Borisg 教授： water heater connected in series and shark bite

connector. 请教

Why is circulation Pump at the 2nd heater water (two connected in series)? 

 

https://www.usinspect.com/blog/technical-jargon-series-vs-parallel-water-heater-configurations/#:~:text=Simply%20stated%2C%202%20water%20heaters,way%20to%20the%20demand%20point.

I googeld. For quick fill of hot tub on 2nd floor?

I googled more. https://www.nrdc.org/bio/pierre-delforge/little-known-power-circulator-pumps#:~:text=Circulator%20pumps%20move%20hot%20water,water%2C%20time%2C%20and%20money.

 

A plumbing guy at Home depot told me shark bit connectors are not code compliant behind walls (what ever that means?). Per him I need to use plumb clamp/ring ($100 for clamp tool). 

Per google, it is good.