It is absolutely impossible to operate an instant or on-demand heater from a 15 amp 120 volt outlet. Do not let the person who made that proposal come back on your property for any reason ever. Incompetent! Some of the on-demand electrical heaters may require 100 amps at 240 volts!

The existing GFCI is grandfathered but is no longer code. Bathroom and kitchen counter require dedicated circuits now.
You can not add this heater to that circuit. As soon as you modify that circuit your entire kitchen and bathroom will need to be rewired to be code compliant to current code.

Is 15A 120V breaker all that is needed for the electrical requirement on a gas tankless? Or does it need to be 20A?

I believe there is some open slots in my panel for a new breaker, and the panel is only about 15 feet from where the unit would go, so running a new one isn't too big of a deal.

The unit would be going on the exterior wall that is part of my garage, so a new electrical run could be ran through the attic, down on the interior of the wall inside the garage, and then drilled through the wall and an electrical box placed on the exterior. Or, it can be ran through the attic and down through the soffit and ran through exterior grade conduit on the exterior wall of the house. Which one of these would be approriate? If running down the exterior wall in conduit, where does the conduit need to start? Just somewhere up in the attic, but it doesn't matter where? I know that code in this area does not require full length of electrical runs to be in conduit

Lastly, is the electrical connection from the gas tankless hardwired in, or is it an actual plug?

The reason I ask these questions is that I have 2 quotes and one of the plumbers doesn't do electrical so I have to source it out, and the other plumber does, but he is the one who said he'd tap the GFCI. They also wanted to charge me $300 to tap the GFCI, run 2 ft of cable, drill a hole, and mount a box, and that is just absurd, so I'd probably do the electrical myself or have my handyman do, it is far cheaper.

If you are going to run this back to the panel, I would recommend you go with a 20 amp circuit, regardless. That will give you plenty of cushion in the event that the tankless electric freeze protection kicks in.

The cleanest install is to go inside the wall. The easiest is to run in surface mounted conduit. You could even do the conduit on the interior of the garage instead, and then go through the wall with it. If you run the conduit on the outside down the wall, use an entrance elbow and then terminate the conduit right inside the attic. Make sure that the conduit is connected to the heater enclosure in such a way that the conduit is properly grounded.

Take a look at the instruction manual for the heater... it will be online. I'm sure it calls for an outlet so that you have an easy disconnect.

I realized I wasn't all that clear. The 2 options I wanted to with were to either run the electrical down inside of the garage, then drill through interior wall, through exterior wall, and mount a box. Or, run the electrical down the eterior wall and mount the box. Either of these I can easily handle myself. Fishing through walls isn't my strong suit.

Sounds like either option will be fine then, and terminating conduit in the attac at an elbow is all I need.

Thanks for the info. Now to figure out which way to go.

The NEC requires that the manufacturers instructions must be followed for LISTED equipment, Article 110.3 (B). So get and read those instructions. Unless those instructions require a dedicated circuit and if the water heater does not require more than 7.5 amperes, 900 watts, the existing 15 amp circuit can be used unless that circuit is dedicated for other purposes, as specified by the NEC.

Bus Driver,

I'm sure we could determine the load on the rest of the circuit and make a code based assessment on if it would be permissible to add it to the load. But in this case, unless it's really hard to install a home run, it should be a home run. You don't want a 15 amp circuit supplying kitchen counter top outlets,bath outlet and the water heater.

It's been a while since I had an NEC code book, (1980) but you don't need to cite chapter and verse to know that this would be better done as a home run.

I don't ever install 15 amp circuits - 20 amp is the smallest I go when installing any residential circuit. I wouldn't think of 15 amp circuits in the kitchen.

It may be permissible (would need more assessment) but it's not a good judgment call to add it to the existing 15 amp circuit.

I would be getting either a Nortiz 751 or a Rinnai R75LSE 7.5gpm rated unit. I found the install guide for the Rinnai and it doesn't say anything about needed a dedicated circuit.

The Rinnai is rated at 50W normal, 2W standbye, and 100W in anti-frost mode. The Noritz is rated 64W normal and 145W anti-frost mode (unsure on standbye)

The Rinnai install guide only specifies 120 VAC 60Hz with a disconnect switch between power source and tankless unit. and that the tankless is electrically grounded. I'm suspecting the Noritz will be the same way.

Someone else braught up a point to me of using a GFCI breaker since this will be electricity where water exposure is possible. I already have a GFCI breaker that I used to replace an original 15A breaker to my bathroom. Reason I have this GFCI is because I retrofit an exhaust fan into my bathroom and electrical for the fan was pulled off the light fixture in the bathroom, so I replace the corresponding breaker with a GFCI breaker. So I was wondering if I could use that pre-existing GFCI for the tankless water heater

I don't think you "need" a dedicated circuit. Mine isn't on one either. I was just saying that I wouldn't add it to the existing load.

Good that you checked the specs. That gives you better information.

Don't mess with a GFCI on this circuit. You don't have a situation where a person is likely to come in contact with water and electricity -- such as a bathroom, kitchen or outdoor area. On the other hand, you want to be sure all the metal parts are properly grounded.

If you decide to connect it to the 15 amp GFCI circuit you have, it will work. And except for the previously noted grandfathered code exception, it should be up to code.
It's just not what I would do.

Must confess that I did not earlier read the original post in detail. Sorry for any confusion resulting. My answers were "correct" but not especially appropriate for this topic. Now from an electrical perspective, the present installation offers a possibility to upgrade other parts of the electrical. First, double check the wire gauge in the kitchen, it might well be 12 gauge. Check it everywhere on that circuit, someone might have extended from 12 gauge with 14 gauge. But the fact that the kitchen circuit apparently extends to the garage and that a new circuit is contemplated to the garage means that an additional wire run to the garage from the panel could be used to feed back to the kitchen from the garage on the existing cable to make two circuits serving the kitchen. The existing circuit would have to be disconnected at one of the kitchen outlets as the circuits (old and new) must not be interconnected since they do not originate from the same cable. If the existing cable is truly #14, 15 amps, the change I propose will not meet the latest code and may not be acceptable to the local inspector since any changes often are required to meet the most recent code in effect. But the change I propose would be an improvement.