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