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haroldo

join:2004-01-16
united state
kudos:1

Central air conditioner on/off? Old wives tale?

A few of the AC repairman I've apoken to say you could leave the central AC on all day as it doesn't waste electricity as compared to turning the temperature up if you're not home.
I took math in school (did well) and figure that not using a major appliance for eight hours saves more energy than using it for ten minutes to cool the house when you return home...(it doesn't even take five minutes for me to feel cool when I return home and turn the temperature down on the hottest days)
is there anything to that old wives tale?
Thanks!



A non

@151.190.0.x

I leave mine on. If I turn it off, it will run continuously from 4 PM to 9 PM to get the house down to 75 degrees again.



Draiman
Let me see those devil horns in the sky

join:2012-06-01
Kill Devil Hills, NC
Reviews:
·Verizon FiOS
reply to haroldo

The best way to figure that is to test both yourself. Pick a time to check your meter before you go to work. Leave the A/C on day1 and check it at a specific time when you get home. On day2 turn off the A/C and check the meter at the same start/end times as the previous day. Simple math will tell you what you want to know. What it won't tell you is what happens when the humidity raises and falls all the time if you choose to turn off the A/C. I prefer to change the temperature before I leave to like 76 then when I get home down to 73. We also have pets so we can't just turn the A/C off. I don't like working 8 hours then spending 45-60 minutes in traffic on the way home only to come into a 65% humidity 80+ degree house either. My life is too short to deal with that over less then a meal out a month.



tschmidt
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join:2000-11-12
Milford, NH
kudos:9
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reply to haroldo

This topic has been beaten to death here.

If the temperature inside the house is different then outside then heat flows between the two. The greater the difference the faster the flow.

Turning off the AC or heat means for that period of time you are using no energy. When the AC/Heat is turned back on it has to work longer to get the higher/lower inside temp back to comfort zone. However that takes less energy then maintaining the set point temperature during that period.

Do a thought experiment, if you were going to be gone a week do you think you would save energy by turning it off or changing the set point compared to leaving it at the normal temperature?

/tom



Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom

said by tschmidt:

This topic has been beaten to death here.

If the temperature inside the house is different then outside then heat flows between the two. The greater the difference the faster the flow.

Turning off the AC or heat means for that period of time you are using no energy. When the AC/Heat is turned back on it has to work longer to get the higher/lower inside temp back to comfort zone. However that takes less energy then maintaining the set point temperature during that period.

Do a thought experiment, if you were going to be gone a week do you think you would save energy by turning it off or changing the set point compared to leaving it at the normal temperature?

/tom

In winter unless you want to freeze for a long time while the heat pump restores the temperature to set point the heat strips will come on negating any savings from lowering the thermostat and more than likely increasing the cost.

I never change mine winter or summer unless we're going to be away for extended periods like a vacation. Lifelong experience in instrumentation and process control business gives me a different perspective on issues like this. The savings (if any) aren't worth the effort and discomfort.


cowboyro
Premium
join:2000-10-11
Shelton, CT
Reviews:
·AT&T U-Verse

1 recommendation

said by Jack_in_VA:

In winter unless you want to freeze for a long time while the heat pump restores the temperature to set point the heat strips will come on negating any savings from lowering the thermostat and more than likely increasing the cost.

I never change mine winter or summer unless we're going to be away for extended periods like a vacation. Lifelong experience in instrumentation and process control business gives me a different perspective on issues like this. The savings (if any) aren't worth the effort and discomfort.

A central A/C has a single source of energy so it will cost pretty much the same regardless of the difference between actual temperature and set point. The heat pump is a bad example for this.
Decent thermostats allow for automatic start earlier so that the set point is reached at the desired time.
I could dump a couple of years worth of hard data which shows without any trace of doubt that keeping the unit off during the day saves energy vs keeping it on. Energy, not necessarily money. Rates may be higher at certain times.


