How to Keep Your Home Cool

The AC pros may know how to install and service a unit, but don’t have any sense of thermodynamics.

Imagine that your home is a bucket, and the “cold” is water. You’d like to fill your bucket (home) with water (cold) up to a certain level, and leave it there while you’re home.

But that bucket has holes, a few small holes, running all the way up to the top. So the higher the water level is in the bucket, the faster the water leaks out. And that’s the only factor that affects the outflow rate — for a given level of the water line at a particular instant, the leakage rate is going to be the same regardless of whether you’re currently filling or not. 

Okay, so if you keep pouring water in, you can keep it the water line at a constant level if you balance the outflow. And consistent with the above, the higher the water level, the faster the outflow so the higher you need the inflow.

So what’s the most efficient approach to deal with the times when you’re not home and don’t care how high the water level is in the bucket?

Let’s start with an easy one. Imagine you’re going away for a month. Is it better to keep filling that bucket, to keep the water line at particular level? No, you just let all the water run out of the bucket. When you get home, you fill it back up to the desired level. That’s going to save a ton of water vs keeping the water line at a given level the whole time.

How about if you’re just gonna be gone for 8 hours? Well, it’s not quite as obvious, but the answer’s the same. Remember, the lower the water level gets, the slower the leakage rate. So the more you let the water level drop, the less water you lose. When you get home, you fill it back up to the desired level and the leakage rate goes back up. But during the day, you lose a lot less water as the leakage rate goes down than if you’d kept it at the same high level all day.

The answer in my water bucket analogy is clear. I suppose the question is, is it a good analogy? The answer is yes — a key concept of thermodynamics is that the higher the temperature differential, the greater the rate of heat transfer. If it’s 90 degrees outside and 75 degrees inside your house, your house is going to “lose its cold” at a much faster rate than if it’s 90 degrees outside and 89 inside your house. And at some point, your house is going to stop “losing cold” entirely (it may or may not be the same temp as the outside temp, depending on greenhouse effects, etc., but it will happen at some point).

Now there may be secondary reasons why it’s a bad idea to let your house heat up to ambient temperature. A big one would if you live in a humid area, which can cause mildew and then indoor condensation when you cool it back down. Or if you have pets, or anything else sensitive to a fairly narrow range of temps.

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