To increase the fractional energy savings achieved with solar thermal combisystems the store volume may be increased Installation of large stores in single-family houses is, however, often limited by space constraints In this article the influence of the store dimensions, as well as internal and external auxiliary volume configurations, are investigated for large solar water stores by annual dynamic TRNSYS simulations The results show that store sizes up to 4 m(3) may be used in solar heating systems with 30 m(2) collector area It is further shown that well-insulated stores are rather insensitive to the geometry Stores deviating from the conventional dimensions still yield high fractional energy savings Furthermore, the simulations show that the performance of an internal auxiliary volume configuration in most cases exceeds that of a solution with an external auxiliary unit The practical limitations of very thin auxiliary volumes must, however, be further investigated. (C) 2010 Elsevier Ltd All rights reserved