Recirculation pump line length

[LukePDX]

I have a renovation of two 8x10 bathrooms on top of each other (8ft ceilings), serviced by one tank right next to lower bathroom. It's all conditioned space. I'm not 100% sure what the plumber plans, but I can't imagine a line longer than 32ft from the tank (probably the upstairs rain shower head). The four faucets are on the same wall as the heater, their supply lines could be 10 feet or less.

I'm considering units with and without internal pumps. Is there a rule of thumb for what distance the recirculation pump starts to become useful?

 

[Jadnashua]

With careful design, you can get water to circulate all on its own. The better you make the insulation, the driving force caused by temperature changes (radiating heat, and thus becoming denser/heavier) it becomes less capable of flow. So, it's somewhat a tradeoff of wasting energy heating the water and having it circulating on its own, or managing it with a pump and only doing it when wanted. A typical recirc pump is quite small, and may not need to run constantly. Getting your system to circulate on its own means following some careful design rules.