Dual temperature piping systems are under an equal threat to chill water systems, with the main differences being the size of the pipe, their location, and an extensive layout within the building property.  They utilize one set of risers to supply both cold and hot water depending upon seasonal requirements.  Dual temperature systems are the primary means of heating and cooling for apartment and condominium complexes, with less presence at high rise office buildings.

As closed systems, they should have low corrosion activity at under 0.5 MPY.  However, over decades and especially when not adequately maintained, significant rust product can be produced, and will often reveal itself in the form of reduced heat transfer.  Turning down chill water temperatures and hot water temperatures up to offset such heat transfer loss solves nothing other than to increase condensation and external corrosion activity during summer months.

With the risers typically installed at the perimeter columns, they are highly impacted by moist air infiltrating through the building envelope.  Most such systems were poorly insulated, with 1/2 in. insulation installed, the insulation missing or incomplete.  The fact that such pipe was never expected to see the light of day behind walls for the next 50 years helps to explain the widespread damage we now frequently document.  Unless constructed of copper pipe, and exceptional rarity, any dual temperature pipe system in place for more than 30 years is likely exhibiting some level of deterioration.

Since dual temperature systems are pressurized, failures, and especially total thread separations, can be catastrophic to many tenants and to multiple floors below.  Once defined, there are no remedial actions available other than total pipe replacement.  Reinsulating the pipe without first rehabilitating it through rust removal and coating, generally impossible to perform safely, accomplishes nothing.  With little pipe remaining, as well as the major corrosion impact to the outer pipe surface, lining the pipe is an equally worthless effort.

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