There are two ways that this type of chimney hurts energy performance. First, the caps on the thimbles and the cleanout door at the base of the chimney are probably leaky, so the stack effect is pulling conditioned indoor air through these gaps and up the flue. Second, the brick chimney probably represents an uninsulated thermal bridge that allows heat to escape via conduction and convection.
Sourced through Scoop.it from: www.finehomebuilding.com
Interesting post on chimneys we no longer use due to spare the air days and the part about keeping them clean.