Tankless water heater in attic?
My contractor recently suggested that instead of having 2 tankless water heaters at opposite sides of my almost new construction single story house (4.5 baths, laundry, and kitchen), I may want to consider a single larger unit in the attic. We had originally planned to have the water heaters on the exterior walls of the house. His reasons include saving the cost of installing the additional water heater and that hot water would arrive at the fixtures faster due to the heater's central location and that the water flows "down" faster. I'm not so sure about the last part since basement water heaters seem to work pretty well.
I'm located in Southern California, no freezing risk, but the unconditioned attic will get quite hot in the summers. I had also planned to have a water softener installed outside "before" the first water heater so I would have soft water throughout the house. If I installed the tankless heater in the attic, I would still keep the water softener outside and run the water to the attic after the softener.
Is the placement of the tankless water heater in the attic recommended for the reasons above or at all? It just seems that the attic would be very difficult place to service the tank if needed and there is always the risk of a leak.
The other choice would be to install a larger tankless (eg Noritz NRC111) outside with the softener, but the kitchen would be about 60 ft away as would be the furthest bath.
Thanks for any guidance.