| Author |
Message |
    
jeffl
Supporter
Username: Jeffl
Post Number: 1714
Registered: 8-2001
| | Posted on Tuesday, May 30, 2006 - 10:51 pm: | 
|
Which uses less energy, i.e. costs less to run?:
1. Turning off your a/c all day long and then putting it back on at night when you get home...
2. Keeping the thermostat at 78 all day and then turning it down at night?
I'm not interested in which will make you more comfortable, only which costs the least to do. |
Scully
Citizen
Username: Scully
Post Number: 564
Registered: 8-2005
| | Posted on Wednesday, May 31, 2006 - 1:26 am: |
|
I'd guess #1 but there are a lot of variables here. How low would 'down' be at night? The humidity would factor in too. Plus weather shifts (rain) while you're out.
FWIW in my house I would definately not leave it running all day in an empty house. |
us2inFL
Citizen
Username: Us2innj
Post Number: 1447
Registered: 5-2001
| | Posted on Wednesday, May 31, 2006 - 5:38 am: |
|
If leaving it off results in your house being over 84 degrees, then it will cost you more to get the house back to cool (78 degree range.) If your house goes from the 78 degree cool to 80-82 with the A/C off, then it would probably be cheaper to shut it down.
The most important variable is how long does it take to cool down the house once you've turned it on. If you have to wait around for an hour for the house to cool, then you have to determine what price that is worth.
A good move would be to get a programmable thermostat that begins cooling 30 minutes before you return home.
We have a dual system Carrier Genesis Puror system here in Florida. Two huge compressors outside and two air handlers and programmable thermo/humidistats inside. The units have very high EER/SEER ratings and we go from 82 degress to 78 degrees in about 15 mins. When we're going to be out for more than an hour or so, the thermostat goes into "away" mode and keeps the temp reasonable while more closely monitoring the humidity. The A/C will come on even if the temp is low but the humidity goes above 60%. |
shoshannah
Citizen
Username: Shoshannah
Post Number: 1267
Registered: 7-2002
| | Posted on Wednesday, May 31, 2006 - 1:48 pm: |
|
According to my a/c guy, #2 costs less unless you are going away for more than two days. |
Tom Reingold
Supporter
Username: Noglider
Post Number: 14541
Registered: 1-2003
| | Posted on Wednesday, May 31, 2006 - 1:51 pm: |
|
jeffl, are you using central a/c? I think #1 would save money if you're using window units.
If you have central a/c, I like us2inFL's idea of setting your thermostat to 84 or maybe even 86 if you'll be gone all day. I also agree that you should turn it off entirely if you're leaving the house overnight.
|
LilLB
Citizen
Username: Lillb
Post Number: 1733
Registered: 10-2002
| | Posted on Wednesday, May 31, 2006 - 1:56 pm: |
|
I'm curious - what temperature do you set your CAC at during the day (while you're gone all day) if you have pets? I know my cats probably just sit around all day anyway while we're at work, so we can raise the temp a few degrees, but is there a recommendation on what to set it at (e.g., 5 degrees warmer than you normally would)? |
us2inFL
Citizen
Username: Us2innj
Post Number: 1448
Registered: 5-2001
| | Posted on Wednesday, May 31, 2006 - 2:29 pm: |
|
LilLB, without taking humidity into account, I would think an interior of a house at anything above 84 would be a bit on the warm side. Before the CAC in Maplewood, our living room would get up to 87/88 and it was plenty uncomfortable.
Down here we tend to run it around 78 - 80 with a relative humidity of about 55%. |
jeffl
Supporter
Username: Jeffl
Post Number: 1715
Registered: 8-2001
| | Posted on Wednesday, May 31, 2006 - 2:35 pm: |
|
Here's the answer I got from my friend who is an environmental engineer:
Turning it off when you’re not there saves energy.
Just think of it this way: If you were leaving your house for the whole summer, would you use more energy keeping it at 75F or turning the AC off? The same basic logic would apply to a shorter setback period.
