How Does Air Conditioning Work

Author Note: Here's
 a link to the original post on
 Energy Auditing Blog. Also, it was written to prove that a Mainer does understand air conditioning, at least for the 9 days a year we need it.

It's time to turn to cooling thoughts, even though Maine didn't actually get a winter this year. It's true; we had an extremely long fall. Then sometime around 2 AM on January 17th, we had winter, then moved right on to spring.

And moving on to spring means pulling down the insulating plastic films and installing air conditioners. In my case, it also means that it's time to write about A/Cs or, more exactly, how does air conditioning work? Just like with walls or insulation or ventilation, understanding a bit about how they work can help clarify your thoughts on how it fits in to your energy savings plans.

How Does Air Conditioning Work - Heat Mechanics

How does air conditioning work? Just
 set the thermostat and grab a glass of wine, right? Not quite. Your air conditioner works basically just like your refrigerator, if you imagine the interior of the house as inside the fridge. The A/C consists of an evaporater, condenser, and compressor with a cooling liquid refrigerant.

The key mechanism is that of sensible versus latent heat. Sensible heat is the one we physically experience every day. The sun comes up, the room rises in temperature. That's sensible heat gain. Latent heat is heat energy gained or lost without changing temperature. The classic example is that of ice melting.

Imagine holding a glass of ice water over an open flame. Depending on its intensity, you may hold the ice water for 10 or 15 minutes before all the ice melts. The ice absorbs loads of heat but stays the same temperature. Eventually the latent heat is overcome and the ice transitions to water.

How Does Air Conditioning Work - Actual Mechanics

Well, any phase transition, including moving to and from a gas moves heat the same way. The air conditioner's evaporator will (and this is the bit of insight you can only glean here), evaporate the refrigerant and, in moving from liquid to gas, it absorbs heat.

The compressor moves this gaseous refrigerant (containing the interior heat) to the condenser. The condenser compresses the gas, forcing it back into liquid form and expelling the heat. Put your hand near the back of an air conditioner; most folks are surprised that it is quite warm.

How Does Air Conditioners Work - And This Matters Why?

So that's the ultra simplified version of how air conditioners work. The heat is extracted from the interior air, cooling it for return to the house and the heat is expelled outside. Why is this important to understand?

First, while I may talk an enormous amount about heating and insulation, most of the country deals with cooling and air conditioning issues to a much greater extent. Someone from Georgia or Louisiana may find some common ground on venting or spray foam as a Mainer, but heating and cooling are entirely different climate-based beasts.

Second, there are a huge variety of air conditioning approaches or ideas built around the concept. There are central air, single room, mini split, evaporative, geothermal heat pumps, cold climate heat pumps and yes, regular air sourced heat pumps.

Those heat pumps are used for hot water production, heating, cooling, making margaritas. They are pretty amazing. Having a smidge of understanding about the concepts can help when thinking about the varied systems.

Lastly, when I was describing the whole evaporator, compressor concept, didn't it seem like it would take a ton of energy/electricity? It does, which is why refrigerators and A/Cs are about the biggest consumers of electricity in residential housing. So how does air conditioning work? By using the simple concepts of phase change heat transfers. Understanding a little about how air conditioning produces cool air can help when envisioning their role in the entire house and in reducing energy use.