An improved model of a heat pipe augmented passive solar space heating system was tested. Computer simulations showed that the new model, featuring key design modifications including a copper absorber, thicker insulation, a rubber adiabatic section, and one condenser exposed directly to room air, has significantly improved heat delivery to the room and reduced losses. The new prototype was tested alongside the previous prototype in a two-room passive solar test facility during January February of 2013. Results showed that modifications implemented for the new model contributed to increased rate of useful thermal gains to thermal storage and to the room, and decreased rate of thermal losses to ambient. Average daily peak efficiencies for the previous system and the new system were 80.7% and 85.0%, respectively. Furthermore, the average storage temperature for the new model, over the entire testing period, was 13.4% higher than that of the previous model; while the average room temperature over the same period was 24.6% greater for the new system. Simulations matched well with experimental data from the new prototype after parametric adjustments were made to the thermal capacitance of the room and conductances between evaporators and condensers, storage tank and room, solar wall insulation, and the load to collector ratio. (C) 2014 Elsevier Ltd. All rights reserved.