In-line water heaters.

[Tmall]

Originally Posted by Papa_Complex

They aren't install and forget, like many people treat their old tank unit. You'll need an annual service, especially when dealing with hard water, and a filter system wouldn't be a bad idea.

This as well, I can't get to the filter in mine. The previous owner must never have cleaned it, because the brass cover will NOT budge on mine. I have to cut the line coming into it, pull that piece out of the heater and replace it, and reinstall the supply line.

If you don't care about price so much, you can get an oil fired water heater. I had one in my previous house and I could not run out of hot water, I tried and failed.