Draiman
Let me see those devil horns in the sky

join:2012-06-01
Kill Devil Hills, NC
Reviews:
·Verizon FiOS

said by cowboyro:

said by Jack_in_VA:

In winter unless you want to freeze for a long time while the heat pump restores the temperature to set point the heat strips will come on negating any savings from lowering the thermostat and more than likely increasing the cost.

I never change mine winter or summer unless we're going to be away for extended periods like a vacation. Lifelong experience in instrumentation and process control business gives me a different perspective on issues like this. The savings (if any) aren't worth the effort and discomfort.

A central A/C has a single source of energy so it will cost pretty much the same regardless of the difference between actual temperature and set point. The heat pump is a bad example for this.
Decent thermostats allow for automatic start earlier so that the set point is reached at the desired time.
I could dump a couple of years worth of hard data which shows without any trace of doubt that keeping the unit off during the day saves energy vs keeping it on. Energy, not necessarily money. Rates may be higher at certain times.

Does that hard data take into account what the humidity can do to stuff in your house? Your comfort level? Life expectancy related to the extra stress a hot house can cause? A pissed off significant other or kids? Stall air on allergies? Way to many things to take into account for hard data to give a realistic answer to the OP's question. It's more of a personal choice given then situation and desired outcome.


rfhar
The World Sport, Played In Every Country
Premium
join:2001-03-26
Buicktown,Mi
Reviews:
·Power-Net Intern..
reply to haroldo

We do shut ours off most nights but I have found that if I shut it off during the day and it gets warm the AC will run continuously for hours to cool of the house.
I have a rather tight house with R50 in the ceiling and R20 in the walls in mid Michigan.



cdru
Go Colts
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join:2003-05-14
Fort Wayne, IN
kudos:7

2 recommendations

reply to tschmidt

said by tschmidt:

This topic has been beaten to death here.

Oh come on. We haven't beaten it to death this week. Maybe we can add in smart meters, government subsidized devices, and building/electrical codes into the mix to get a real discussion going.


dosdoxies
Premium
join:2004-12-15
Wallingford, PA

1 recommendation

said by cdru:

said by tschmidt:

This topic has been beaten to death here.

Oh come on. We haven't beaten it to death this week. Maybe we can add in smart meters, government subsidized devices, and building/electrical codes into the mix to get a real discussion going.

Don't forget whether to put electrical tape on wire nuts or not.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to cowboyro

said by cowboyro:

example for this.
Decent thermostats allow for automatic start earlier so that the set point is reached at the desired time.
I could dump a couple of years worth of hard data which shows without any trace of doubt that keeping the unit off during the day saves energy vs keeping it on. Energy, not necessarily money. Rates may be higher at certain times.

Yes and when it's cold the recovery has to start early in the time that's supposed to be saving energy in order to keep the strips off if it's able to do it without strips. When it's cold a heat pump relies on strips to supplement the heat that it can't provide. Turning the entire unit off in most cases would cost far more than just keeping it running.

Example in my case the heatpump pulls 1.4 kW running which is about 14 cents/hour. 8 hours would be $1.12. Heat strips 15kW cost about $1.58/hr to operate.

I would be interested in seeing your hard verifiable data.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to cdru

said by cdru:

said by tschmidt:

This topic has been beaten to death here.

Oh come on. We haven't beaten it to death this week. Maybe we can add in smart meters, government subsidized devices, and building/electrical codes into the mix to get a real discussion going.

OH COME ON you must be reading the posts

Given that people are posting to this thread means that some are interested. You realize you don't have to read them. Does this mean that a couple of people have the power to determine a subject has been "beaten to death" therefore choking it off?


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to dosdoxies

said by dosdoxies:

said by cdru:

said by tschmidt:

This topic has been beaten to death here.

Oh come on. We haven't beaten it to death this week. Maybe we can add in smart meters, government subsidized devices, and building/electrical codes into the mix to get a real discussion going.

