We’ve been exploring a thankless solution, but the company that quited us said it isn’t a good idea in our area because the ground freezes in the winter. We don’t live in a super cold area, but it does snow a few times a year and it can get into the single digits of degrees Fahrenheit.
One thing to consider also is that tankless water heaters need a considerable amount of space around it because it needs adequate air flow. So if your water heater is in a closet, it might not work anyway.