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Steve
I know your IP address
Consultant
join:2001-03-10
Foothill Ranch, CA
kudos:5

1 edit

Running A/C w/ windows open

Hello all,

Under what circumstances does it make sense to keep the windows open while trying to cool a house IF:

1) it's 74F outside (and dropping, it's evening)
2) it's 80F inside
3) A/C set for 72F

Should one keep the doors/windows open to help aid in cooling, or are the outside influences counterproductive to the A/C doing its job?

The above temps should be read relatively (outside cooler than inside, but still slightly above the A/C setpoint).

Under what circumstances are open-doors-and-windows effective while running A/C, or should it never be done?

Thanks...

Steve
--
Stephen J. Friedl | Unix Wizard | Microsoft Security MVP | Orange County, California USA | my web site

bemis

join:2008-07-18
Reading, MA
Reviews:
·Comcast
I don't think it ever makes sense to run the a/c w/ the windows open.

Though I do sometimes turn the central air fan on to help circulate the air through the house. Our living room has 4 windows and the main intake is located there as well so when the fan is running the room has negative pressure and pulls a noticable breeze in.

I just did that before bed--dropped from ~75* to 61* over the last 4 hours... but the house was still 78*... opening windows and running the fan helped bring it down 5* in 15 minutes.


davidg
Good Bye My Friend
Premium,MVM
join:2002-06-15
none
reply to Steve
with the windows open you are bringing in humid air as well.

if you are setting back the stat, 8 degrees is too much for most modern units. they design them to run long for more humidity removal, but to do so the system is perfectly or slightly under sized. when this is the case, 8 degrees is too much to easily overcome. if i set ours back more than 4 degrees it takes it until well after dark to catch back up. i checked the run times, and it actually runs MORE each day with a 8 degree set back than with a 4. YMMV, but that is my experience with several various size/style units both at home and at work.
--
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SandShark
Long may you run
Premium,MVM
join:2000-05-23
Santa Fe, TX
kudos:3
reply to Steve
According to Weather Underground, the temperature in Yorba Linda is ~64 degrees F, relative humidity is 83% and the dew point is 61 degrees F, so leaving the windows open and running the A/C would be counterproductive with those readings.


Jahntassa
What, I can have feathers
Premium
join:2006-04-14
Conway, SC
kudos:4
reply to Steve
In a house? Never, that I can think of. Several reasons already given, just echoing them.

matt5

join:2001-10-06
Lagrangeville, NY
reply to Steve
said by Steve:

Under what circumstances does it make sense to keep the windows open while trying to cool a house...
Trying to give your power co the most money you can!

if it is nice out... stick a box fan in the window(s)...


Steve
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join:2001-03-10
Foothill Ranch, CA
kudos:5

3 edits
reply to Steve
Ok, I'll be a bit more specific - I wanted to make my request neutral to avoid spinning it towards the answer I was looking for.

I rent a room from the guy who owns the house, and I have a wide comfort range: I have a fan that moves the air in the part of the house I work in, so I never run the A/C on my own account.

But my housemate runs the A/C at the drop of a hat, and had it running while it was 74F outside (and dropping) but 80F inside. My contention is that getting a bit of flow-through through the house will help carry some of the heat away - to

So: If he's determined to run the A/C - which I cannot do anything about - is my opening a window helping or hurting? Does it depend on the temperature differentials?

Note: My housemate runs the A/C when it's 58F outside (!!!) to cool a too-hot house because he won't open a window in his own bedroom, and he won't even consider running a fan in his room.

Edit - it's 10:36PM, 69F outside, 76F inside, and the A/C is running with a 73F setpoint. It's hard for me to imagine anything I could do that would make this worse...

Edit #2 - I guess humidity matters: at 10:52PM, it's 68F / 67%RH outside; 75F / 52% RH inside, with the A/C still running.

