Different variants of a cost-effective solar thermal drainback system were designed, which can be used as an add-on to conventional domestic hot water systems in multi-family houses (MFH). Expert interviews were conducted to identify the most important renovation situations in MFH in which the drainback system can be used. The comparison of levelized cost of heat (LCoH) of four investigated solar thermal drainback systems (DBS) with a conventional solar thermal system, shows relevant cost reductions for the DBS of up to 33 %. A demonstration DBS was installed in the laboratory, where the transfer of solar heat was implemented through the cold water connection pipe of the hot water storage tank of the existing DHW system. The system was designed using thermal hydraulic simulations (THS). THS was used to formulate design steps for small DBS with one row of collectors, which can be used by manufacturers to design DBS with high operational reliability.