First, your system efficiency rating — whether it be SEER, EER, AFUE are all laboratory ratings and are not reflective of the real world efficiency of your system. Just as a quick example, if you have any sort of airflow restriction in your supply or return, your blower motor will have to work harder, increasing the power it uses and reducing your efficiency.
Second, systems may lose as much as 1% efficiency each year, depending on a variety of factors. While furnaces certainly can decrease in efficiency over time, it’s the A/C or heat pump systems you should be most concerned about. They are most susceptible to reduced efficiency, especially when they aren’t maintained properly.
While the older compressors are more susceptible to losses of compression efficiency due to a reduction in the reciprocating of piston compressors ability to seal, scroll type compressors can still lose efficiency. As bearings wear and the electric motors that power compressors become less efficient, the performance of the entire compressor is diminished.
Refrigerant Undercharge / Leaky Coils
Evaporator coils are also very prone to pinhole leakage as a result of corrosion over time. When your system becomes undercharged, efficiency is reduced greatly.
The evaporator coil (your indoor coil in an A/C system), is what actually transfers the cooling produced by the expansion of compressed refrigerant, to the air circulating through your house. The dirtier or the more oxidized your evaporator coil becomes, the less efficient this transfer becomes.
Poor Airflow/Duct Issues
While poor airflow or duct issues may exist when a system is first installed, one thing we can be certain of is that these issues won’t correct themselves. As time goes on, air leakage becomes greater, insulation value is decreased, and physical impediments can manifest themselves. One example that significantly impacts performance we see quite frequently, especially in package units, is loose insulation impeding airflow. This impediment can be so great that it likely reduces unit performance by 50% or more and even causes the unit to freeze up.
Reduced Motor Performance/Efficiency
Over time and the difficult life an HVAC system endures, motor performance can be significantly reduced, to the point where just prior to failure, huge amounts of energy can be wasted. Old and improperly maintained motors start slowly and never perform to their original design. This can be a result of wear in the bearings and the windings that actually create the magnetic field that makes a motor turn at all. Motors can be tested for their amperage draw, and when connected to other system data, an approximation of the system’s overall efficiency can be produced.