DIYbanter - View Single Post - Central A/C - leave running or use set-back thermometer?

Clark W. Griswold, Jr.

wrote:

The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
outdoor heat.

Well, sort of. A/C compressors are nothing more than heat transfer devices. As
someone else posted, they aren't working harder of easier based on what the
outdoor temps are. They consume the same amount of electricity.

A little refresher course: I'm making up the numbers, but the principles hold.

An AC compressor takes a freon like gas and compresses it. Basic thermodynamics
states that when you compress a gas it heats up. Lets say its 95 outside. By
compressing the gas it goes up 15 degress to 110. If it were 70 outside, it
would go up to 85. The 15 degrees from compressing doesn't change - that's basic
thermo again.

We run the compressed gas through an outdoor radiator and blow a fan across the
radiator. That cools the compressed gas back down to outdoor temp - say 95.
Bring the gas inside and through a heat exchanger, only this time we let the gas
expand.

Basic thermo says the gas gets colder, lets say by the same amount, so the heat
exchanger temp is now 75. By blowing the 90 degree house air through a 75 degree
exchanger, the house gets cooler and the cycle repeats.

Will the house cool faster if the heat exchanger is 50 degrees (ie outdoor
temp/compressed gas is 65)? Sure. Is the compressor running less? Probably. Is
the AC working easier? No. Its working the same.

Will you run the compressor more by leaving it on all day compared to turning
the thermo up while you are gone? Absolutely, because the house isn't perfectly
insulated.

Will the house take a while to cool down? Sure - there's a certain amount of
thermal mass involved besides the air that has to be cooled. But that same mass
also acts as a brake to keep the house from heating up.