Recovery of energy carried by greywater is possible, for instance, through application of Drain Water Heat Recovery (DWHR) units at the drain water outlet from a shower unit. This paper reports results of a study concerning assessment of rationale behind the use of two different horizontal DWHR units. The analysis, carried out in laboratory conditions proved that equipping of the unit with properly shaped baffles can significantly increase its effectiveness. The effect was observed for all flow rates and device body bottom slopes adopted for the analysis. The paper points out the effect of difference of temperatures between the preheated water and the greywater giving up the heat. The effect turned out to be small in case of the compared devices. Results presented in the paper are of a practical value and can be used as guidelines for designers and potential investors. (C) 2019 Elsevier Ltd. All rights reserved.