Another low-cost and fairly simple way to reduce energy use in your building is to prevent air exchange between the indoor conditioned space and the outdoors. To do this, you will want to make sure the air sweeps at the bottom of your doors are in good condition, and replace them if not. Similarly, check the weather stripping on the sides of outer doors to ensure that it isn’t degrading, and replace it if it is.
Take a close look at your windows. If you notice cracks around the window frame, these can be filled with caulking to prevent air flow.
Lastly, walk around the exterior of the building. Any place where mechanical or electrical equipment leaves or enters the building you may find gaps where air can escape and enter. Fill these with insulation or caulking.