Regulation (EU)813/2013 does not exclude synthetic fuels, which are thus in the scope, although there are not any specific provisions for those combustibles in the current Regulation (EU) 813/2013. The standards that could be used are, for gas fired boilers EN15502-1, and for liquid fuel fired boilers EN304. Such standards could be amended to include provisions for synthetic fuels, if needed.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.