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AKA06
join:2011-02-10

AKA06 to haroldo

Member

to haroldo

Re: Central air conditioner on/off? Old wives tale?

Not seen it mentioned yet, but it should be noted that air conditioning units are actually working more efficiently running to cool down a space versus maintaining the temperature once the target is reached.

Unless there is some additional factor not mentioned, the repairman is incorrect, in terms of energy/money used.

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA

Premium Member

said by AKA06:

Not seen it mentioned yet, but it should be noted that air conditioning units are actually working more efficiently running to cool down a space versus maintaining the temperature once the target is reached.

Unless there is some additional factor not mentioned, the repairman is incorrect, in terms of energy/money used.

Really? When trying to cool a hot space lots of BTU's are exchanged loading the compressor raising the head pressures and electrical load. This condition probably would represent the max loads the system would ever see. When maintaining there is little extra load on the system thus lower energy used.
Jack_in_VA

Jack_in_VA to cowboyro

Premium Member

to cowboyro
said by cowboyro:

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.
[att=1]
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.

Yes a picture is worth a thousand words. From your graph it appears that your unit is running only slightly less with the setback during the week vs continuous on weekends.

On a 90 degree day you show almost 6 (5.8?) hours vs slightly less during the week. (5.6?).

Personally IMO that little bit of difference is too little to have much impact.
Mango
Use DMZ and you get a kick in the dick.
Premium Member
join:2008-12-25
www.toao.net

Mango to haroldo

Premium Member

to haroldo
We got a thermostat at work that turned our HVAC system off after closing and on just before opening. On average, we reduced our total energy usage by 40%. We're only open eight hours a day and we typically don't have extreme weather, so it seems quite reasonable to ascertain that it uses less energy to bring the office up/down from ambient temperature than to hold it at the set temperature for 16 extra hours.

KrK
Heavy Artillery For The Little Guy
Premium Member
join:2000-01-17
Tulsa, OK
Netgear WNDR3700v2
Zoom 5341J

KrK to Draiman

Premium Member

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?

Um... NOBODY'S HOME.... and it's only for a few hours, not enough for everything to start falling apart.... if you don't want it completely off, just turn up up to 85 or so.
KrK

KrK to Jack_in_VA

Premium Member

to Jack_in_VA
I find the longer runs makes the place feel more comfortable because the extended run pulls much more humidity out of the air vs the short cycling of maintaining temperature.

I currently have the Thermostat set for 78 when home and use fans, and 85 when not home.

Just got my latest bill and it was $103, but on AMP that was $53 for June 11th to July 12th or so.

It saves money for me, but then again, going from 72 to 78 saved a lot. Having a fan that can move air around you makes a huge difference. At night I could probably set it up 80+ and just use the fan and still be fine.

PSWired
join:2006-03-26
Annapolis, MD

PSWired to Jack_in_VA

Member

to Jack_in_VA
said by Jack_in_VA:

Really? When trying to cool a hot space lots of BTU's are exchanged loading the compressor raising the head pressures and electrical load. This condition probably would represent the max loads the system would ever see. When maintaining there is little extra load on the system thus lower energy used.

That's exactly the point. Of the energy input into the AC system, a certain amount is used to provide refrigeration action (the part you mention as raising the head pressures, etc.) and a part is used to maintain the processes inside the equipment- friction between moving parts, fans, controls, etc. When the refrigeration load is low, overall efficiency is naturally going to be low since the constant losses--compressor friction, fans, etc.--make up a bigger percentage of the system's energy usage.

Taking a step back, it's clear to anyone who understands heat transfer that turning the system off when the conditioned space doesn't need to be cooled *will save energy*. Comfort factors are an entirely different discussion, and a whole new set of variables are introduced if we're talking about saving money when electric rates are not constant throughout the day.

The OP asked about energy usage. Some of the posters here seem to be evaluating the entire philosophy of using air conditioning.

Lurch77
Premium Member
join:2001-11-22
Green Bay, WI

Lurch77 to haroldo

Premium Member

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...(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)

You have a seriously well insulated home, or a very over sized system if it only take 10 minutes of run time to get it to your set point after being off for 8 hours. How is the humidity in your home with such a short run time?

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA

Premium Member

said by Lurch77:

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...(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)

You have a seriously well insulated home, or a very over sized system if it only take 10 minutes of run time to get it to your set point after being off for 8 hours. How is the humidity in your home with such a short run time?

I would guess over-sized. Not only is the ambient air inside higher the walls, floors and furnishings will also be at ambient.
Critsmcgee
join:2011-12-02

Critsmcgee to KrK

Member

to KrK
said by KrK:

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?

Um... NOBODY'S HOME.... and it's only for a few hours, not enough for everything to start falling apart.... if you don't want it completely off, just turn up up to 85 or so.