Don't forget whether to put electrical tape on wire nuts or not.

N/M


cdru
Go Colts
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join:2003-05-14
Fort Wayne, IN
kudos:7

3 recommendations

reply to Jack_in_VA

said by Jack_in_VA:

Given that people are posting to this thread means that some are interested. You realize you don't have to read them. Does this mean that a couple of people have the power to determine a subject has been "beaten to death" therefore choking it off?

Lighten up Jack. It was in posted in humor.


cdru
Go Colts
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join:2003-05-14
Fort Wayne, IN
kudos:7

1 recommendation

reply to haroldo

said by haroldo:

I took math in school (did well) and figure that not using a major appliance for eight hours saves more energy than using it for ten minutes to cool the house when you return home...

In a general sense, there is no absolute answer to the question as the problem requires far more information then you can post in this form. Physical characteristics of the house, weather patterns, environmental conditions, energy rates including time of use billing, equipment type, efficiency, etc.

This doesn't even consider perceived effects and overall comfort. 80 low humidity can feel more comfortable then a very muggy 75. Even just air blowing on you increases the evaporation rate on your skin so you feel cooler even though the surroundings are warmer.


djrobx
Premium
join:2000-05-31
Valencia, CA
kudos:2
Reviews:
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reply to Jack_in_VA

I never change mine winter or summer unless we're going to be away for extended periods like a vacation. Lifelong experience in instrumentation and process control business gives me a different perspective on issues like this. The savings (if any) aren't worth the effort and discomfort.

So says the man with the low electric rates.

If you had the punitive tiered electric rates that we have, you'd be looking for ways to cut a few percent off the top, as small reductions in usage result in more significant changes on your bill.
--
AT&T U-Hearse - RIP Unlimited Internet 1995-2011
Rethink Billable.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to cdru

said by cdru:

said by Jack_in_VA:

Given that people are posting to this thread means that some are interested. You realize you don't have to read them. Does this mean that a couple of people have the power to determine a subject has been "beaten to death" therefore choking it off?

Lighten up Jack. It was in posted in humor.

I know yours was. This is an interesting topic as there are so many facets to consider.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to cdru

said by cdru:

said by haroldo:

I took math in school (did well) and figure that not using a major appliance for eight hours saves more energy than using it for ten minutes to cool the house when you return home...

In a general sense, there is no absolute answer to the question as the problem requires far more information then you can post in this form. Physical characteristics of the house, weather patterns, environmental conditions, energy rates including time of use billing, equipment type, efficiency, etc.

This doesn't even consider perceived effects and overall comfort. 80 low humidity can feel more comfortable then a very muggy 75. Even just air blowing on you increases the evaporation rate on your skin so you feel cooler even though the surroundings are warmer.

+1


cowboyro
Premium
join:2000-10-11
Shelton, CT
Reviews:
·AT&T U-Verse

1 recommendation

reply to Jack_in_VA

said by Jack_in_VA:

said by cowboyro:

example for this.
Decent thermostats allow for automatic start earlier so that the set point is reached at the desired time.
I could dump a couple of years worth of hard data which shows without any trace of doubt that keeping the unit off during the day saves energy vs keeping it on. Energy, not necessarily money. Rates may be higher at certain times.

Yes and when it's cold the recovery has to start early in the time that's supposed to be saving energy in order to keep the strips off if it's able to do it without strips. When it's cold a heat pump relies on strips to supplement the heat that it can't provide. Turning the entire unit off in most cases would cost far more than just keeping it running.

Example in my case the heatpump pulls 1.4 kW running which is about 14 cents/hour. 8 hours would be $1.12. Heat strips 15kW cost about $1.58/hr to operate.

I would be interested in seeing your hard verifiable data.

