Author Topic: Air conditioning question  (Read 3008 times)

stoaX

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Air conditioning question
« on: May 19, 2020, 02:09:17 PM »
Any HVAC experts out there?  In January I moved to South Carolina. I've never lived in the southeast United States before. When the summer heat and humidity strikes, will I be better off letting in the cool night and morning air and just running the AC in the heat of the day and evening?  Or will doing that let in so much humidity that it will offset the savings of letting the cooler air in.

Inquiring minds want to know.  Thanks!

marty998

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Re: Air conditioning question
« Reply #1 on: May 19, 2020, 02:39:55 PM »
https://www.mrmoneymustache.com/2012/06/14/the-worlds-most-efficient-air-conditioner/

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Sibley

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Re: Air conditioning question
« Reply #2 on: May 19, 2020, 03:02:32 PM »
I not aware that MMM has ever lived in the South. Visited, sure. But that's very different from living there year round. Heat + humidity are oppressive. They can be deadly, especially as you age or if you have compromised health. Plus, having too high of humidity in the house can cause problems with mold. I think MMM's article is correct, to a point. But he was writing from a perspective of Canadian and Coloradoan climate, which is quite different from the American South.

OP: You SHOULD work on acclimating yourself to your new climate. Only being comfortable when it's 68 and 30% humidity is not a good thing.

You SHOULD work on maintaining/improving your overall health. In particular, maintaining a healthy weight will help you regulate your body temperature. If you have chronic diseases, keeping them properly controlled can help.

You also SHOULD work on figuring out how to maintain comfortable temps and humidity in your home, holistically. Ceiling fans, landscaping that shades the house, etc can be helpful. Remember that modern air conditioning is modern, and people have lived in the South for hundreds of years, if not thousands, quite successfully. History can give you some ideas. If air conditioning is part of your solution, fine. But it should not be all of your solution.

As for your particular question - my understanding is that A/Cs will work harder with swings in temp than to maintain a constant temp. However, ideally, you'd use your AC to help bring down the humidity more than to bring down the temperature.

stoaX

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Re: Air conditioning question
« Reply #3 on: May 19, 2020, 03:03:22 PM »
https://www.mrmoneymustache.com/2012/06/14/the-worlds-most-efficient-air-conditioner/

Please don't forget the message of the blog if you can ;)

Thanks - that was a good blogpost that MMM did, along with the "how not to use your air conditioner" post that is linked inside it.  I will certainly do what I can to acclimate. And what you linked reminded me that  I sometimes don't drink as much water as I should, so doing better there will be important. 

But I still wonder how much of an air conditioner's cooling effect is achieved via cooler air vs. drier air. 

NotJen

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Re: Air conditioning question
« Reply #4 on: May 19, 2020, 03:13:20 PM »
I'm in the southeast, but not South Carolina specifically.  Where I live, it does not really "cool down at night" during the summer in a way that would have a positive effect on the inside temperature.  Yes, humidity control is the main reason I use the A/C (I keep it at 78-80 in the summer).

I'm enjoying this time right now when I can have the windows open - yes, it's warming up and the inside temp has been in the 80s a few days, but the humidity isn't at peak yet so it's comfortable.  I'm resisting turning on the A/C, because once I do, there are very few days where I'll be able to turn it off and open the windows again.

terran

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Re: Air conditioning question
« Reply #5 on: May 19, 2020, 03:49:15 PM »
I'm also a fairly recent transplant to a hot and humid place. The open the windows at night and never use A/C plan doesn't work nearly as well here. I don't know if it's right or not, but I've taken to freely opening the windows if the dew point is below 50-60 degrees and the temp is something I'd like to let inside, but that means windows are closed and A/C is used most of the time for several months in the summer. We cool to 80 degrees. We still have higher winter electric bills than summer.

KungfuRabbit

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Re: Air conditioning question
« Reply #6 on: May 19, 2020, 05:19:48 PM »
If the "cool" air is roughly the same, or even 5-10 degrees below, your air conditioner setpoint AND that cool air is 100% humidity (which is often is), you are sadly before off not letting it in.  If you want to get technical...

