First of all the information I'm providing is based on my local codes, your local codes my very.
Quick Answer
You will need to feed the lower floor manifolds with 3/4" PEX inlets and 1/2" outlets. No manifold would be required for the upper bathroom as only 1/2" PEX is needed for both the hot and cold, just tee off the 1/2" lines to feed the sink, tub and water closet. All individual fixture runs would be 1/2" as well.
Detailed Answer
Using manifolds and running to each fixture individually is called a homerun system. This can be less labour but may have a higher material cost. It all depends on how far the groups of fixtures are from the manifold. This method is typically used when the pipes are run through the slab.
Depending on the location of the fixtures, you may want to look at a truck and branch system. This is were you would run two mains and branch off to the fixtures as it passes them. The mains would get smaller as the fixtures are taken off.
Water lines are sized so that the velocity within the pipe is kept below a set point. This is to limit the wear and tear on the pipe material. Different materials can handle different velocities at different temperatures. The max velocity for copper and PVC is 5ft/sec for cold and 4ft/sec for hot. PEX pipe can handle a maximum velocity of 8ft/sec for both hot and cold.
Fixture units (FU) are given to typical fixtures. A FU is a design factor that is used to represent the typical requirement of a particular fixture (it is not a flow rate but).
Fixture Units for Private Use Fixtures (no flush valves):
|--------------------------------------------------------|
| Fixture | Hot FU | Cold FU | Combined |
|--------------------------------------------------------|
| Bathroom Group | 4.50 | 4.50 | 6.00 |
| Bathtub/Shower | 1.50 | 1.50 | 2.00 |
| Clothes Washer | 2.25 | 2.25 | 3.00 |
| Dishwasher | 3.00 | - | 3.00 |
| Hose Bib | - | 7.00 | 7.00 |
| Kitchen Sink | 2.00 | 1.50 | 1.50 |
| Water Closet | - | 3.00 | 3.00 |
| Lavatory | 0.75 | 0.75 | 1.00 |
|--------------------------------------------------------|
The above table lists the common residential fixtures and their FU. You can see that a bathroom can be considered one group and has a lower FU then the sum of it's parts. This is because they are typically used by one person at a time so it is unlikely that all fixtures will be running at the same time.
Once we have the FUs, we can look up a sizing chart that will tell us the maximum FUs that a pipe of a set size and material can handle while staying below the maximum velocity. You can view these tables in the Domestic Water Sizing Tables (For Small Buildings) section below.
Based on this information, we can size your distribution system as follows for PEX:
|----------------------------------------------------|
| Fixture Group | Hot | Cold |
| | FU Size | FU Size |
|----------------------------------------------------|
| Upstairs Bathroom | 4.50 1/2 | 4.50 1/2 |
| Downstairs Bathroom | 4.50 1/2 | 4.50 1/2 |
| Kitchen | 4.50 1/2 | 1.50 1/2 |
| Half Bathroom | 0.75 1/2 | 3.75 1/2 |
|----------------------------------------------------|
So you can see that the pipes feeding each group of your fixtures will only need to be 1/2". Each individual fixture would also be 1/2". However, depending on how the groups of fixtures are located relative to each other, you may require 3/4" pipes to feed more then one group. The total FU of your house is 14.25 Hot FU (3/4"), 21.25 Cold FU (1") and 28.00 Combined FU (1"). The Combined FU is used to size the section of pipping from the cold line into the house to the hot water tank. Don't be alarmed that the cold and combined are sized at 1". This is due to the allowance of the hose bib, which our code now calls for 7 FU. This is too high and has been causing issues in large buildings with hose bibs on small decks, in that the lines are oversized and do not properly flow, allowing for growth in the water. Without the hose bib, your cold is only 3/4".
Domestic Water Sizing Tables (For Small Buildings)
- Minimum pressure available - 60 PSI at property line. (Greater
acceptable).
- Pressure reducing valve set at 60 PSI minimum.
- Pressure loss for meter (3 PSI), maximum building height 25 ft. (10.82 PSI)
- Minimum 0.115 PSI for friction loss. If less than 0.115 PSI, system
must bp fully engineered by detailed method or there will not be
sufficient water to supply the fixture.
Pipe Flow Velocity Table For: Copper & PVC:
|---------------------------------------------|
| Pipe Size | 5ft/sec (cold) | 4ft/sec (hot) |
| | GPM FU | GPM FU |
|---------------------------------------------|
| 4" | 186.65 850 | 149.32 600 |
| 3" | 106.16 400 | 84.93 295 |
| 2-1/2" | 74.37 245 | 59.50 170 |
| 2" | 48.23 120 | 38.58 81 |
| 1-1/2" | 27.72 46 | 22.18 34 |
| 1-1/4" | 19.59 29 | 15.67 22 |
| 1" | 12.86 18 | 10.29 14 |
| 3/4" | 7.54 9 | 6.03 7.5 |
| 1/2" | 3.64 3.5 | 2.91 2.5 |
|---------------------------------------------|
Pipe Flow Velocity Table For: PEX, PE, PB, CPVC & Ductile Iron:
|-----------------------------------|
| Pipe Size | 8ft/sec (cold & hot) |
| | GPM FU |
|-----------------------------------|
| 4" | 300 1800 |
| 3" | 170 750 |
| 2-1/2" | 152 500 |
| 2" | 78 265 |
| 1-1/2" | 44 102 |
| 1-1/4" | 30 54 |
| 1" | 20 30 |
| 3/4" | 12 17 |
| 1/2" | 5.8 7 |
|-----------------------------------|
It's really hard to read your question, but I think you are essentially asking: "Is it okay to replace galvanized pipe and connect it with copper?"
The answer is yes, but there are a couple things to keep in mind:
Copper and galvanized pipe will suffer from galvanic corrosion where they meet unless they are isolated. This requires use of a dielectric union
Galvanized pipe also is generally cited has having a 40 to 50 year lifetime. Generally they will rust from the inside out, restricting the flow in the process.
This restricts the flow, and is often the reason that older homes have such poor water pressure. It would be a shame if you get your new shower installed and then find out the pressure is awful, and it could very well be that it is the galvanized supply lines and not the shower itself.
There is also the possibility that there are metals leeching out into the water supply from this corrosion, but to know if this is happening for sure you'd have to get your water tested.
Once you start seeing corrosion on the outside of the pipes, it's just a matter of time until you have a leak. I've actually hard of some home insurance policies not covering damage from leaking galvanized pipes, so that might be worth checking out.
There's also no reason you have to use copper, pex is a perfectly valid option, and for replacing a long in-wall run it's significantly easier to install than copper (it bends, and comes in spools up to hundreds of feet long).
Best Answer
I home run cold tap water lines 1/2" as close to supply as possible so I can get cold water ground temperature for drinking with the shortest wait possible - assumes your tap water is good quality. Not sure if you'll be drinking the cold water from the bathroom.
I home run hot water lines as close to the hot water tank as possible so again I wait the minimum amount of time to get the water at the tank temperature.
If I was only re-doing this bathroom I might do the last mile here as if it was homerun plumbing up to your take off point and if/when you redo the rest you could update it.