This is mundane but practical.  In the course of having a minor but critical repair done on a failed air conditioner, I learned that it had never had enough freon.  I had just accepted that the vent temperatures in the low 70s were what we could expect, but after our friendly neighborhood repairman added some freon, our vent temperature is 59°F now.  It’s strikingly analogous to what happens to humans when they’re missing blood volume – they don’t tend to work as well.  And just like with humans, optimal efficiency is achieved by the precise amount of fluid in the system.  If you put in too much, then it doesn’t move because there’s no room for the pressure differentials that allow pumping in a closed system.

The environmental and economic benefit is much like proper tire pressure’s effect on fuel economy.  We’ve been here for three years, and on every hot day, our second-floor air conditioner was running for longer than it should have, simply due to the fact that it was inefficient. All that time, we used more electricity than we needed to.  This simple cheap step would have paid for itself long ago.  As someone who loves efficiency, this really hurts, but I’m glad it’s fixed now.

I encourage all of you to measure your vent temperatures.  You should get somewhere around 60°F – anything more is inefficient and costly and needs to be addressed, possibly with more freon.