1/2" copper main to 3/4" Pex main runs

Hi there,

Newer member here. I'm about to install a Pex-a system in my 70's ranch style, one-level home. This system will be replacing an old galvanized system.

What I'm interpreting as the main line from the meter is copper. It appears that this main line bends up and forms a Tee with a 3/4" Galvanized and 1/2" galvanized ends. The solid black line is the wall to my garage where the water heater is. A run of 1/2" galvanized comes out of the wall...I'm assuming from the water heater.

Like this:


My question is whether it would make sense to make my main runs to distant manifolds for fixtures in 3/4". I'm about to buy my supplies, so I want to make sure I am getting the right stuff. I was thinking 3/4" Pex to distant manifolds, then have 1/2" run from these manifolds to fixtures. Good idea? Or, since it is a 1/2" main coming in, should I just run the whole house in 1/2" Pex.

I believe the water heater requires 3/4" to it by code, so I'd still have to put some 3/4" in.

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Second question:

I'll be moving a toilet a few feet parallel along joists. The main vent stack is currently servicing two toilets with an all hub double sanitary tee.

By moving the toilet back, it'd put a 90 deg bend right at the sanitary tee joint. Does this matter? Am I asking for problems here? Will it be better just to rebuild the drain stack...that'd be a lot of work...there are some other connections into it along its vertical run.

Crudely drawn:

Thanks for any advice.

If it'd help, I can crawl under the house to get actual photos of these areas.

3/4" PEX and 1/2" copper have comparable inner diameters.

I've seen drinking straws with more space inside than 1/2" PEX. Single fixture use only.

IMO, and a lowly DIYer, but I think you want 3/4" copper where you currently have 3/4" galvanized.

kcodyjr said:

3/4" PEX and 1/2" copper have comparable inner diameters.

Click to expand...

Not to distract too much from the original topic of my post, but 3/4" Pex-a ID = 0.671 and 1/2" Copper M ID = 0.569.

1/2" Pex is something like ID 0.47

Best bet is to put a PEX manifold with shut off valves then all your hots and colds come off it in red and blue PEX.
You can home run each branch circuit with 1/2" PEX.
Here is a Watts manifold with the valves and push connect fittings make it very easy.
Google for P-10PORT

3/4" copper has 50% more capacity that 3.4" PEX, and 3/4" PEX has about 35% more capacity than 1/2" copper. 1/2" PEX, because of the dynamics of reducing the diameter of a circle is MUCH more limited.