I am remodeling an early 1960's slab on grade 2 story home.

The water supply line comes in from under the slab, directly in the middle, and runs to the attic. The kitchen, laundry and existing bathrooms (8 appliances) are at one end of house and a new master bathroom (5 appliances) has been added at opposite end. The original, existing water supply plumbing layout is an electric heater with trunk and branch, one hot and one cold, to just one side of the house. The span between the ends is a little over 50 feet.

I have a new Rinnai RUR98iN, and there is plenty of room in the attic.

I am redesigning the plumbing system to run water supply lines to the new master bathroom. My current plan is to replace the old trunk and branch system with 4 manifolds, one hot/dedicated recirc and one cold at each end of the house. I end up with a cold, a hot, and a dedicated recirc line for each end of the house. Six lines in all. In the center, where the water heater is, will be at least 3 main manifolds, one for the cold mains, one for the hot mains, and one joining the dedicated recirc lines. Almost all the old copper is being replaced with Uponor Wirsbo hePEX-a.

I am concerned about the possibility of unbalance flow in the two dedicated recirc loops.

I am considering installation of a throttle valve, upstream of the check valve, on the "shorter" dedicated recirc loop. The shorter loop is the one servicing the new master bathroom. I say "shorter" because it will easily have a lower dynamic flow pressure drop than the "longer" loop to the kitchen/bathrooms/laundry. It is physically shorter and has fewer turns in the hot and recirc lines. My theory is without some means to throttle flow in the "shorter" loop, I may not get much recirc heating in the "longer" loop. I definitely do not want to discover a need to retrofit a throttle valve.

All main lines are 3/4 inch. All appliance lines are 1/2 inch.

I am hoping someone else out there has had some experience with my idea and can provide an opinion, even if you think I've lost my marbles.

balancing valves in each and thermometer on each is the only sure way of achieving perfect balance.
yeah, one balancer may be left wide open but to be sure it should be there.

I used 2 - Caleff 132 Quicksetter 3/4" balancing valves with temp gauge, with one Grundfos recirculating pump on set up similar to yours. Two loops of unequal lengths etc. back two 50 gallon water heaters hooked up in series for a 3 unit apartment that I re-piped. Very happy with them and easy to set.

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When setting up the recirc lines ensure that the water flow does not excees 2 FPS. Maximum water temperature 140. Follow these two guidelines and the recirc should work perfectly for years