Heat gain is proportional to temperature and moisture differential between indoors and outdoors. If there’s a bigger differential (i.e. its cooler and drier inside than outside), you’re losing more BTUs of cooling that need to be replenished by your AC. When you turn off the AC, there’s less (or zero) differential, so you’re losing less BTUs.
|
LilLB
Citizen
Username: Lillb
Post Number: 1734
Registered: 10-2002
| | Posted on Wednesday, May 31, 2006 - 3:44 pm: |
|
84? Anything above 72 seems too warm for me...and I don't have fur  |
Tom Reingold
Supporter
Username: Noglider
Post Number: 14550
Registered: 1-2003
| | Posted on Wednesday, May 31, 2006 - 3:46 pm: |
|
Would you really cool a house just for a cat? Anyway, our cat seems to love the heat. The hotter, the better, and she has lots of fur.
|
Bob K
Supporter
Username: Bobk
Post Number: 11677
Registered: 5-2001
| | Posted on Wednesday, May 31, 2006 - 3:51 pm: |
|
Jeffl, sometimes I just love engineers logic. If you are away all summer you just have to lower the temperature once when you return. If you turn off the A/C everyday you have to lower the temperature everyday.
I don't have the math to figure out which way is better, but the example seems flawed to me.
Did I ever tell you the one about the engineer who figured by putting two logs on a fire it would burn twice as long?  |
mjc
Citizen
Username: Mjc
Post Number: 1151
Registered: 10-2004
| | Posted on Wednesday, May 31, 2006 - 3:56 pm: |
|
Cats and hot weather: our old guy did fine in our no-A/C Southern California home. In the hot weather (100+ outside, probably 90 inside) he just didn't move much in the daytime, and tried to find tile to sleep on. Temps in the 80s are not going to be any challenge to a cat IMO, unless maybe if it has respiratory or other health problems. |
Zoesky1
Citizen
Username: Zoesky1
Post Number: 1534
Registered: 6-2003
| | Posted on Wednesday, May 31, 2006 - 3:58 pm: |
|
I turn my central air to 74 at night and 72 in the daytime. We are all cold-lovers. I cannot stand it inside much above 76. Plus we like to use duvets all year long. |
us2inFL
Citizen
Username: Us2innj
Post Number: 1449
Registered: 5-2001
| | Posted on Wednesday, May 31, 2006 - 4:47 pm: |
|
I'm glad I don't get your electric bill. The Florida P&L we get is enough for me, especially in the summertime.
When I worked up there at ABC, they use to keep the studios and control rooms at an artic 66 to 68. Anyone who has been in the audience for the Letterman show can attest to that.
Down here, we find 77 to 78 and a low humidity setting works well. |
jeffl
Supporter
Username: Jeffl
Post Number: 1716
Registered: 8-2001
| | Posted on Wednesday, May 31, 2006 - 4:50 pm: |
|
Bob, this engineer specializes in this area, i.e. environmental issues and saving money on energy costs. I believe he knows what he's talking about. |
MittenReckitt
Citizen
Username: Mittenreckitt
Post Number: 23
Registered: 4-2006
| | Posted on Wednesday, May 31, 2006 - 5:22 pm: |
|
I have a two zone - upstairs/downstairs.
Wife works from home so we keep it at 72 during the day downstairs, where she works. What is the most efficient temp to keep upstairs at during the day? |
Tom Reingold
Supporter
Username: Noglider
Post Number: 14555
Registered: 1-2003
| | Posted on Wednesday, May 31, 2006 - 5:52 pm: |
|
bobk, I think you just posted an absurd statement, attributed it to an engineer, and thereby implied that engineers are wrong about everything. Please explain how I'm reading you wrong or why you said something so absurd. Turning off the A/C does save energy. Discreting engineers doesn't change that fact.
|
Bob K
Supporter
Username: Bobk
Post Number: 11679
Registered: 5-2001
| | Posted on Wednesday, May 31, 2006 - 6:15 pm: |
|
Tom and Jeff,
I spent ten years working with a bunch of graduate engineers and from that experience (and a few others) I developed a love hate relationship with that profession.
