House as a system?

This phrase is used over and over with professionals referring to energy efficiency in homes, but what does it mean?  The basic idea is that your house is a system of components all working together in concert.  Just like a car has many parts that accomplish its designed task so does your house.  When considering whether or not you need to replace your HVAC unit you may need to also consider other parts of the house or system.  Would you consider changing one tire on your car to a larger size?  Not without first thinking about how that would affect its performance right?  Unfortunately this is not always the case where our houses are concerned.  The amount and type of insulation for example will affect the sizing of the HVAC units.  What the leakage rate on your house will also affect the comfort and ability of the HVAC system to effectively condition the air in the house.   Improperly installed or leaky duct work  can be just as responsible for high energy bills as the old windows and doors that you can see need to be replaced.  This technique can be used to compare different options and how they increase or decrease energy consumption.  With the aid of sophisticated software, and careful data collection, any house can be evaluated for potential upgrades.  Most importantly before you make any changes, the estimated energy usage and savings needs to be calculated.