Thanks,
Steve
--
Stephen J. Friedl | Unix Wizard | Microsoft Security MVP | Orange County, California USA | my web site


ironweasel
Weezy
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join:2000-09-13
Belen, NM
kudos:1
reply to Steve
While probably not the case in a humid environment, running a swamp cooler (evaporative cooler) with one or two windows slightly open in a drier environment can make a noticeable difference.

Here, we have to keep a window or two open or else the entire house gets humid real fast. Was 101F today with a relative humidity of something like 11% outside, but with no windows open, the humidity inside was closer to 70% until I went and opened a window.
--
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leibold
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join:2002-07-09
Sunnyvale, CA
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reply to Steve
Don't ask me whether it is right or wrong because I don't know, but I do something very much like you describe.

I do have an old central A/C that I use rarely. I almost never run it during the day as that would be a waste with the poorly insulated home. On most days, just opening the windows in the evening is enough for me to cool down the house sufficiently.

Occassionally this is not enough (day was too hot or not enough of a breeze in the evening to move the hot air out of the house) and I turn on the A/C without bothering to close doors and windows. Because it is colder outside than inside (possibly even colder then the A/C set temperature) that seems ok to me. I don't run the A/C very long either (between 15 minutes to an hour) since it brings relief from the heat in very short time.

I'm very interested to hear what the experts have to say about this.
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koolman2
Premium
join:2002-10-01
Anchorage, AK

1 recommendation

reply to Steve
My room mate is the same way. He turned the portable A/C unit on last night when it was 77 °F (25 °C) inside and 55 (13) outside. I promptly turned it off and inserted a box fan in the window, which cooled the entire apartment to a comfortable 68 (20) in 15 minutes, at 1/50 the cost (A/C unit would take about 75 minutes to cool to the same temperature, A/C pulls 850 W, fan pulls 75).

My opinion is that if a window is within cooling range of the A/C, it should be closed when it's on. Perhaps you could convince him to put a box fan in his window blowing in, and then put a box fan in your window blowing out until inside temperature equalizes with the outside, and then run the A/C. Either that or make him pay an extra 10-20% on the electric bill.


Jahntassa
What, I can have feathers
Premium
join:2006-04-14
Conway, SC
kudos:4
reply to Steve
Aah, the specifics make more sense then. If only because you're talking about a roomate who seems to be lacking in common sense.. (no offense, but really..)

So: If he's determined to run the A/C - which I cannot do anything about - is my opening a window helping or hurting? Does it depend on the temperature differentials?
Well it mostly depends on the return system, and how much your room affects the rest of the house. In this particular case, i'd say the humidity matters more than the temperature. The AC does a lot of work to remove the humidity from the air (as far as I understand it).

You can try and explain the economics of running the AC when it's cooler outside. For example, my TED5000 shows that my AC uses about 4 kWh whenever it's on. However, just running the fan is a fraction of that.

Whenever the outdoor temperature is equal or lower to my comfort temperature, I open the windows. Especially if there's a breeze. Unless it's rainy / humid, then i'll run the AC.

In your case, I don't think the humidity difference would really be 'hurting' anything more than he's already wasting electricity by running the AC to cool the inside to hotter than the inside.

Just out of curiosity, does his running the AC affect you negatively (i.e. paying part of the power bill?), are you trying to get some ammunition to show him his backwards ways, or are you just trying to make sure you aren't compounding the heat issue by keeping your window open?

Cloneman

join:2002-08-29
Montreal
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reply to Steve
1) you can leave the windows open when you JUST turned on the air conditioner. For the first 5 minutes the A/C spits out warm air anyway and you dont want that trapped in your house

2) You can open the windows when the air quality has deteriorated due to windows being closed for too long. MOst people wait for a cool day to do this and turn off the AC while the windows are open. If this is not an option, some people leave one window open to ensure fresh air is present in the home at all times (this makes your A/C work harder though...) Commercial systems have intakes that use outside air, otherwise it would really smell inside large buildings.

c1590

join:2010-02-28
I used to do that, crack a window and turn on the AC. It makes it less dry, and you still get fresh air. At the time I was paying for electricity as a flat rate since it was included in the utilities package for my apartment so I didn't care about wasting.

