Respect this humility. Something can be engineered to maximize efficiency or performance, and the engineers job was done perfectly if he succeeds at maxxing these out. However the ability to perform repairs can determine if a car or heating system is totalled or not. Often affecting a customers wallet way more than a slightly higher utility bill would.

For example, To my customers I recommend cast iron boilers with an indirect tank over a high efficiency combi boiler. Too many components that are hard to find when they fail and you need them most. Keeping it simple saves my customers money despite a slightly higher gas bill.