Dirty coils reduce efficiency and cause your unit to work harder, causing parts to wear out faster, and shorten the life expectancy of your unit. Your unit will cost you more month after month on your utility bill if the coils are not kept clean. Test performed by the EPA explain that as little as 1/10th of a centimeter of dirt or dust on a coil could reduce efficiency by 21%. Since the average homeowner’s heating and cooling bill is $1,500 per year, a 21% loss in efficiency means you could be wasting up to $315 each year.