The heat capacity of dry air (energy required to change the temperature) is about 1 kJ / kg / C
The heat of condensation of water (energy required to turn it from vapor to liquid) is about 2500 kJ / kg

If those units and terms are not familiar that is fine, just notice that 2500 is >>>>>> 1, so taking water vapor out of the air is a LOT more energy than cooling down air. 
 
Humid air is somewhere around 2% water vapor, so roughly speaking dehumidifying humid air is about the same energy required as lowering the temperature of dry air by 50 degrees! 

So, again, if humidity is high you are better off keeping all of the windows closed and keeping the humidity in the house down, because even if you cool off the house slightly by letting night time air in the energy to take out that water FAR surpasses the savings of not running your AC at night. 


Cranky

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Re: Air conditioning question
« Reply #7 on: May 19, 2020, 05:43:52 PM »
I have lived in Florida, and I have lived in Colorado, and the night air situation is pretty different.

In the south “cool off at night” means “get down to 80°”. In Colorado, it means that you might well want a blanket at night in July.

I have lived in Miami without a/c and honestly - it’s more comfortable to just get used to being hot than it it to go in and out of the a/c.

But in SC, I think your best strategy is to choose a comfortable temperature, like 78°, and keep the thermostat at that all summer. Dress lightly. Use fans. Drink plenty of water. Exercise outside first thing in the morning or in the evening. If you are gone all day, turn the thermostat up a couple of degrees, but don’t make the a/c work too hard.

Fishindude

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Re: Air conditioning question
« Reply #8 on: May 20, 2020, 05:23:39 AM »
Set the thermostat at whatever temp you are comfortable with and just leave it there while you are home day and night.   If your utility bills are high or you sense the equipment is working too hard, you are likely much better off spending money on insulation and weather sealing measures rather than turning the equipment on & off.

stoaX

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Re: Air conditioning question
« Reply #9 on: May 20, 2020, 06:41:22 AM »
I not aware that MMM has ever lived in the South. Visited, sure. But that's very different from living there year round. Heat + humidity are oppressive. They can be deadly, especially as you age or if you have compromised health. Plus, having too high of humidity in the house can cause problems with mold. I think MMM's article is correct, to a point. But he was writing from a perspective of Canadian and Coloradoan climate, which is quite different from the American South.

OP: You SHOULD work on acclimating yourself to your new climate. Only being comfortable when it's 68 and 30% humidity is not a good thing.

You SHOULD work on maintaining/improving your overall health. In particular, maintaining a healthy weight will help you regulate your body temperature. If you have chronic diseases, keeping them properly controlled can help.

You also SHOULD work on figuring out how to maintain comfortable temps and humidity in your home, holistically. Ceiling fans, landscaping that shades the house, etc can be helpful. Remember that modern air conditioning is modern, and people have lived in the South for hundreds of years, if not thousands, quite successfully. History can give you some ideas. If air conditioning is part of your solution, fine. But it should not be all of your solution.

As for your particular question - my understanding is that A/Cs will work harder with swings in temp than to maintain a constant temp. However, ideally, you'd use your AC to help bring down the humidity more than to bring down the temperature.

Thanks, all good points.  Fortunately the health situation is excellent for both Mrs. stoaX and I and she has planted a dozen or so shrubs that will provide shade in years to come in addition to what we already have. 

stoaX

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Re: Air conditioning question
« Reply #10 on: May 20, 2020, 06:45:35 AM »
Thanks to everyone else for your replies.  They have really helped me think about how I'm going to deal with the inevitable summer weather.  The amount of energy needed to de-humidify vs. cool was most enlightening...and as always, you mustachians have come through with even more. 

FLBiker

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Re: Air conditioning question
« Reply #11 on: May 21, 2020, 06:07:45 AM »
I live in Tampa.  For a while, I didn't use the AC except for a few peak days in the summer.  Then we had a bunch of mold issues.  Now, we use AC (kept between 77 and 79) all "summer" long.  Our electric bill (1800 sqft, 1960s windows, so-so insulation) is about $100 for about 5 months of the year, and about $75 the rest of the year.

