# Water pressure and supply line question

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Mike_B, Oct 11, 2006.

1. ### Mike_BNew Member

Joined:
Oct 10, 2006
Hello! I'm adding a shower and the fixture recommends 3/4" supply lines. The city line comes in at 3/4" and then it is reduced before the water heater. If I change it to 3/4" all the way up to the bathroom, can I "T"-off to the existing 1/2" lines to the kitchen, 1st floor bathroom, laundry, etc. How would this effect the water pressure throughout the house?

2. ### hjModerator & Master PlumberStaff Member

Joined:
Aug 31, 2004
Occupation:
Plumber
Location:
Cave Creek, Arizona
pressure

The dynamic pressure, which is what you are asking about, would depend on many more factors than just the size of the pipe. The distance from the largest water source, the amound of water flowing at the time, where it is flowing, etc., all determine what the pressure will be. When no water is being used, the pressure is equal everywhere in the house.

4. ### Mike_BNew Member

Joined:
Oct 10, 2006
Thanks HJ. So now I guess this becomes a volume question.
With the shower fixture needing 3/4" for volume of water, do I need to run it before reductions to 1/2"?

If I change it to 3/4" all the way up to the bathroom, can I "T"-off to the existing 1/2" lines to the kitchen, 1st floor bathroom, laundry, etc.

The washing machines seems to start running when I take showers If
I run 3/4" to the washing machine, will it make a difference or just leave it 1/2"?

I want to be as effective as I can while I have access.
Any help will be appreciated!!

5. ### jadnashuaRetired Defense Industry Engineer xxx

Joined:
Sep 2, 2004
Occupation:
Retired Systems engineer for defense industry.
Location:
New England
3/4" pipe has 2.25x the volume of a 1/2" pipe. So, by running the 3/4" stuff to the high volume users, you've got much more available volume with the slightly larger lines. You'd be much less likely to notice a big drop in pressure with the larger line as long as you don't try to exceed the supply.

6. ### hjModerator & Master PlumberStaff Member

Joined:
Aug 31, 2004
Occupation:
Plumber
Location:
Cave Creek, Arizona
pipe

You cannot use any more water than the valve or faucet will supply, regardless of how large the pipe feeding it is. Run 3/4" to the shower and branch off 1/2" as needed. The shower valve should compensate for any minor pressure variations. If there are major pressure variations, then the water supply source is inadequate.