We had a decent ice storm here Friday afternoon. It wasn't huge, but it was enough to get me out of work early. So, we're sitting around, munching on our dinner, drinking on our wine, watching some "East Bound And Down" DVDs, when I hear a thump-thump-thump-thump-thump from one of the AC units just outside the kitchen window.

AC units making noise during an ice storm?

Why is the fucking AC unit doing anything when it's 18 degrees outside?

So, I bundle up, grab a flashlight and go see what's happening. As I suspected, the fan blades are spinning, but are covered in ice, throwing the whole thing off balance. It's shaking like a washing machine. It stops spinning, so I grab a screw driver and a metal tomato stake and knock the ice off the blades. I fuss around with the thermostats inside and determine that the AC unit is tied to the upstairs heater somehow, and that the ice was the cause of the thumping. Minor problem solved.

Then I'm back to the important question: why is the fucking AC unit doing anything when it's 18 degrees outside? After the long list of things that didn't go right with the construction of this house, I'm fully confident that the heating/AC people have screwed up the upstairs heating system and that we're simultaneously heating and cooling the house, that we're spending a fortune on power to make it happen, and yet despite the hot and cold air streams blowing around there are no mini-hurricanes flying around the stairwell! At least that would be entertaining! But, testing with my expertly-calibrated back-of-hand thermal energy detector confirms that there is no cold air blowing upstairs; it's all hot air.

Hmm. And then I remembered something that the builder mentioned many months ago: the phrase "heat pump".

Google, google, wiki: oh, so that's what a heat pump is!

Reversible heat pumps work in either thermal direction to provide heating or cooling to the internal space. They employ a reversing valve to reverse the flow of refrigerant from the compressor through the condenser and evaporation coils.

In heating mode, the outdoor coil is an evaporator, while the indoor is a condenser. The refrigerant flowing from the evaporator (outdoor coil) carries the thermal energy from outside air (or ground) indoors, after the fluid's temperature has been augmented by compressing it. The indoor coil then transfers thermal energy (including energy from the compression) to the indoor air, which is then moved around the inside of the building by an air handler. Alternatively, thermal energy is transferred to water, which is then used to heat the building via radiators or underfloor heating. The heated water may even be used for domestic hot water consumption. The refrigerant is then allowed to expand, cool, and absorb heat to reheat to the outdoor temperature in the outside evaporator, and the cycle repeats. This is a standard refrigeration cycle, save that the "cold" side of the refrigerator (the evaporator coil) is positioned so it is outdoors where the environment is colder.
In cooling mode the cycle is similar, but the outdoor coil is now the condenser and the indoor coil (which reaches a lower temperature) is the evaporator. This is the familiar mode in which air conditioners operate.

Learning. Always learning.