3/4" has almost double the available flow as 1/2".
A toilet and sink only need a 1/2 supply off of that 3/4 main.
If I had a choice I'd be using 3/4 to the tub.
You'd notice more pressure at the shower head.
I'd be sticking with a 1/2 copper line to the shower head from the valve, it will have a larger ID then 1/2 Pex and I've never seen a 3/4 Pex drop leg elbow.

 

3/4" has almost double the available flow as 1/2".

3/4" PEX only has an I.D. of 0.67" so it shouldn't have that much more flow than 1/2" copper, yes?

A toilet and sink only need a 1/2 supply off of that 3/4 main.

So it wouldn't fill any faster using a 3/4" PEX line?

Thanks for all your feedback! I guess my main question is, will it be ok to upsize the 1/2" hot water copper line to a 3/4" PEX?
I didn't want to downsize it to 1/2" PEX (0.4" I.D.) considering that I will have 2 lavs + shower + tub connected to it.

 

If possible I would continue with 3/4 to the higher demand showers and tub baths.

 

You could run 1/2" to all of those fixtures off the 3/4" and have no issues. I've never seen a lack of pressure at a tub/shower from using 1/2".

You can use pex up to the shower head but I don't recommend it down to the tub spigot, we've had issues with pressure backing up to the shower head even when the diverter is open.

 

The valve size itself will restrict any flow gains by increased pipe size. Rule of thumb is 2 fixtures max on 1/2 inch line. Unless your installing more then a 2 1/2 bath 3/4 main trunk is all you need.

 

3/4" pex trunks on both sides would be best practice. It might sound great to want to oversize things, but bear in mind that the hot side will cool down and you'll have to empty all that (not so) hot water out before getting properly hot water. That's why some people do home-run plumbing, but it doesn't make a ton of financial sense and is pretty difficult to keep it looking nice and avoid a lot of drilling.

You can even share 1/2" in some cases, like if you have a double lav in a master bath for example. The only times I would say absolutely don't share a 1/2" with any other fixture, is for showers and laundry.

But to be ideal, just use 3/4x1/2 tees and a 3/4x1/2x1/2 for the last 2.

ETA:
It just occured to me that I'm speaking out of habit of using the Uponor (we like to still call it Wirsbo) system, which expands the pex allowing for bigger fittings. Maybe this makes some difference in why some people like to oversize pex?

 

It might sound great to want to oversize things, but bear in mind that the hot side will cool down and you'll have to empty all that (not so) hot water out before getting properly hot water.

That is a very good point. Thank you

But to be ideal, just use 3/4x1/2 tees and a 3/4x1/2x1/2 for the last 2.

Just for me to be sure, could you re-clarify this statement? I think what you are saying is that before reaching the last 2 fixtures, reduce to 1/2" from 3/4"?

My issue is that the hot water comes through the slab on the complete opposite end of the room as the cold water. The trunks would run past each other and the first fixture on the cold would be the last fixture on the hot. The sequence I would be plumbing these off the trunks is the following:

Cold:
3/4"--->tub---->toilet---->hose bibb on outside wall--->shower--->lav1--->lav2

Hot:
1/2" upsized to 3/4" PEX--->lav2--->lav1--->shower--->tub

Thanks to EVERYONE for your comments!:thumbsup:

 

Ive noticed no difference at the faucet over copper systems or pex systems.

 

Hope this phone - made illustration explains what I meant about the tees.

 

You're going to have 3/4" all the way to the water heater, correct? I'm not sure of the effectiveness of using any 3/4" at all if you're tying it all into a 1/2" line. I guess the bigger pipe would amount to something like a tiny pressure tank? Or it might make pressure loss worse, I've never seen pipe go up in size down the line so I've no idea what would happen.

It didn't occur to me before just now that that's what you were saying (I think)

 

You're going to have 3/4" all the way to the water heater, correct? I'm not sure of the effectiveness of using any 3/4" at all if you're tying it all into a 1/2" line.

I have 1/2" copper all the way to the water heater. I want to use 3/4" PEX for both bathrooms, because it has an I.D. of 0.6". Going to 1/2" PEX would result in a reduction of size.

The main is 3/4 to the shower with 1/2" branches to the shower and tub valves.

I do understand the illustration but I am a bit confused about your statement since you mention the shower twice.

 

The TRUNK LINE is 3/4" at least as far as the shower. You can go to a 1/2" trunk afterwards if you like.

Each fixture is still served by a 1/2" branch, even the shower. Just you need to bring 3/4" trunk at least as far as the shower.