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.

Electronics do not like excessive heat, and water still freezes at 32 F.

I agree about the summertime heat, because even though they're intended for outdoor installation, adding an addition 20+F won't help any.

The AHJ may not like the idea of an outside unit in an attic.

What happens if the power goes out? Is your tankless heater toast? Maybe a popsicle would be a better term. If it is in the attic, it then thaws out and floods the house. I saw that happen in a recently-rennovated laboratory building. It was not with water heaters but with water to air heat exchangers after the heating supply went out on a very cold night.

I do recall that before I discovered that they are too expensive to make any economic sense, I noticed that one brand uses a turbine in the water supply to make spark to ignite.

This winter's been a cakewalk here in GA, but it was really cold last year. A new industrial plant I worked on had some big outdoor pipes freeze and burst (4" and 6"). If someone had cracked the hose station at the end of the line, it wouldn't have happened, but no such luck. We went back and added freeze protection valves that will open up and trickle water through the line if it gets below freezing.

Kind of a slick idea. Very little expense for that use. Might be pricey for the house, but it is kinda dummy proof.

OP says there is no freeze risk, why all the concerns of freezing?

To hot, are you kidding me? Countless commercial units are run at 185 degrees, it's just as hot in that cabinet.

Mines in the attic, who assumed a outdoor unit would be used in a attic? OP never said that

Go for it but have a pan with drain to protect from water damage.

1. The water heater must be located within 20ft of the attic hatchway.
2.There must be at least a 24" wide solid floor catwalk from the hatchway to the water heater location.
3. There must be a 30" workspace in front of the water heater.
4.At the water heater location there must be a 24" solid floor walkway all around the heater.
5.There must be a 120v electrical outlet in the near proximity of the water heater to permit operating power tools when servicing the unit.
6.There must be an electric light in the near proximity of the heater and the light must be controlled by a switch that is placed near the attic hatchway.
7.There must be an approved drip pan with a minimum 3/4"dia drain line that is run outside and terminated by code specification or it must terminate in an approved indirect waste.
8.The TPRV must have a full bore discharge line that goes outside and discharges in an obvious location per code spec or it must discharge into an approved waste receptor at and obvious location. TPRV MAY NOT discharge into the pan.
9. The water heater must remain accessible for service or inspection, (must not be blocked by personal items stored in the attic.)

Under the IRC the attic hatch must be a minimum of 22"x30"
Under the UPC the attic hatch must be a minimum of 30" x 30"

Note, those dimensions are the minimum, but code also says the water heater MUST fit through the opening. That would generally not be an issue with a tankless but with a tanktype you need to check the tank size.

I need about 5 of those for my house.