi, a 10' piece by 3.7 psi, and so on.

Anyway, I am not sure what my point is here other than the drawing will yield drastically different amounts of water.

 

[RBKM]

Here's a point you may wish to consider. If you have a long distance from the water heater to the end of that 3/4" pipe, it will take much longer to purge the cool water from the line than it would with 1/2". A 3/4" pipe holds over 8 times as much water as a 1/2" pipe if the same length. (A=DxDx.7854) A recirculation pump/line would remedy this if you want to stay with 3/4".

I didn't realize that flushing out the cold water was what took the time--I figured it was warming up the pipes too, but maybe that's not a factor. It looks like almost a straight run from our water heater to our shower, so let's figure at most 50 ft of 3/4" pipe. That's 600 inches x .44 inches = 265 cu inches = 1.1 gallons. If the shower runs at 2-3 gallons/minute, the cold water should be purged in 20-30 sec. Not too bad. With 1/2" there would be about 44% less volume (I'm not sure why you say that 3/4" pipe holds 8 times the amount of water of equiv length 1/2" pipe--it's just the ratio of the diameters squared, or 2.25).

rshackleford:
That sounds more reasonable from what I remember of fluid flow stuff. Putting in a short stretch of 1/2 should increase the total resistance but not as much as if the whole thing were 1/2"

 

[Gary Swart]

Thank you for catching my math error! At my age I should know enough to check my answers a bit closer. You're correct, the difference is about 2.25 times, not 8. But, it is a very noticable time difference. I plumbed my bathroom with 3/4" and it was awful! It didn't take long before I switched back to 1/2" and put in the recirculation pump/system. Fortunately, my plumbing is all in the open in the basement so changing pipes and adding a return line did not create too much problem.
I think the pipe warming time would not be a significant factor.