Again you are giving a BAD EXAMPLE THAT DOES NOT APPLY.
An air conditioner runs with pretty much the same efficiency regardless of where the thermostat is set at. The efficiency of the heat pump + strips assembly is not constant and highly depends on which is running. You simply cannot compare the two.


cowboyro
Premium
join:2000-10-11
Shelton, CT
Reviews:
·AT&T U-Verse
reply to Draiman

said by Draiman:

Does that hard data take into account what the humidity can do to stuff in your house? Your comfort level? Life expectancy related to the extra stress a hot house can cause? A pissed off significant other or kids? Stall air on allergies? Way to many things to take into account for hard data to give a realistic answer to the OP's question. It's more of a personal choice given then situation and desired outcome.

It does nothing in a daily scenario. We are not talking about leaving the A/C off for a week, we are talking about leaving it off for few hours when nobody is home. No stress, no pissed off "others". If humidity becomes an issue then it's likely a cat can escape too...


cdru
Go Colts
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join:2003-05-14
Fort Wayne, IN
kudos:7

1 recommendation

reply to cowboyro

said by cowboyro:

Again you are giving a BAD EXAMPLE THAT DOES NOT APPLY.
An air conditioner runs with pretty much the same efficiency regardless of where the thermostat is set at. The efficiency of the heat pump + strips assembly is not constant and highly depends on which is running. You simply cannot compare the two.

When considering cooling only, heat pumps and air conditioners operate the same because AC is a one-way heat pump. And neither type remain at a fixed efficiency, that's why there are set standards to specify how the efficiency is measured.

garys_2k
Premium
join:2004-05-07
Farmington, MI
Reviews:
·Callcentric
·callwithus
reply to haroldo

I ran some Simulink simulations of this and concluded that, yes, it saves energy, no question. How much depends on a few things, including:

• (Obviously) the amount of time it's set back
• The setback temperature vs. "normal" temperature
• The capacity of the AC vs. the heat load (the heat load being at both "normal" and setback temperatures)
• What time the setback starts vs where the AC is on its cooling cycle (did it just get caught up and turned off, did it just begin cooling after the temp. reached the "normal" turn on point)
• How much tolerance the occupant has for it not being caught back up to "normal" after the setback ends
• The heat capacity, thermal inertia, of the house and its contents

It's not a simple calculation and the results vary widely depending on the above, but other than for a few weird combinations of short setback and the unit being caught going into setback just when it would have started in normal mode, it will definitely save energy.



Draiman
Let me see those devil horns in the sky

join:2012-06-01
Kill Devil Hills, NC
Reviews:
·Verizon FiOS
reply to cowboyro

said by cowboyro:

said by Draiman:

Does that hard data take into account what the humidity can do to stuff in your house? Your comfort level? Life expectancy related to the extra stress a hot house can cause? A pissed off significant other or kids? Stall air on allergies? Way to many things to take into account for hard data to give a realistic answer to the OP's question. It's more of a personal choice given then situation and desired outcome.

It does nothing in a daily scenario. We are not talking about leaving the A/C off for a week, we are talking about leaving it off for few hours when nobody is home. No stress, no pissed off "others". If humidity becomes an issue then it's likely a cat can escape too...

I didn't think you'd have a good way to wiggle out of that and you don't. I'm just glad it's all public so everyone can see your response.


OmenQ
Spazz
Premium
join:2003-03-21
Continuum

2 recommendations

reply to haroldo

I have two zones, one upstairs and one downstairs. Being in Texas, I only concern myself with A/C most of the year. Since the house is unoccupied from 8AM to 5PM, excepting the cats and fish tank, the thermostats for both zones are set to 83 during that time.
At 5PM, the downstairs unit cools to 77, and rarely is it not at 77 when I get home at 5:30. (Usually just shutting off)
At 11 pm, the upstairs thermostat goes to 77.
At midnight, the downstairs thermostat goes back to 83. (Residual cooling from upstairs keeps the downstairs cool enough)
At 8AM, both units are back to 83.
If someone's home, they can manually change the set point below 80. If the wife were to be stay-at-home, she could set the unit for whatever she wants during the day.

