There are other questions such as this that ask about a point of use heater to provide quicker hot water, but they don't seem to fit what I want.
Our kitchen sink takes a while to get hot, and I would like this to be more immediate. If I use a small tank water heater (2.5 gal) under the sink, and connect that to the hot water, it seems that I would get 2.5 gallons of hot water immediately, followed by 20 seconds of cool water (the time it normally takes to get hot water) that would then gradually warm back up, right?
What I would love to have is an inline tankless, plug in (120v) water heater that would immediately provide warm (ok if not super hot) water, and then automatically turn off once hot water from the tank arrives so that the water is not over heated.
It seems like such an obvious solution, but I haven't been able to find such a device. Is there a reason why this does not exist?
Best Answer
TLDR: they all do that, it's a safety feature to avoid scalding.
It takes a lot of power to make heat.
You won't get much heat out of a 120V on-demand heater unless you really, really, really restrict water flow, like, you could get a cup of tea out of it if you're patient. Don't even bother with on-demand unless you are willing to run 240V to the heater and at least 20A preferably 30.
An on-demand heater made to run alone on a 15A circuit, is 1440W. It will only embarrass itself and its entire class of heaters. You will be disappointed, swear them off for good, and that will be a shame because they are wonderful when you size them properly.
As others discuss, they have safety controls to prevent outputting scalding water. You should not set your on-demand heater to scalding to avoid legionella, legionella can't grow in an on-demand. You should set your tank heater to kill legionella.