This study develops recommendations on how efficiency indicators can be considered and designed as part of the amendment of the Ecodesign Regulation (EU) No 813/2013 on space heaters and combination heaters. Efficiency indicators should be used for the early detection of efficiency problems in heating appliances as well as efficiency-reducing defects. They are based on data that is mostly collected for control purposes anyway. To date, there are no standards for the interpretation and evaluation of efficiency. This study fills this gap and develops a method for efficiency indicators for heating appliances such as boilers and heat pumps.