And?!?!?!? 9 to 11 hours a day isn't a 'few' hours for a typical person. That's 1/3 to 1/2 the day! It's enough time for a house to go from 71 to 90+ and that's not significant? Humidity can go from 45-50% all the way up to 65-70% in that time as well. Not sure about you but that's significant to me!
tcope
Premium Member
join:2003-05-07
Sandy, UT

1 recommendation

tcope to haroldo

Premium Member

to haroldo
$20 says we all come to the same conclusion arrived in all the past threads on this same subject.

KrK
Heavy Artillery For The Little Guy
Premium Member
join:2000-01-17
Tulsa, OK
Netgear WNDR3700v2
Zoom 5341J

KrK to Critsmcgee

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to Critsmcgee
You know houses survived for decades without air conditioning. It's not like suddenly they will fold up into a pile of rot and mold because the AC is off for 6 hours a day.

Like I said, if not off, just up. You can have it turn back on before anyone gets home and cool back down. It's not rocket science.
Critsmcgee
join:2011-12-02

Critsmcgee

Member

said by KrK:

You know houses survived for decades without air conditioning. It's not like suddenly they will fold up into a pile of rot and mold because the AC is off for 6 hours a day.

Like I said, if not off, just up. You can have it turn back on before anyone gets home and cool back down. It's not rocket science.

They didn't have the same content decades ago so that doesn't hold any weight as an argument. Not worried about the house itself either.

Draiman
Let me see those devil horns in the sky
join:2012-06-01
Kill Devil Hills, NC

1 recommendation

Draiman to Mango

Member

to Mango
said by Mango:

We got a thermostat at work that turned our HVAC system off after closing and on just before opening. On average, we reduced our total energy usage by 40%. We're only open eight hours a day and we typically don't have extreme weather, so it seems quite reasonable to ascertain that it uses less energy to bring the office up/down from ambient temperature than to hold it at the set temperature for 16 extra hours.

No extreme weather there make a BIG difference. In the span of 60 minutes the weather changed 21 degrees here yesterday. When you have stuff like that happening it changes the picture.

KrK
Heavy Artillery For The Little Guy
Premium Member
join:2000-01-17
Tulsa, OK

KrK to Critsmcgee

Premium Member

to Critsmcgee
I give up. Obviously you're just trying to argue.

I guess you live in fear of a power outage.
Hotch
join:2012-06-12

Hotch to haroldo

Member

to haroldo
All I can say, and it may have been said all ready but one of the biggest factors in determining how to set or run your a/c, especially central a/c is how "good" or "efficient" is your system that includes the a/c units themselves as well insulation and the duct system.

It is common in tract homes especially the lower priced or modest homes to have a/c that are way too small for the house. Same for regular middle of the road condo units.

Worse, of course, is often apartments that have only one wall unit for the entire apartment. (A cruel joke by building contractors.)

Even if you choose the route to leave the a/c off while gone and come home to find it incredibly warm a proper or optimal central a/c system should be able to cool down the place in a relative short time. If you got a poor system it can run full bore for hours to get the placed cooled down.

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA

Premium Member

said by Hotch:

Even if you choose the route to leave the a/c off while gone and come home to find it incredibly warm a proper or optimal central a/c system should be able to cool down the place in a relative short time. If you got a poor system it can run full bore for hours to get the placed cooled down.

On hot days even when the AC units are left on even a properly sized unit(s) more or less run continuously so how do you think that one could cool down a hot house in a "relatively short time"? It's just not physically possible.

mattmag

join:2000-04-09
NW Illinois

1 recommendation

mattmag to Draiman

to Draiman
said by Draiman:

A pissed off significant other or kids?

'nuff said right there, end of discussion and Amen to that!

AKA06
join:2011-02-10

AKA06 to Jack_in_VA

Member

to Jack_in_VA
said by Jack_in_VA:

Really?

Really. Running at full load, a compressor delivers its best efficiency.
iknow
Premium Member
join:2012-03-25

iknow to A non

Premium Member

to A non
said by A non :

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.

you'd have to leave it off for longer than 5 hours in your case to see any savings. if you leave it off for 8 hours you'll save the electric used in 3.(off for 8 hours and 5 hours to recover).

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

1 edit

Jack_in_VA to AKA06

Premium Member

to AKA06
said by AKA06:

said by Jack_in_VA:

Really?

Really. Running at full load, a compressor delivers its best efficiency.

Yep and also it is using it's maximum kW. The more kW the more the cost.

