Moen 263 Shower system and piping questions

We recently started a bathroom remodel and for the shower my wife wanted to replace our current showerhead with a shower system. Below is what we have in our possesion now.

Moen 263ORB Double Handle Vertical Spa with Moen 3320 1/2 Inch Rough In Valve

After doing some research and finding that we will need a bigger (or 2nd) water heater to supply the system, I have another question and after searching for hours haven't found the answer.

Right now, our supply lines for the hot and cold to the shower valve are 1/2". Our master bath is on the lower level and the water heater is in the attic above the second floor (texas home). Replacing the supply lines with 3/4" copper is impossible due to the lines being behind existing drywall on the 2nd floor(don't need another project). We have never had a pressure problem in the house. We can shower, have the sink running and flush the toilet with no noticeable drop in water output. Will the 1/2" supply lines be able to adequately supply enough water to have the shower on and the 4 body sprays(out to 4-5 feet or so) at the same time??

All of the inlets on the valve are 1/2" and the body sprays are 1/2". I will be installing a pressure balance loop as recommended by moen for the body sprays.

Hopefully I didn't leave anything out...

Greg

Add up the total volume expected from the shower system and compare that to the available output from the 1/2" version of the valve. If it is below the max, you should be okay. If it is above the max, then the performance of the sprays will not be as good as you want. Most 1/2" valves are spec'ed for a maximum flow of around 6 gallons/min. A 1/2" line can flow a little more than that. Keep in mind you may be using some cold, so the total load on the 1/2" hot line may not be the total flow. You can make your existing WH provide more if you turn the thermostat up. If you do that, you may want to put a tempering valve on it. This mixes enough cold water to keep the temperature where you set the valve (until the WH output decreases enough so it doesn't need to temper it any more). This will make it look like you have a bigger WH. If you don't get carried away, that may be enough. If you want long showers with everything on...you'll almost certainly need more or bigger supply.

multiple shower sparys/prssure

I think I just posted almost the same question as you had - while I am sorry to say I do not have the answer I can tell you that pressure notwithstanding have you thought about a tankless hotwater heater?
We have tankless hotwater system that is a gift from heaven !!
Endless amouts of hot water and certainly easier than adding a 2nd hotwater unit/sytem.
On a different note, can you tell me what a "pressure balance loop" is?
Thanks
Craig

The Moen system you describe will work best with a 3/4" supply reduced to 1/2" just before the valve. 5 - 2 1/2 GPM heads will work better with that configuration.

Yes, that 50 gallon water heater won't cut it!

The pressure balancing loop is needed to ensure that the 4 body sprays are equal. See the diagram in your instruction book.