I say go for it, worst thing that will happen is nothing and you waste electricity.

bemis

join:2008-07-18
Reading, MA
Reviews:
·Comcast
reply to Steve
said by Steve:

I rent a room from the guy who owns the house, and I have a wide comfort range: I have a fan that moves the air in the part of the house I work in, so I never run the A/C on my own account.

But my housemate runs the A/C at the drop of a hat, and had it running while it was 74F outside (and dropping) but 80F inside. My contention is that getting a bit of flow-through through the house will help carry some of the heat away - to

So: If he's determined to run the A/C - which I cannot do anything about - is my opening a window helping or hurting? Does it depend on the temperature differentials?

Note: My housemate runs the A/C when it's 58F outside (!!!) to cool a too-hot house because he won't open a window in his own bedroom, and he won't even consider running a fan in his room.
I'm confused--Does the guy turning on the a/c own the house, or just rent?

Who pays the bills? Do you all split them? If so, then I'd say the one(s) with common sense about not running the a/c when it's 58* outside need to speak to the one who runs the a/c and find out his motivation, maybe he has valid reasons in his mind?--allergies, outside noise, fear of insects coming in the windows (I didn't say the reasons had to be rational )

If it's 58* outside and the guy is running the a/c you might be helping because an open window will quickly allow in cooler air from outside, but in general probably not helping. But, if you shut your doors and the a/c vents in those rooms, then open the windows, you probably aren't hurting all that much either?


Steve
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join:2001-03-10
Foothill Ranch, CA
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said by bemis:

I'm confused--Does the guy turning on the a/c own the house, or just rent?
I rent. He owns the house and runs the A/C.
If so, then I'd say the one(s) with common sense about not running the a/c when it's 58* outside need to speak to the one who runs the a/c and find out his motivation, maybe he has valid reasons in his mind?
He says can't stand the noise of a fan when he's asleep - which can't be any worse than the sound of the A/C running just outside his room - but that doesn't explain why he won't run a window fan while he's awake.

Basically: he seems to think A/C is free unless I open a door, at which point I'm costing him money. I think he went to the Old Wives Tale school of thermodynamics.

My work area is in a common part of the house that can't be isolated, but right next to the back door where I can get a nice airflow. My area is the farthest away from the A/C thermostat.

But the air vent in my room is always closed, and at night I just run a window fan.

My motivations: I'm mainly trying to use energy efficiently.

Steve
--
Stephen J. Friedl | Unix Wizard | Microsoft Security MVP | Orange County, California USA | my web site

Bobcat79
Premium
join:2001-02-04
When I went to bed last night, it was 60 F outside and 75 F inside, but I kept the A/C on. One reason was that it was quite breezy, and I didn't want all the pollen in the air to make it difficult for me to breathe.


ElminsterOld

join:2009-03-04
reply to Steve
My sister runs the HVAC system year around due to allergies. They have a humidifier, air filter, UV light, etc. on the system so it's on heating or cooling all year reguardless of what's happening outside. Some people like or need to have hospital grade air in their house.


compuguybna

join:2009-06-17
Nashville, TN
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reply to Steve
Sounds like it might be time to find a new "housemate".

said by Steve:

said by bemis:

I'm confused--Does the guy turning on the a/c own the house, or just rent?
I rent. He owns the house and runs the A/C.
If so, then I'd say the one(s) with common sense about not running the a/c when it's 58* outside need to speak to the one who runs the a/c and find out his motivation, maybe he has valid reasons in his mind?
He says can't stand the noise of a fan when he's asleep - which can't be any worse than the sound of the A/C running just outside his room - but that doesn't explain why he won't run a window fan while he's awake.

Basically: he seems to think A/C is free unless I open a door, at which point I'm costing him money. I think he went to the Old Wives Tale school of thermodynamics.

My work area is in a common part of the house that can't be isolated, but right next to the back door where I can get a nice airflow. My area is the farthest away from the A/C thermostat.

But the air vent in my room is always closed, and at night I just run a window fan.

