Solar hot water - Heat

There's really no such thing as cold.

Unlike politics, just because it can make you miserable, numb, or even dead, cold doesn't actually exist.
What you are feeling when you feel cold is actually heat moving away from you and toward something else. Things lose heat or gain heat but don't lose or gain cold. Heat is so generous that it simply wants to share itself evenly everywhere. Unfortunately for you, that means if you are warmer then the winter around you, all that generous warmth you make is going to try to spread out to everywhere else.

"My air-conditioner makes cold" I would sometimes hear myself say. "But it doesn't" I might reply. It turns out an air-conditioner is what is known as a heat pump. All it does is finds some heat and shifts it to somewhere else. In the case of cooling, it takes the heat out of your room, and throws it outside where it belongs. In the case of heating it grabs heat from outside and sticks it in your room. That bit is most amazing as far as I'm concerned. Its incredible to think that there is so much heat out there in chilly winter, that an air-conditioner or a solar hot water system can pick up great wads of the stuff and jam it into my house to make me warm.

Thanks science.

Another important and useful thing about heat is that it rises. That's because when stuff is hot the little stuff that stuff is made of move about a lot. This means the stuff expands a bit. When an amount of stuff is bigger than the same amount of the same stuff, it weighs less. Which is to say it displaces more of some other stuff. I'm glad we cleared that stuff up.

Hot stuff rises above its neighbouring less hot stuff. That means we can move heat for free. If hot air rises and we need to move it up, all we have to do is let it move there. If we need to move it up and to the side, all we need to do is put it in a pipe.