PEX Homerun vs T&B with 1/2in Supply

Question

I'm adding a small bathroom to my detached garage. I've read lots on home run vs trunk and branch, I've got a pretty good idea of the pros and cons of each.

Issue for me is, I've got 1/2in copper already in the garage as the starting point. It is what it is, we work with what we've got. But I know I'll need to make the most of the pressure I get from it.

Many have commented that home run systems, while providing more stable pressure when multiple fixtures are in use (which will never be my use case), they can provide less pressure at individual fixtures. Is this just because home run systems use smaller pipes and they expect you'll be running farther through this smaller pipe? (As opposed to a short branch close to a larger trunk?)

This is going to be a small (5x7) bathroom with a small water heater, a shower, one sink, and a toilet. So there is not going to be much pipe being used in either configuration, and the trunk or manifold supply is going to have to be 1/2in either way. Will one configuration give me better water pressure at the shower?

Answer 1

You are saying that from meter to garage you already have a single 1/2 inch pipe, and the question is where, within the garage, you create further branching for the fixtures. Answer: it depends on the details, and requires measurements and calculations that you may or may not be able to do.

Think of the entire system from the meter to the new shower.

Say you have 100 feet of 1/2 inch pipe from meter to garage, and another 10 feet to the new shower. Should you put a manifold in the new bathroom, or do you put it in the garage but outside the bathroom ten feet closer to the meter? Then it's either 100 or 110 feet before branching. What will the pressure difference be? Will it be perceptible by users? Some detailed calculations would be required, but probably not worth even thinking about.

Say on the other hand the meter is on the other side of the garage wall, you have two feet of half-inch pipe, and you have a huge garage and it's another 50 feet to the bathroom. Then it could make a big difference, and you could put a manifold at the garage entrance or run 3/4 inch pipe the 50 feet, upsizing then downsizing, and put a manifold or T&B in the bathroom.

There is no generic answer to whether a new bathroom would best benefit from one style of piping or another when branching off an unknown existing installation.

Note that in this answer I, like you, am using "pressure" colloquially to refer to the perceived pressure of the water flowing outside the fixtures, what is technically really flow rate. Let's not get too hung up on language.