My motivations: I'm mainly trying to use energy efficiently.

Steve


Steve
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Consultant
join:2001-03-10
Foothill Ranch, CA
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said by ElminsterOld:

My sister runs the HVAC system year around due to allergies.
He and I both have minor allergies, but he's never voiced this as a reason for any of it. When the temperature differentials are different, we have the doors and windows open.
said by compuguybna:

Sounds like it might be time to find a new "housemate".
I generally have a pretty decent arrangement here, but I assure you that very point crosses my mind weekly.

bemis

join:2008-07-18
Reading, MA
Reviews:
·Comcast
reply to Steve
said by Steve:

Basically: he seems to think A/C is free unless I open a door, at which point I'm costing him money. I think he went to the Old Wives Tale school of thermodynamics.

...

My motivations: I'm mainly trying to use energy efficiently.
Sounds like the guy is somewhat unreasonable, but also that you aren't bothered by the hvac use except for energy... This has become a personal issue not a technical one

if it were me, and I otherwise liked the house and housemate/owner, I'd probably just not open the windows


gregamy

join:2003-05-22
Middletown, CT
reply to Steve
Use dewpoint, not temperature and humidity (though it's calculated based on those).

My breakpoint is 60*: any time the dewpoint is 60 or higher, I turn on the A/C. Any time the dewpoint is below that the windows are open...

tcope
Premium
join:2003-05-07
Sandy, UT
kudos:2
reply to Steve
The house AC should be a closed system. It should be drawing air in from return vents and it maintains a static pressure. So opening a window would not cause the unit to pull air in from the outside or push it out. Air will enter the same as if the unit was on or off. Of course, hot air will enter and cool air will leave. So leaving a window open is like throwing dollars out the window.

Swamp coolers benefit from leaving a window open in that they blow air into the home from outside (this is not what a house AC unit does). Leaving a window open allows pressure to be released and that cooler air to circulate through the entire house.

Two different systems.

Kearnstd
Space Elf
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join:2002-01-22
Mullica Hill, NJ
kudos:1
reply to Steve
windows open because the home HVAC system is a closed loop the supply air is coming from the space to be cooled. so it does not help to have new air leaking in. a properly working central air system will get that house cooled down pretty quickly.

however another side consideration if you get lots of cool nights is something called a "whole house fan" with these you can leave the AC off, open the windows and turn this fan on and it will suck in the cooler outside air.

cracking open windows works in cars because it is an open system and its supply of air is from the outside. so having a window cracked open allows the hot air to be pushed out.
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morbo
Complete Your Transaction

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reply to Steve
said by Steve:

I generally have a pretty decent arrangement here, but I assure you that very point crosses my mind weekly.
Weekly? Then move. Stop subsidizing his wasteful ways if it means that much to you. Be sure to tell him why you are leaving as well as maybe it will stick with him.

Just know that you leaving this "pretty decent arrangement" may cost you more money in the short and long run than putting up with quirky a/c beliefs.


djrobx
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Valencia, CA
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reply to Steve
I do this somewhat often. Usually when the ac has been off all day, it WAS hot but has cooled off and I just want to cool the house down rapidly. But only the upstairs windows. We also live in a dry climate.

Consider:

1) it's probably 85+ upstairs, heat trapped by good insulation.
2) it's now 70 degrees or less outside
3) We're going to spend the next few hours downstairs
4) using attic fan exclusively will render the house uncomfortable for a long time. Home has very good insulation that's acutally woking against us at this point.

If I JUST use the AC, it will be on all night and it will be pretty uncomfortable upstairs when we go to sleep. If I run both the ac and attic fan with upstairs windows open, the downstairs is comfortable for the evening and by the time we go to bed the upstairs has exhausted all of the heat built up throughout the day.

Typically I can turn the AC back off within an hour.

--
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fixrman
Premium
join:2003-02-10
Hatboro, PA
reply to Steve
Warmer inside than outside? It would be utter stupidity to run the air con unless someone has allergies that prevent exposure to the outside air. A giant money waster.

