I have a few quesitons on a new build using Pex.

Is there a reason to run an HR system for cold lines if there is no concern on waiting for hot water? My thoughts are that a trunk and branch system seems the most practical.

I planned on using an HR system for hot supply using 1/2" for most areas. One area that I am concerned with is the master bath that will require roughly 50' of piping runs to most fixtures. From reading it seems that a 1/2" pex line will hold around .46 gallons at 50'. This seems that it would still take a decent amount of time to clear the cold water. Although this is not as wastefull as a 3/4" trunk line it is more of a concern on wait time.

Here is what I thought as a solution to this problem of waiting on hot water. I was looking at having a 2nd 10-20 gallon electric water heater in my closet (not wifes) adjacent to the shower. I would have it drained under the bench seat in shower (out of sight). If I run a 3/4" hot supply into the cold end of WH the room temp water in the line is mixed into the 2nd WH which is set slightly hotter than main to adjust for room temp water. The hot end of WH would then run to a manifold where 1/2" pex lines are run to each fixture in master bath. This would cut the runs to ~6'-15' compared to 50'. The 2nd WH would not use as much electricity b/c it is always fed by a hot line (room temp at worse). I feel this is a better solution than a re-circ system b/c the master bath is the only place where hot water use is most important.

The rest of the homes hot water supply would be run on 1/2" HR where lengths are considerbly less than master bath. I was thinking of a 66 gal main WH with a 20 gal 2nd giving plenty of hot water for us and guests.

So it seems that my thoughts are using Trunk and Branch for cold supply, HR for most hot supply and remote manifold (off 2nd WH for master bath).

Hopefully some of you can make sense of this and provide some input in a kind manner. I attached picture if that helps of master bath. Thanks