@ Debbie, as far as I can ascertain a regular "Gastherme" is what they call a "Gasdurchlauferhitzer" i.e. only for hot water. And then there is a so-called "Kombitherme" for heating and hot water. So a "gas hot water heater" or a " gas heating and hot water system."

Yes! And/or warm water boiler

Thanks guys.
Any ideas on "Grundabtretungsvertrag"?

... und nicht notwendigerweise eine Zentralheizung, sondern auch (oder sogar eher?) dezentral, pro Wohnung.

@ DEbs, the other one would be "real estate assignment contract."

@ Helmi. Thanks again.