Send your question to Umbra!

Q. Dear Umbra,

We are in the midst of a heat wave. I have a window unit air conditioner in my tiny apartment. There is an “energy saver” option that shuts down once the temp reaches the desired setting (generally 79 degrees), then starts back up again when the mercury rises. It’s been so hot that it seems to only take a few minutes till it fires back up. Is this really saving energy?

Brooklyn, N.Y.

Grist thanks its sponsors. Become one.

Reader support helps sustain our work. Donate today to keep our climate news free. All donations DOUBLED!

Photo by Jan Tik.

A. Dearest Emily,

With hope, the heat will have broken by the time you read this. But we all know it will rise again, and we’ll have plenty more days when we can hear the entire country lean back in its chair, wipe its brow, and mumble, “Hot enough for ya?” Except here in Seattle, where we get approximately one hot day a year, usually the third Tuesday in September.

Before we get to your question, a few eye-opening facts about America’s love of indoor climate control: In the late 1970s, 23 percent of American homes had some form of air conditioning; today, 87 percent do. We have become so addicted that 9 out of 10 new homes are built with central air. We spend $40 billion a year air-conditioning our buildings, says the EPA, and cooling our homes accounts for 17 percent of household energy use.

Grist thanks its sponsors. Become one.

In return, we get — well, I’ll let author Stan Cox say it: “Air-conditioning buildings and cars in the U.S. has the climate impact of half a billion metric tons of carbon dioxide a year. That exceeds the total annual carbon dioxide emissions of any one of these nations: Australia, France, Brazil, or Indonesia.”

Wait, you mean the thing we use to get through the record heat is … helping to cause the record heat? I believe that is what the kids call ironic.

So my first piece of advice is, when you can stand to go without the AC, please do. Keep your blinds closed against the sun, use a fan, unplug appliances that generate heat, eat popsicles, drink water, place wet washcloths upon your fevered brow, sleep naked. When you use the AC, please set it at a reasonable temperature (your 79 is very civilized) and do not — do not — crank it all day while you are gone, people. Your dining room set does not need to be comfortable.

Emily, you’ve perhaps passed out while waiting for me to get to your actual question. I have experienced “energy-saving” mode, and I’ll admit I find it rather annoying. The mechanical hum stops. Your brain adjusts to the quiet. And then — ga-glunk — it starts up again. On or off? On or off? It can’t seem to decide.

Here’s what is actually happening: Air conditioners remove heat and moisture from your air by passing it over coils filled with refrigerants. In a window unit, a compressor moves the refrigerants through the coils, and fans move the air where it needs to go. In energy-saver mode, the compressor shuts off when, as you say, the desired temperature is achieved. The fans turn off too, but they come back on every so often to check in on you, see whether you need any lemonade, etc. When the fans find air that’s too warm, the compressor comes back on. So what you might be hearing every few minutes is the fans turning on and off, while the compressor is laying low.

According to the EPA, this mode uses about 10 percent less energy than the conventional “cool”mode, so that saves you a little money. I’ve had trouble getting more detail than that from several manufacturers I contacted, none of whom could provide specifics on power savings by my deadline. Some even seemed a bit stymied. Generally I can count on geeky online chats to yield at least one insanely detailed calculation in these situations, but here, too, they all just shrug and say, “Eh, doesn’t save you much.” I guess even the geeks are feeling hot.

If I do get more details, I’ll add an update here. Meanwhile, I can tell you that choosing a unit that’s the right size for your dwelling is the real key to efficiency and comfort. Make sure your home is properly insulated, too — that will keep you comfy in summer and when the now-unimaginable chill of winter descends.