The way I see it if you leave for the summer and turn the AC off when you return you have to use X kilowatts of electricity to cool your house. If you turn off the AC every workday for the summer you are going to have to cool the house sixty times or so when you return, using X kilowatts (or close to it) everyday. Possibly I misunderstood Jeff's post, or something was lost in the translation.
The story about the firewood is a true one. When LittleK was about ten we did an overnight Cub Scout trip at Camp Conklin in the late fall. The cabin had one of those 55 gallon drums turned into a woodstove. One of the Dads an engineer used the logic I outlined earlier. At about 1:00am everybody woke up in a ninty degree cabin. Yeah, the Dad was an engineer, a great Dad and a great guy, but, as I said before, I have a love hate relationship with engineers.
Honest, some of my best friends are engineers. |
Tom Reingold
Supporter
Username: Noglider
Post Number: 14556
Registered: 1-2003
| | Posted on Wednesday, May 31, 2006 - 11:05 pm: |
|
Sure, and they'll try to produce a baby in one month by impregnating nine women. Yuk yuk.
But please clarify what you're saying. You say leave for the summer and turn the AC off. How does that consume X kilowatts? I think it consumes zero. When you return at the end of the summer, the house is 90F inside. It consumes Y kilowatts to bring it down to 76.
Your neighbor, however, leaves for the summer and sets the thermostat for 80. The house warms to 82, the AC cools it to 78, repeatedly over the summer. Are you saying it would consume Y kilowatts? That's crazy. It will consume Y+Z kilowatts, with Z being very high.
Imagine you have a muddy slope, and you perch a boulder at the top. If you let it slide over a day, it falls to a hard surface at the bottom. Rather than let it slide, you pick it up every hour and put it at the top of the slope. Then you leave it for a week and find it at the bottom. You heave it up. It's a lot of work, but it's LESS WORK OVERALL. One key reason is the overhead of the work, not the work itself.
|
Scully
Citizen
Username: Scully
Post Number: 566
Registered: 8-2005
| | Posted on Wednesday, May 31, 2006 - 11:15 pm: |
|
Good grief! I think US2inFL had the best answer:
'A good move would be to get a programmable thermostat that begins cooling 30 minutes before you return home.' |
Smarty Jones
Citizen
Username: Birdstone
Post Number: 689
Registered: 10-2005
| | Posted on Wednesday, May 31, 2006 - 11:37 pm: |
|
The programed Thermastat doesn't address the problem question, it simply makes the house comfortable, which was not relevant.
Bobk is right, the engineer is basically solving the wrong equations, as is demonstrated by his erroneous example. (The example being- Which uses more power, an A/C unit in the OFF position, or one running for 3 months? Of course there's an easy answer to that question).
I put this very dilemna to the test with my own walet the summer of 1999 living in a huge loft with 20' ceilings. The first 2 months I ran the AC only as needed (when I was home on weekends, and nights that were too hot to sleep). Avg electric bill was $1,000 a month for June/July. After that, I felt I had nothing to lose but set it at 74 degrees and leave it there. My bill was cut in half for Aug/Sept.
After seeing it myself, I stopped trying to challenge why it does or doesn't work. It just works.
PS. it needs to be one constant temp....even small ups and downs in temp negate the benefits of the approach. |
jeffl
Supporter
Username: Jeffl
Post Number: 1719
Registered: 8-2001
| | Posted on Thursday, June 1, 2006 - 8:22 am: |
|
I guess this argument is not going to get settled here. I'll keep believing that running an a/c for a shorter period of time is cheaper in the long run. I asked my engineer friend if he wanted to chime in. He declined stating that debunking these kinds of myths is what he has to do all day.
He was intrigued, however, by Tom R's concept of an engineer making love to 9 women at a time to produce a baby in a month. He is pretty confident that no engineer has ever done that in his lifetime, much less in a month. |
LilLB
Citizen
Username: Lillb
Post Number: 1736
Registered: 10-2002
| | Posted on Thursday, June 1, 2006 - 8:35 am: |
|
My cats actually rule our house... we just live there.  I guess I should just ask them what that want....but I stepped on the tail of one of them this morning so she's not speaking to me. |
Closed: New threads not accepted on this page