Can I save money by getting a window air conditioner and using my central air less?
August 8, 2012 11:18 AM   Subscribe

Will turning up the thermostat on my central air and buying a window AC unit for the one room I use the most make a significant difference in my electric bill?

This is hopefully a simple question, but I'm not confident that my assumptions are correct.

I recently bought a house, it's a 2-story that's about 1750 square feet, located in Michigan USA. The house has a central air unit that looks very very old, but still functions well. The thermostat in the house is on the ground floor, and I've kept it set to about 70 degrees Fahrenheit when I'm home, since the upper floor tends to be at least 7-8 degrees warmer than the ground floor, and I spend more of my time upstairs. I am currently the only person living here. The outside temperatures during July were generally in the 90s, occasionally in the 100s.

I knew that cooling a 1750 square foot house was going to cost more than cooling my old 1-bedroom apartment. But I was surprised by just how much more. My electric bill last month was $260, twice the highest amount I ever paid while living in an apartment! I'm guessing that the AC is the main difference between the bills at these two places, since everything else I own is more or less the same, and is getting the same amount of use (same computers and TVs, comparable refrigerator, etc).

I'm trying to figure out how to save money on my electric bill while at the same time cooling my upstairs bedroom/office more effectively. I'm thinking I could put a window air conditioner in my bedroom upstairs, and then turn the downstairs thermostat up to 78 or so.

My question is though: Would this actually save me any money? I'd still be using the central air to cool the rest of the house, just not as much. Would the difference be enough to justify the cost of the window unit? In particular, I was looking at purchasing the Frigidaire FRA086AT7, which costs $200. To justify that expense in the short-term, I'd hope to save at least $40/month on my electric bill by using this strategy. Is that realistic? I really don't have a good feel for that the impact on my energy usage would be. Especially since I don't intend to turn the central air off altogether.

If this wouldn't be an effective money-saver, do you have any other suggestions?

If you have any hard data, or you've done something like this and can tell me about your experience, I'd love to hear it! Thanks MeFites.
posted by Vorteks to Home & Garden (14 answers total) 5 users marked this as a favorite
Before you spend the money and go through the hassle, try this first. Close the vents downstairs and open the vents upstairs. Heat rises, so you want the cool air upstairs, it will naturally be cooler downstairs.

We bought a programmable thermostat and that seemed to help a lot.

We do this when the seasons change, reversing for winter, and we've saved about $100 per month on our bill. We also bumped the temp up a bit, from 72 to 74.

I think you'll find the house more comfortable over all.
posted by Ruthless Bunny at 11:22 AM on August 8, 2012 [3 favorites]

Agreeing with teh above. In addition, we use two of the Booster vent fans and we are able to get it MUCH cooler upstairs compared to downstairs. Increasing the flow upstairs (or encouraging it?) seems to be the key.

We have all downstairs vents covered, 2 x boosters on the top floor, 1 x booster on the middle floor (3 story townhouse) and we are able to get the middle floor and the upper floor at the same temperature like that (thermostat is on middle floor). They switch on and off with air flow if you get the right ones and seem pretty basic but effective. I wish we could put a vertical booster fan in the ductwork, but we're renting, so that option is out.

I've kept it set to about 70 degrees Fahrenheit when I'm home, since the upper floor tends to be at least 7-8 degrees warmer than the ground floor, and I spend more of my time upstairs.

Alternatively (or additionally), move downstairs during the day.
posted by Brockles at 11:27 AM on August 8, 2012 [1 favorite]

Response by poster: Thanks for the answers thus far.

I still wonder if anyone knows whether my specific idea would work or not?

@Ruthless Bunny - I'll try again to adjust the vents. I've monkeyed with the upstairs vents but it hadn't occurred to me to close the downstairs ones. I may have to replaces some of the registers since it seems some of them don't have a way to close them.

@Brockles - The booster vent fans are an interesting idea. I'll look into those. Thanks.

