Too long in between posts here. My apologies. That soupy Northeastern summer is upon us here in Philly. The houses I visit that are lucky enough to have air conditioners are running them unapologetically, and at full force. And if you want to know a dirty little secret, so am I. The "energy man" runs his A/C a lot. A LOT. No shame in my game. It's a luxury I'm willing to pay for...to a point.
One of the questions I'm asked nearly every day at work is about how people should be using their air conditioners. Warmer is always better when you're sitting right there. But people always want to know if they save more money by leaving the AC running all day, or turning it off when they leave and back on when they come home. There's a lot of conflicting information about this. A LOT. Sorting through all of it took me the better part of two days. If you talk to an AC repairman, they will say leave it on, because they want you to wear it out (and then pay them to repair it). But if you talk to the engineers who design HVAC systems, they'll always do what I do and turn it off. The reasons require us to do a little bit of SCIENCE!™, but I'll try to simplify things.
Nature is always trying to balance itself out. Not trying to get all hippie drum circle, the rules of this universe just work that way. Things like air, water, pressure, and heat all want to even themselves out as much as possible. In order to do this, they need both a way to move and a reason to move. If there's not a hole, wind can't blow through it. If it's 70°F inside and 70°F outside, there's no reason for the heat to try and get through your walls. The bigger the hole or the bigger the difference, the faster things will move. So, if it's 70°F inside, the heat will come in faster on a 100°F day than it will on a 75°F day.
For this example, let's say your air conditioner has to run for three minutes to bring the temperature down 1°F. You keep it set to 70°F all day, and today, it's 90°F out, so that heat really wants to get in. You set the thermostat and leave for work. The AC runs for three minutes to bring the house down to 70°F, and then five minutes later, it's back up to 71°F, so the AC kicks on again. Three minutes on, five minutes off. All. Freaking. Day. 24 minutes out of every hour. By the time you get home from work, you've removed 60°F of heat from the house, and the air conditioner ran for more than three hours. That works out to $42 a month for an average central air system, not even counting weekends or what you're spending when you're actually home.. That's a lot of money to spend with no one getting anything for it. A LOT.
Now instead, let's say you turn the AC off when you leave for work that day. The house starts at 70°F, and it heats up. By the time you get home, it will probably be right around the 90°F that is is outside. Feels kind of gross. Hope you left plenty of water out for the cats. You switch the AC on the second you get in the door. Remember, it runs three minutes to take every 1°F out of the air, so that means it's gonna run for an hour straight, and remove 20°F from the house all at once. The math gets a little easier here: one hour is much less than three hours, and 20°F is much less than 60°F. And that saves you about $30 a month.
With the AC running, the heat has a reason to keep coming in, so the AC has to keep removing it; with the AC off, it has much less of a reason, so it is removing less over the course of the entire day. The missus came up with a great analogy for this: Running the air conditioner is like grocery shopping. It takes less "work" to bring them in from the car one item at a time, but you have to do it over and over again. Keeping the groceries in the bag and bringing them in all at once is far more efficient in the long run.
That's your physics lesson for the day, all. Time to go taste test some cookies in the kitchen. Of course, I'll be turning the air conditioner in the living room off when I go.
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