The cats don't seem to mind it being 83 during the day, as I've kept this schedule for the past 3 years with no ill effects on them or the rest of the house. My electric bill only exceeds $200 in July and August, when it's 110 degrees out. Not exactly verifiable, empirical data, but it's my experience. It makes no sense to me to keep the house at my comfort level for the 9 hours when no one's home. The unit's still going to use energy, but it seems like it would use less maintaining 83 than it would to maintain 77.

Regardless, the goal of an air conditioner is occupant comfort. If the house is occupied, set it to your comfort level. If it's not occupied, well, what's that line about a tree in the forest?
--
Cogito Ergo Nom



cowboyro
Premium
join:2000-10-11
Shelton, CT
Reviews:
·AT&T U-Verse
reply to cdru

What I mean is that with 80F inside (for example) the efficiency is the same whether the thermostat is set at 78F or at 65F. As opposed to heat pumps where if you have 74F inside the efficiency may dramatically decrease is you change the thermostat from 75F to 76F and it starts the auxiliary heating.



cowboyro
Premium
join:2000-10-11
Shelton, CT
Reviews:
·AT&T U-Verse
reply to haroldo

And since a picture is worth 1000 words, here is the data collected from my lower level unit in the summer of 2010. Average daily compressor run times for weekdays (with setback) and weekends (all day constant) vs the maximum outdoor temperature for the day. Since the numbers are average it pretty much flattens daily variations in humidity/sun/clouds/temperature.


I use intelligent recovery where the setpoint must be reached at the given time. It increases the usage a bit vs just starting 1 hr early to feel "just OK" but the house feels nice and cool when I get home.


morbo
Complete Your Transaction

join:2002-01-22
00000
reply to haroldo

FACT: You will save money if you turn off your AC or set it to a higher start point (from 72 to 82).



sivran
Opera ex-pat
Premium
join:2003-09-15
Irving, TX
kudos:1
reply to haroldo

We have an ancient thermostat--two switches and a dial. No buttons, no fancy display, no programming.

We leave the AC off during the day (in TEXAS!) and if I'm home, I use my window unit. We turn the AC on usually around midnight. We did an experiment with turning it on earlier (like, shortly after sundown) and progressively setting the thermostat lower. Used roughly the same amount of energy as just cranking it all the way.
--
Think Outside the Fox.



Snakeoil
Ignore Button. The coward's feature.
Premium
join:2000-08-05
Mentor, OH
kudos:1
reply to cowboyro

quote:
Rates may be higher at certain times.

I don't know about rates, but demand varies through out the day. When I lived in GA, the power company was offering a discount on the electric bill. All you had to do was allow them to install a remote "kill" switch to your AC unit. If they triggered the kill switch, you got a discount that month on your power bill. I think it was 25, maybe 50 bucks. That was so that during the peak hours during the say, the risk of "brown outs" was reduced/prevented.
Our current power company in Ohio hasn't made that type of offer, though even so often they offer to buy the old "inefficient" basement/garage refrigerator/freezer. They offer 50 to 150 bucks, plus hauling it away. I guess they want to lure people into buying more energy efficient appliances.
--
Is a person a failure for doing nothing? Or is he a failure for trying, and not succeeding at what he is attempting to do? What did you fail at today?.


Snakeoil
Ignore Button. The coward's feature.
Premium
join:2000-08-05
Mentor, OH
kudos:1
reply to rfhar

quote:
We do shut ours off most nights

This makes sense. As night air is typically cooler then daytime air. One reason why I wish we have a whole house fan. We could kill the AC at about 9pm, open a few windows, turn on the fan. Draw in the cool night air and save some energy running the AC.
--
Is a person a failure for doing nothing? Or is he a failure for trying, and not succeeding at what he is attempting to do? What did you fail at today?.