I actually have moved downstairs for a time, but I'm trying to find a long-term solution that doesn't require me to be downstairs, since (1) it's harder to keep the house company-ready when I've got all my stuff (including bedding) downstairs all the time, and (2) although I live alone now, I will have family moving in with me in the near future, and my personal space will be limited to my upstairs bedroom at that time.
posted by Vorteks at 11:55 AM on August 8, 2012

It sounds like it might help, and I'd be interested to know the results if you do.

Some other tips:

* Close the doors and vents in rooms you aren't using much.

* Do you have ceiling fans? I've found they help a lot more than I ever expected. We keep our central A/C set a little higher and run the fans in addition.

* I don't know about Michigan, but in many states, you can now shop around for your electric supplier. You're still billed by the normal utility as they take care of the "last mile" of service. Right now I'm paying 7.9c/kWh for supply whereas the normal utility charges about 10.5c/kWh for supply. It's not much of a savings but it does help, particularly in the winter as our A/C system is also a heat pump.

* Consider installing a whole house fan in the attic.
posted by tckma at 11:56 AM on August 8, 2012

I still wonder if anyone knows whether my specific idea would work or not?

It will definitely work, but its whether having both AC systems working will be cheaper. In the booster fan example, you are using just electric fans to introduce more cold air into the upstairs compared to the base air distribution.

With an extra unit, you're cooling extra air that is pumped into the upstairs. My personal (baseless, most likely) feeling is that unless you isolate that upstairs room entirely (doors closed, etc, small AC inside it) then the top AC will just work harder to cool the air of the whole house - cold air will 'fall down the stairs' hot air will rise up into, and need cooling in, your bedroom. This will mean that the smaller AC system is effectively taking the lion's share of the cooling. I can't imagine that'd be more cost efficient.

It may work, but without isolation, the cold air will still fall and your base issue is to get that cold air to stay upstairs, which brings you back to the booster fan argument.
posted by Brockles at 12:18 PM on August 8, 2012

Best answer: You just bought the house in the last few months? One key piece of data is how much your electric bill is for a month that you're not running the central AC at all. If you can last for August and September as-is, or testing the vent-closing ideas, you can decide in the spring whether to put your window-unit plan into motion.

(July electric bill with AC)-(October electric bill with no AC or heat)=approximate amount the central AC is costing you. Maybe check a few months to verify this data.
Maybe by looking at other months (September) you'll get an idea of what the bill's like when the AC is working less hard, say halfway in between those values?

Websitessuggest that a window unit AC will be chilling 25%-50% of the time it's on at moderate temps (80-90) up to near-constant when temps outside are >95deg. If you leave it on 24 hours a day, and nighttime temps are 80ish, call it 12 hours active chilling per day.
The unit you linked is 8000 BTU with a EER of 10.8, or (divide) 740 watts.
740W * 12 hours *30 days = 266 kWh/month
266kWh*$0.13/kWh electric charge = $35/month.
This seems like not very much, but remember that depending on how you use it (is the door to the room closed?) and how hot it is, this could basically double (running 24hours/day). I also don't know the rules for how Energy Star sets the EER - is that on high/med/low? If the number is for med and you run on high, of course things change.

Once you've calculated some numbers you can compare, and you can make your decision as to whether you want to give this a try next spring.
posted by aimedwander at 12:27 PM on August 8, 2012 [1 favorite]

A vent fan in the attic to remove hot air is a good idea. Does your attic have insulation up to current standards? Attic insulation might save you winter heat bills also.
posted by Cranberry at 12:27 PM on August 8, 2012

The booster vent fans look like a good idea, but a simpler and cheaper approach is to use magnetic vent covers to completely or mostly cover the vents in the rooms you want to be warmer than they are currently, then leave them open in the room you want cool. Then turn up the thermostat. You will have to fiddle with how much of the various vents are covered, but it's a very low-invetment approach.
posted by Mr.Know-it-some at 12:52 PM on August 8, 2012 [1 favorite]

