Monday, August 03, 2015

Two Mistaken Mental Models: Thermostat and Air Conditioning

It has been often said that we learn chiefly by analogy, by comparing the thing to be learned with something we are already familiar with. Thus, for example,  EEPROM (Electrically Erasable, Programmable, Read-Only Memory) is like a marker board: When you write on it, the information will remain until you erase it. Turning the power off does not erase it the way main memory is erased.

The analogy-based understanding you create is known as a mental model. It's your intellectual explanation of how the thing works or what it does.

It follows, then, that if you apply a wrong or inaccurate analogy to something you are trying to understand, you will construct a wrong mental model of it. Here are two common examples.

First misunderstanding: A thermostat is like an accelerator or a water faucet: As you press down on the accelerator or open the water faucet,  you get more heat or cooling volume. What's wrong with this mental model? Except for houses with multi-stage heating or cooling, a thermostat is actually just an on-off switch. If it's 60 degrees F in the house and you turn the thermostat up to 70, the heater comes on. If you turn it to 80, the heater also comes on, but at the same output capacity as at 70. It is true that many modern systems have variable heat output depending on the difference between current temperature and desired temperature, but if you are in a house with a system more than, say, five years old, don't assume that you are controlling heating power or output by turning the temperature up or down farther.

The second misleading mental model involves home central air conditioning. You might have seen ads for an "A/C tuneup," which in themselves seem to promote the analogy that central air conditioning units are like automobile engines. So if a technician comes to your house and ends up telling  you that your unit was a bit low on refrigerant, so he added a couple of pounds, you might be thinking that, just as your car sometimes gets a quart low oil, it's  understandable that your A/C unit might get low on refrigerant.

But this is where the model is wrong. Automobile crankcases are open systems, where oil can leak out or even be combusted if it gets past the piston rings.

Or suppose you reason a pari that, since your automobile's air conditioning system needed to have some refrigerant added, it's quite believable that you home system would, too.

But note what is in common with these analogies. The car needs oil because the oil leaks. The car AC system needs refrigerant because the refrigerant leaks out (at the compressor seals).

 A home air conditioning unit is a sealed system that cannot use up either oil or refrigerant. The only time a technician can add refrigerant could be (1) that the system was undercharged when it was installed or worked on or (2) that there is a leak. So, if you have your  unit checked and the service tech says your system needed some refrigerant, ask him, "Where is the leak?" or "Did you fix the leak? Show me where."

So, after all, the analogies are informative after all, if  you remember that the engine and car A/C need oil or refrigerant because of a leak, not because of normal use.