Many conventional chemically treated closed loop hydronic systems suffer from corrosion associated with oxygen. The failures have been approached through the use of traditional chemicals such as oxygen scavengers/metal passivators and thermal mechanical deaeration pretreatment. While these approaches have worked well for high temperature hot water heating under minimal makeup conditions, the chemical eradication process eventually fails under high volume makeup
conditions. Also, deaeration is not a suitable solution for smaller low temperature systems. This paper will discuss the conditions, which lead to corrosion in these systems. An outline will also be provided for corrosion inhibitor performance expectation. A number of 'tell-tale' signs or observations that can be made without extensive chemical testing or exhaustive system study will also be presented.