Response by poster: @Mr.Know-it-some Wow, about $5 for three? I'll be trying magnetic vent covers for sure. That's a very low-risk investment. Thanks!
posted by Vorteks at 1:20 PM on August 8, 2012 [1 favorite]

Best answer: We use a window unit to cool an upstairs room whose door is always kept closed, and it is very effective. It allows us to get by without AC for the rest of the house almost all the time. If you're able to keep the door to your room closed, you might consider this strategy. The rest of the house will be warm, for sure, but it's manageable if you are spending most of your time in the cooler room.

Also be sure you are doing the obvious low-tech things: fully covering all windows during the daylight hours (blinds plus curtains, or as much opacity as you can get), making use of cool nights plus window fans as much as possible to cool the house overnight without using AC, attic fan if possible.
posted by LobsterMitten at 2:27 PM on August 8, 2012

Best answer: It's worth bearing in mind that any calculation you make here has to include the cost of buying the window unit, which is not a particularly cheap object. You might also take into account the inconvenience of having to install/uninstall said window unit when the seasons change, and of not being able to use that window when it's installed there. Also, those accordion-style wings on a window unit don't really provide any insulation to speak of -- they're only barely better than just having an open hole in the window for cold air to leak out of.

Others have made suggestions for other things you can do that would probably be cheaper and/or more satisfactory solutions to your problem. Even improving attic insulation and ventilation is not necessarily more expensive than buying a window A/C, would help keep bills down in the winter as well when you have to heat the house (rather than only in the summer) and would be more elegant, quieter, and would improve the value of your home. Do try those vent covers first though, they might be all you need.
posted by Scientist at 2:28 PM on August 8, 2012

We live in the sultry, hot, humid coastal south. We use programmable thermostats and keep our house @ 78 when we're home, up to 85 when we're at work. That's fine for me, but my husband likes it cool, and his office heats up *a lot* with all his computers running. We spent about $125 for an efficient window unit (eer rating of 11). It's not on all the time, just when he's using the room & needs it. The remote control makes it super convenient for him to use while he's sitting at his desk.

I can't say how much cheaper our electricity has been, because we implemented this solution our first summer in the house. But it definitely works for our purposes and allows me to be strict about the thermostat temperature. And our house is about 1900 SF, and we paid about $116 for 1113 KWH last month. (that number is useless to you, like comparing apples to oranges, but it's something, maybe? ). Consumer Reports helped us by ranking the window unit AC's by efficiency & price for the SF we were looking to cool.
posted by Kronur at 1:25 AM on August 9, 2012

Another thing that may help, is if you can point that window unit to blow the cool air directly where you are sitting. You wouldn't have to have the whole room be as cool, as long as you are feeling the cool breeze directly.

What you want to do should work, although if you are going to do this and are serious about saving money, make sure you completely close up the room you are in so that no air can get in or out (block the vents when you have the window unit running), and make extra sure that the window unit seals in the window. You might also want to set your house AC a bit higher (80 or 82) during the hours that you are going to be working in your closed off space, there's no reason to get the whole house down too far if you are primarily going to be in a different room. A few degrees can make a significant difference in your electricity bill.

Also, definitely close the registers in any rooms you won't be using, or in rooms that feel cooler than others. In our house, even with the downstairs registers closed, it's always a little cooler than upstairs when the AC is running. If I leave the downstairs vents open, it's at least 10 degrees cooler.
posted by markblasco at 8:12 AM on August 9, 2012

The thermostat in the house is on the ground floor...

You can also fix this. You'd need to extend the wires (or hire someone to do so) to a new hole in your wall upstairs, but if you then close the door to "your" area (=nice and cold) and cover the other vents (=liveable temp) you'd achieve both your goals.
posted by teremala at 9:37 AM on August 9, 2012

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