Water pressure drop over house-long run to bathroom

[wwhitney]

If your goal is to stop the hot water temperature variations in the shower, then the best and only reasonable intervention is to change the shower valves to pressure-balancing or thermostatic shower valves.

Only once you've done that is it worth thinking about repiping if the fixture are still behaving in ways you don't like. [E.g. when the toilet is flushed, the now-constant-temperature shower flow is reduced too much.]

Cheers, Wayne

 

[Reach4]

Separate 1/2 inch hot for each bathroom will be best, and better still if you add separate colds to each bathroom. I am not a pro.

 

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[Gundraw]

@wwhitney - Yes, the plan is ultimately to swap these fixtures as time allows. Both bathrooms are tiled, so planning to do that right away is a little daunting. However, there are two other problems. #1 - The noise from the hot water copper pipes and #2 - Some kind of recirculation system to quit wasting so much water when showering in Bath 2.

I have the two 1/2" copper lines to convert to cold water, do I
A.) Combine them so both bathrooms feed cold from both lines
B.) Run each bathroom cold on its own line.

Then when it comes to hot water, do I :
A.) Run one insulated 3/4" Hot Pex line and split to each bathroom.
B.) Run two separate 1/2" Hot Pex lines, each feeding a different bathroom.

I would guess that with a recirculation pump, the 3/4" would be better for letting both bathrooms recirc with the same pump.

 

[Jadnashua]

You want a 3/4" copper equivalent sized pipe for each bathroom...running 1/2" pex will not help...it's too small for a supply to a bathroom.

To prevent significant temperature swings, you really should consider replacing the valves with anti-scald versions. There are stand-alone pressure balanced valves but you'd need access behind the tub/shower walls to install them. Retrofitting a modern valve is a fairly common job, and they make renovation plates to cover the larger hole you need to make that happen. They come in various sizes, shapes, and finishes https://www.homedepot.com/s/renovation%20plate?NCNI-5

The dynamic pressure drop starts to go way up when you have a smaller line that is longer. Plus, there's a functional limit on how much water can be drawn through any sized pipe. While PEX allows a faster flow, its ID is smaller than copper. On copper, for the hot water, the Copper Institute's design guide calls for NGT 5fps, which is 4gpm on 1/2" and 8gpm with 3/4" on hot. So, in a single bathroom, if someone was showering and another using the sink, you could easily exceed those limits. The faster the flow, the more friction, the lower the pressure, the more you'd notice it which is where a pressure balanced valve can help.