We have one of those small 1/3rd gallon instant water heaters under the sink, with its separate little tap on the sink. To make instant hot drinks or to clean stuff. Worked flawlessly for many years. Recently it started splattering, air bubbles come out. That wouldn't be so bad but sometimes it splatters across the cup you are holding and it can burn fingers. It's this one (3MB file):
I unplugged it, ran it for gallons, much more than the thing holds. It's plumbed into the regular faucet next to it and that doesn't splatter. No leaks anywhere. Given that I ran more water than could ever be in that boiler plus short 1/4" branch-off line and it still has air bubbles I am puzzled. Where could those come from? Why does the faucet on the same supply line not produce any bubbles?
The sediment content of our water is very low, so that's unlikely.