I could debate you as the manufacturers are going to 2-stage and variable speed compressors to increase efficiency/costs. I have a 2-stage 16i Trane now and I just received my latest bill. It appears that my daily use is 10 kW less/day than the same period last year with my 14i even though the temperatures here for June were the second highest since they started keeping records. Thermostat set to 75 deg 24/7. That's the only efficiency that has any merit.
Hotch
join:2012-06-12

Hotch to Jack_in_VA

Member

to Jack_in_VA
Sorry dude, but you are mistaken. It's all about moving air and cooling air. I know from experience.

I used to live in a modern "state-of-the art two-story house with a state of the art optimal central air conditioning system.

Now I don't claim to be even close to an authority on air conditioning but I know from using the available system in that house did what I said it did.

There was a top of the line central a/c system and top of the line a/c unit for the first floor and a top of the line second unit for the second floor. Appropriate vents, ducks, intakes the whole nine yards plus optimal insulation.

It could lower the house to about 78 degrees in a relatively quick time. It would take it a bit longer to get it down to the desired to 72 degrees.

Now if you say it was impossible than either I was delusional or it was fracking miracle.
AKA06
join:2011-02-10

1 edit

1 recommendation

AKA06 to Jack_in_VA

Member

to Jack_in_VA
said by Jack_in_VA:

Yep and also it is using it's maximum kW. The more kW the more the cost.

Running longer cycles is more efficient and less costly than multiple short cycles. This is precisely why having an over-sized cooling unit is a problem, because you'll end up short-cycling and never reaching a longer cycle. I'm not going to go out of my way to convince you - it's your AC and your bill.

norbert26
Premium Member
join:2010-08-10
Warwick, RI

norbert26 to Snakeoil

Premium Member

to Snakeoil
said by Snakeoil:

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.

this depends on humidity levels . If high humidity / sultry this may not be comfortable.

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA to AKA06

Premium Member

to AKA06
said by AKA06:

said by Jack_in_VA:

Yep and also it is using it's maximum kW. The more kW the more the cost.

Running longer cycles is more efficient and less costly than multiple short cycles. This is precisely why having an over-sized cooling unit is a problem, because you'll end up short-cycling and never reaching a longer cycle. I'm not going to go out of my way to convince you - it's your AC and your bill.

No need to try. Anyone who thinks that running an AC 2 hours is cheaper than running it in cycles for 1/2 hour total is cheaper can't be convinced. The only benefit from longer cycles is the de-humidification is more effective.

Yes you are right "It's my bill" and I'm very comfortable with it. 75 degrees 24/7.

Majestik
World Traveler
Premium Member
join:2001-05-11
Tulsa, OK

2 edits

Majestik to haroldo

Premium Member

to haroldo
I have 2 6yr old Heil.ACs for my 1500sf split level condo.15 or 16 seer. Nothing fancy. Programmed set at 80 degrees when at home and 86 when away. Anything below 80 is too cold for me when ceiling fans are on.
I get home from work near midnight or 3am when working ot.
With the upstairs unit It takes around 10 minutes reach 80 from 86 then on three times the first hour and every hour until about 10am then doesn't come on again until sometime after I leave for work at noon.
Downstairs unit is set at 80 degrees and programmed to run at 78 degrees for 10 minutes every day at 9pm. This ac only runs maybe 10-20 minutes a day total in this heat. All of my bathrooms are downstairs.
This is in 100 degree daytime temps and 86 low. Humidity in my home reads 44%. I'm under a comforter right now. Humidity reads 54% outside presently.

Last year my highest monthly bill ever was $82.92 @ 817kwh and that's with every single day of that billing period at 101-106 degrees.
Usually it's around $50/month in the summer. This is one of the reasons I chose this condo.

RipTides
join:2002-05-25
Dallas, GA

RipTides to haroldo

Member

to haroldo
I think this topic would benefit from discussing code and the pulling of proper permits, but currently I can't figure out a way to segue into that realm of discussion.

But I'd like to suggest everyone chill.

PSWired
join:2006-03-26
Annapolis, MD

1 recommendation

PSWired to haroldo

Member

to haroldo
I hate to take this off-topic since the original question was about energy usage, but here's a plot of the temperature and humidity yesterday inside and outside my house. My AC is set for 74F when occupied, 82F when unoccupied, and 76F when sleeping. Occupied is from 6:30 to 8:30 AM and 4:30 to 11:30 PM. Unoccupied from 8:30 AM to 4:30 PM, and sleeping 11:30 PM to 6:30 AM.

The indoor humidity definitely doesn't get out of control during the setback periods.

House was built in early 60s and has original windows but is otherwise well insulated.

garys_2k
Premium Member
join:2004-05-07
Farmington, MI

garys_2k

Premium Member

It looks like the AC runs from about 4:30 to 8:00 pm to catch up. Any "wife acceptance factor" issues with that?

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA to PSWired

Premium Member

to PSWired
What happened around 1730? Outdoor temp went down 15 degrees and humidity went up from 60 to 95+? Thunderstorm?