On demand heats the water at the point of use .
So a heater, usually electric for convenience sake, in every room with a faucet.
Tankless, simply replaces your current water heater with a super fast ( high btu ) heater.
Mine is gas, 174,000 btu. Btu size has to be high enough for the number of faucets being used at one time, AND …. For the desired temperature rise needed. I’m in the north, so water enters my basement at 45*F, year round. It pumps out 135* water all day, but I have it set at 123*.
The water still has to flow from the heater through the pipes to the faucets , so you still have to wait for the hot water to get there.
It works for us, it will keep up when we are using 2 showers, and the dishwasher.
Another option I did not go with, is to add recirculating pumps, and return pipes. That would give us instant hot water. But then you are running the pumps and continuously heating the water in the pipes.
