In order to solve or relieve the blockage and fouling problems of sewage heat exchangers which have limited the development of sewage source heat pumps for a long time, a novel de-foulant hydrocyclone with continuous underflow and reflux function was proposed. Sand-water experiments and domestic sewage experiments were conducted to study the effect of underflow pipe diameter on the separation performance of the novel de-foulant hydrocyclone. Experimental results suggested that separation efficiency increased with increasing underflow pipe diameter. However, as long as the underflow pipe diameter was not too small (i.e. >5 mm), the novel de-foulant hydrocyclone provided effective separation performance, i.e. the separation efficiency of 94.3-97.2% for the foulant (<4 mm) in the untreated domestic sewage, and 99.96-100% for the sand (75-250 mu m). Compared with the novel de-foulant hydrocyclone with 0 mm underflow pipe diameter (i.e. a closed pipe), which was similar to the conventional hydrocyclone with a closed "grit pot" under its underflow orifice, the novel de-foulant hydrocyclone with continuous underflow and reflux function showed higher separation efficiency (41.6-46.2%) and slightly higher energy consumption (036-1.29 kPa), which could be ignored. In addition, a new concept of separation efficiency (i.e. the comprehensive separation efficiency), which takes into account both separation efficiency and split ratio, was proposed in this paper. Experimental results suggested that only when the ratio of the underflow pipe diameter to the vortex finder diameter was between 12.5% and 25.0%, did the novel de-foulant hydrocyclone have a high comprehensive separation efficiency for foulant (<4 mm), i.e. it had high separation efficiency (92.6-94.3%) and a low split ratio (1.32-2.54%) at the same time. With the increase of the underflow pipe diameter, the split ratio increased quickly and energy consumption increased slightly, whereas the concentration in overflow decreased slightly. (C) 2016 Elsevier B.V. All rights reserved.