Relating energy demand
to degree days

Normally, a significant deviation can be traced quickly to some easily-remedied cause such as a mis-set time control or minor equipment malfanction.

However, should the problem prove elusive (as may be the case with a small-percentage shift in performance), the building will continue to operate on its new adverse performance characteristic and ultimately you will be able to discern what the new pattern is: see the red line in the figure.

In this example, the excess consumption is approximately the same regardless of degree-day value, giving a new performance line parallel with the old. This kind of diagnostic evidence is very useful because it rules out whole categories of possible causes and implicates others.