Go with one of these, pricey but the ROE comes fast in gas savings.
https://www.amazon.com/Rinnai-RUC98...true&ref_=as_li_ss_tl&tag=shopperz_origin2-20
This is the way to go...I have 4 in my house and they are great.
It also helps to filter the water first (I have an EWS) and then make sure you flush them annually.