Abstract: Many leaks successively occurred in the fan coil pipes of a hot water boiler after about three years of use. The fan coil pipes in this batch all adopt Q215 hot-dip galvanized steel pipes. In order to find out the cause,some of the steel pipes cut on site were respectively subjected to macroscopic observation, chemical composition analysis, detection of the attachments on the inner wall, microscopic structure inspection, mechanical property test, morphology analysis of the opening part, energy spectrum analysis, etc. The results show that the chemical composition and mechanical properties of the steel pipes meet the standard requirements, the oxygen content in the circulating water exceeds the standard, there are many attachments on the inner wall of the pipes, and the pipes fail due to under-deposit corrosion.