The performance of a novel dry-expansion evaporator developed for a wastewater source heat pump (WWSHP) was experimentally tested and the test results are reported. The evaporator is a retrofitted shell and tube heat exchanger where bio-fouling build-up on tube surface can be easily removed from outside of the heat exchanger. An experimental WWSHP containing the novel evaporator using waste bath water as heat source for water heating was built up. A one-month long test to examine the effect of tube bio-fouling on the operating performance of the WWSHP was conducted. The test data suggested that with the growth of bio-fouling on the evaporator surface during the testing period, the daily averaged refrigerant temperature leaving the expansion valve decreased gradually from 11.4 to 9.4 degrees C; the daily averaged heat exchange capacity of the evaporator dropped from 8.2 to 5.8 kW, and the daily averaged COP of the WWSHP decreased gradually from 3.09 to 2.50. However, after performing cleaning at the end of the one-month long testing period, the averaged refrigerant temperature leaving the expansion valve was increased from 9.4 to 11.2 degrees C; the averaged heat exchange capacity was increased from 5.8 to 8.0 kW, and the averaged COP was also increased from 2.50 to 3.04. These comparison results demonstrated clearly that the novel evaporator with defouling function was effective in easily removing the bio-fouling build-up at a low cost. (C) 2012 Elsevier Ltd. All rights reserved.