Perhaps a copper cold water line is being warmed by heat in the surrounding airspace, such as could occur in an attic. Attic heats up from the sun, cold water copper line in the attic absorbs the heat, passes it on to the first place it empties into.
Edit to add: In your locale, this might not be as likely as it could be in a warmer climate.
Last edited by There_Ya_Go; 01/03/20.
