Do 120v on demands generally require a new breaker?

Do on demand 120v water heaters require a new breaker installed in the box? I know with the larger units you need a 240v/30 amp.

Definitely YES. In order to get ANY hot water from them, and even that is problematical, you need a LOT of watts and they can only be supplied with a dedicated line and circuit breaker. Personally, I think even most 240 volt units are worthless, so you can tell what I think of 115 volt units. Unless you reduce the flow to a "pittance" using the supplied faucet aerators, you will NOT get any hot water, but warm maybe, from ANY "on demand water heater".

What he said. Only to add...

...at the cool incoming water temps in WA, there aren't any 120V electric on-demands out there that can truly handle more than a single faucet-flow (and nothing like a shower-flow.) You can get away with them in the warmer parts of TX or FL, but they don't have glacier-fed reservoirs or 40F artesian sources, and the temp at the water main on the street on the coolest day of winter in FL will be warmer than that in Tacoma on the hottest day of summer. To get water up to 105F shower temps from 65F takes ~40% fewer watts than from 40F, at any given flow, so what may work marginally in warm-water states doesn't really cut it in the cool-water PNW.

I agree that the point of use water heaters are pretty much worthless. If you only need a couple of gallons or less at a time, you would be served much better by a small standard tank heater. A 6 gallon tank heater doesn't take up much room either.

quote; If you only need a couple of gallons or less at a time, you would be served much better by a small standard tank heater. A 6 gallon tank heater doesn't take up much room either.

Actually the small 1 and 2 gallon heaters hang from a bracket on the wall, BUT, because they are pressure vessels they DO need a T&P valve and discharge to install them legally.