"Double duct" room air conditioners (DD-AC) are compact systems with no external heat exchanger unit. Outdoor air flow is conveyed through two ducts crossing external walls. Design and optimization of a DD-AC operating with R290 are reported. A suitable 3D simulation code was developed for the analysis of finned coils heat exchangers in order to optimize system efficiency, still minimizing refrigerant charge. In particular, the effect of refrigerant condensate subcooling on refrigerant charge and the optimal number of circuits on system performance were investigated. The R290 system performance was compared in summer conditions, according to EN 14511, with a similar off-theshelf system operating with R410A. The results showed that, with a R290 charge lower than 150 g, it is possible to achieve at least the same energy efficiency with the same cooling capacity of the R410A system.