Blinded by Science blog #5: Should I turn the AC or heat off in my home when I'm away at work during the day? Do I save any energy and $ on cold and hot days by doing this?

Well everyone, it’s that time of the year again.  The birds are singing, the sun is shining, and copious amounts of fluid are leaking from my nose and eyes (allergies suck!).  That’s right; spring has arrived (and hopefully gets to stay for awhile, allergies and all).  In honor of the changing of the seasons, I’m going to answer a question from a friend of mine that is pretty topical (plus it could save you money and everyone likes that).  Chris H. asks, “Should I turn the AC or heat off in my home when I’m away at work during the day?  Do I save any energy and $ on cold and hot days by doing this?”

Well Chris, to answer your question, we’re going to have to look to the physics of heat/energy transfer which is a topic I haven’t thought about since I took AP Physics in high school (Sorry Mr. Thompson!)*.

Let’s imagine a simple scenario.  You have a house you wish to be heated to 75°F from a starting temperature of 35°F and the outside temperature is 35°F.  Your heater turns on and begins to heat the house (depending on the heater, it will probably use natural gas or electricity).  The temperature in the house eventually reaches 75°F and the thermostat sends a signal for the heater to shut off.  And now, because no place has perfect insulation, heat begins to be lost to the outside (heat is transferred from high temperature to low temperature).  This causes the temperature in the house to begin to drop.  When it does, the thermostat sends a signal for the heater to turn back on and bring the temperature back up to the set point.  When the temperature again reaches 75°F, the heater is shut off and heat again begins to be lost to the outside**.  This series of events will continue as long as nothing changes (like the outside temperature reaching 75°F or you turning off the heater or you changing the set point).

This pattern uses up a lot of energy (and when you are paying the heating bills, a lot of money) and it makes sense when you are actually in the house (let’s assume the temperature you set the thermostat at is the lowest temperature you feel comfortable at since you could otherwise save some money by just lowering the set point).  But the question asks about when you are not in the house and here is where physics comes in.  Because the amount of heat lost to the outside is a function of the difference in temperature between the inside and outside, it actually requires more energy to keep the temperature at 75°F than it would at, say, 65°F and you would actually have a slower heat loss at the lower temperature (so says the Department of Energy).  I realize this is counter-intuitive, but it is true.  In fact, according to that link, setting the thermostat 10-15° back for 8 hours a day (like when you are at work or even sleeping) can translate to a 5-15% savings on your heating bill.  This is why using a programmable thermostat is such a good idea.  You can set it to automatically lower the temperature during the times you are away and don’t need the inside to be so warm and to raise the temperature again when you get back (Pro-tip: have the thermostat start raising the temperature 30 minutes or so before you get back from work or wake up in the morning.  That way the house won’t be as cold when you get home/wake up).

And this takes us back to the specifics of the original question.  If lowering the temperature set point by 10-15° can save a good amount of money, what about just turning off the heater when you are at work?  Yep, just completely turning it off would save you more money . . . with an important caveat.  In the winter, if you let the house get too cold, your water pipes could freeze.  Believe me; you don’t want that to happen.

So it would make the most sense to just set the temperature lower than normal instead of turning off the heater altogether.

“But wait!” you say, “Doesn’t setting the thermostat lower just mean the heater has to work harder to get back up to the temperature I want when I get home?”

“Oh you,” I say as I chuckle a little and pat you on the head, “A heater doesn’t work ‘harder’ if there is a greater difference in starting temperature and set temperature.  It just works longer.”  This is the same reason why jacking up the thermostat doesn’t actually make the room heat up any faster (a fact, I must admit, I was unaware of before researching for this blog post).

In fact, today’s heaters are more energy efficient if they work for longer periods of time at once.  This means it will actually take less energy to raise the temperature a large amount all at once as opposed to keeping the temperature at the set point all day.  And since energy in this case translates to money you spend, it also means you spend less money.

And just so we’re all on the same page, everything I said also holds true for lowering the temperature in the summer with the A/C.

So yes, you can definitely save money by turning off the A/C or heater (Well, as I said, just turn down the heater in the winter.  Don’t turn it all the way off) when you are not at home.

*I asked my friend Jeff M. to look over my answer since it has been awhile and he studies physics.  He said it “looks fine” which makes me feel better since that means I didn’t just make it all up.

**Just to be clear, in real life heat would be lost the entire time, not just when the heater is off.  But the amount of heat the heater is generating is greater than the amount of heat lost to the outside which is why the temperature in the room increases.