A heat exchanger system comprising: a heat exchange main unit having a gas channel and a fluid channel (60, 70) configured such that a fluid in the fluid channel (60, 70) can exchange heat with the flue gas in the gas channel in use, a housing accommodating the heat exchange main unit; a drain collecting part arranged inside the housing and configured to collect condensate from the flue gas; and a siphon (200, 300) attached to the drain collecting part to prevent the release of the flue gas from a condensate outlet (204, 304) formed therein, wherein the siphon (200, 300) has a tubular body (210, 310) being attached to the drain collecting part and having a first portion (240, 340) and a second portion (250, 350), the first portion (240, 340) having a condensate inlet (202, 302), the second portion (250, 350) having the condensate outlet (204, 304) and an operation member (252, 352), the tubular body (210, 310) being configured such that, when the tubular body (210, 310) is attached to the drain collecting part, the first portion (240, 340) is connected to the drain collecting part and is at least partially located inside the housing, the second portion (250, 350) is at least partially located outside the housing, and the operation member (252, 352) is located outside the housing, characterized in that the second portion (250, 350) is detachably attached to the first portion (240,340) without opening the housing of the heat exchanger system. a heat exchange main unit having a gas channel and a fluid channel (60, 70) configured such that a fluid in the fluid channel (60, 70) can exchange heat with the flue gas in the gas channel in use, a housing accommodating the heat exchange main unit; a drain collecting part arranged inside the housing and configured to collect condensate from the flue gas; and a siphon (200, 300) attached to the drain collecting part to prevent the release of the flue gas from a condensate outlet (204, 304) formed therein, wherein the siphon (200, 300) has a tubular body (210, 310) being attached to the drain collecting part and having a first portion (240, 340) and a second portion (250, 350), the first portion (240, 340) having a condensate inlet (202, 302), the second portion (250, 350) having the condensate outlet (204, 304) and an operation member (252, 352), the tubular body (210, 310) being configured such that, when the tubular body (210, 310) is attached to the drain collecting part, the first portion (240, 340) is connected to the drain collecting part and is at least partially located inside the housing, the second portion (250, 350) is at least partially located outside the housing, and the operation member (252, 352) is located outside the housing, the first portion (240, 340) having a condensate inlet (202, 302), the second portion (250, 350) having the condensate outlet (204, 304) and an operation member (252, 352), the tubular body (210, 310) being configured such that, when the tubular body (210, 310) is attached to the drain collecting part, the first portion (240, 340) is connected to the drain collecting part and is at least partially located inside the housing, the second portion (250, 350) is at least partially located outside the housing, and the operation member (252, 352) is located outside the housing, characterized in that the second portion (250, 350) is detachably attached to the first portion (240,340) without opening the housing of the heat exchanger system.