Environmentally Assisted Cracking (EAC) of copper tubes in high rise residential HVAC systems is an under-recognized problem. The authors have worked on three cases in the Midwest within the last few years. Failures found in two buildings occurred during the construction process and in another case five years after completion. The tubes were surrounded with an elastomeric PVC/nitrile insulation and joined together by brazing at high temperatures to create risers that circulate water from floor to floor. The EAC failures were determined to be the result of a combination of high residual stresses formed during the manufacturing process of the copper tubes exposure to moisture on the exterior of the tubes and exposure to harmful EAC-promoting chemicals including ammonium and nitrate inherent within the as-constructed system. Results of on-site investigations and laboratory testing using a variety of physical and chemical techniques and a review of the pertinent literature will be presented.