Open all the windows and enjoy the air flow.


Steve
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Foothill Ranch, CA
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said by fixrman:

Warmer inside than outside? It would be utter stupidity to run the air con unless someone has allergies that prevent exposure to the outside air. A giant money waster.
Yes, that's what I thought too.

But if he's going to run the A/C in a foolish way, I was hoping that opening the windows would help that process along, but it's not so clear that it does. So I'll just leave doors/windows closed, be warm, and keep the peace.

Steve
--
Stephen J. Friedl | Unix Wizard | Microsoft Security MVP | Orange County, California USA | my web site


NYR 56
Premium
join:2000-12-05
Smithtown, NY
reply to tcope
said by tcope:

The house AC should be a closed system. It should be drawing air in from return vents and it maintains a static pressure. So opening a window would not cause the unit to pull air in from the outside or push it out. Air will enter the same as if the unit was on or off. Of course, hot air will enter and cool air will leave. So leaving a window open is like throwing dollars out the window.
The thing is, the "hot air" won't enter because it's actually cooler outside than inside. Given the circumstances, I can't imagine any reason why Steve's open window would be wasteful and in fact agree that it is probably beneficial. I'm not sure how much the humidity affects things, but if it's 59 outside and the AC is set for 70-something, it is definitely useful to have the window open.

dsanfte

join:2005-03-15
UK
reply to Steve
Most of these guys seem to be energy-saving- or efficency-zealots. Don't listen to them.

Inside 80F, outside 72F or similar? Yea, open the windows and leave the AC running. You'll cool down quicker.

Once inside temp is stable around 72, close the system again to resist heat rise.

A window air conditioner costs about $2-3 /day to run. It's not an insane cost.

devlista

join:2010-05-23
Boonton, NJ
American homes are built with so much fake wood and they contain so many chemicals of various kinds that keeping a house closed up in either the winter or summer is asking for health problems. People may have also noticed that most window air conditioners have a drip pan that holds water underneath the AC coils and many no longer have a adjustable vent for "outside air" - which is probably all for the best because those drip pans tend to accumulate dust and mold.

A few years ago some scientists at Texas Tech analyzed a bunch of common window air conditioners and they discovered that as they come from the factory, they are basically mold machines. they can make people very sick.

So what is commonly recommended is that people take pains to seal the air path between inside and outside, (disable any vent directly inside the AC unit!) clean the indoor parts of the coils well and frequently, and put a better filter on the intake (at least an MERV 7 -if the compressor can take it) so that the indoor portion of the coils don't accumulate dust as quickly (which gets wet and grows mold)

Clean them every few weeks during the AC season with a foaming cleaner too.

its also a good idea -if it doesn't have one- to drill a small drip hole at the lowest, most distant end of the drip pan (taking EXTREME care not to nick the coils!!) so that condensed water doesn't sit there inside the AC.

That will reduce the cooling numbers slightly but it will dramatically improve the ability of the AC to dehumidify your home and it will also reduce the amount of mold that it grows substantially.

And last but not least, set up some way to VENTILATE year round..

Not just an exhaust or intake fan, because both have issues.. you need balanced ventilation, an equal amount going in and out..

A good way to do this is by using a heat recovery ventilator. An HRV with an aluminum care, kept clean (important!) can recover as much as 65-75% of the heat in the winter and substantially cool the air coming in in the summer.. You can buy controls that allow the HRV to cycle on - on the low speed, so its ventilating as little as 20% of the time.. (if you are on a budget, some cheap light timer can be set up to do this.)

Your modified AC will remove the humidity.. Modern houses are too tight - its a health disaster because people are getting exposed to a huge amount of junk that nobody is going to tell them about. But its there. And its making many people very sick.

When its not really hot or really cold, you can use an HRV to cool your home, saving a bundle on electricity.. (an HRV takes less energy than a light bulb)

Its a lot cheaper than getting cancer from the toxic stuff in your home (which all emits more nasty stuff when it gets hot)