Takk

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Re: Air conditioning question
« Reply #12 on: May 21, 2020, 01:05:47 PM »
Sorry for being late, in Florida here as an HVAC engineer you do not want to keep any place warmer than 80F in general due to moisture and mold problems that cost a lot more than the energy. Removing moisture is the major issue here, which is done by cooling the air down to sub-55f, then blows that air back into the house, this causes the condensation and drain at the unit, which is a nice way you could confirm if you have a moisture problem. Mid day, run the AC, see how much condensate comes out and collect it. if your house is super tight, you may not need to run it often, if it is a leaky home (or you leave a window open) then the moisture removal will be substantial and obvious to you.

I usually only open windows when I look at the temperature outside and see it's 55F or less. But I could be a bit more selective by looking at wet bulb or dew point to go a little warmer.

Why 55F? Because that is the optimum comfort level for most humans. 70-75f dry bulb, 55f wet bulb, which translates to about 50% Relative humidity.





 

stoaX

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Re: Air conditioning question
« Reply #13 on: May 21, 2020, 02:30:32 PM »
Thanks - I'm glad I asked since my habit in the drier climes of the west was to open things up if it was just a few degrees cooler outside and that sounds like a bad idea in the humid southeast. 

terran

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Re: Air conditioning question
« Reply #14 on: May 21, 2020, 03:21:15 PM »
Thanks - I'm glad I asked since my habit in the drier climes of the west was to open things up if it was just a few degrees cooler outside and that sounds like a bad idea in the humid southeast.

I think it comes down to humidity rather than temperature. We opened the window last time with dew point around 55 degrees, but we're up over a 60 degree dew point for the next week so the windows will stay closed. If you can get cool dry air inside then why not, but as others have explained in detail cool moist air might not be worth it. My understanding is that dew point is a better measure than humidity because it adjusts for the different amounts of moisture than air can hold at different temperatures.

Takk

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Re: Air conditioning question
« Reply #15 on: May 27, 2020, 08:11:03 AM »

I think it comes down to humidity rather than temperature. We opened the window last time with dew point around 55 degrees, but we're up over a 60 degree dew point for the next week so the windows will stay closed. If you can get cool dry air inside then why not, but as others have explained in detail cool moist air might not be worth it. My understanding is that dew point is a better measure than humidity because it adjusts for the different amounts of moisture than air can hold at different temperatures.

Dew point is a great measurement if you have it, as is wet bulb and enthalpy for direct measurements. Relative humidity is also good, but it requires an understanding of psychrometrics and the psychrometric chart to be useful.  as a guideline, 75F at 50% RH has about the same amount of water in the air as 55F at 100% RH.

terran

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Re: Air conditioning question
« Reply #16 on: May 27, 2020, 08:25:47 AM »

I think it comes down to humidity rather than temperature. We opened the window last time with dew point around 55 degrees, but we're up over a 60 degree dew point for the next week so the windows will stay closed. If you can get cool dry air inside then why not, but as others have explained in detail cool moist air might not be worth it. My understanding is that dew point is a better measure than humidity because it adjusts for the different amounts of moisture than air can hold at different temperatures.

Dew point is a great measurement if you have it, as is wet bulb and enthalpy for direct measurements. Relative humidity is also good, but it requires an understanding of psychrometrics and the psychrometric chart to be useful.  as a guideline, 75F at 50% RH has about the same amount of water in the air as 55F at 100% RH.

Looks like dew point can be roughly approximated as: DP = ((Tf - 32)x5/9 - (100 - RH)/5)x9/5 + 32 where Tf is temperature in fahrenheit and RH is relative humidity. Or if you're already working in celsius then it's the easier DP = Tc - (100 - RH)/5. The more precise version seems to use natural logs: https://en.wikipedia.org/wiki/Dew_point

I just use the values reported on the weather website I use.

stoaX

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Re: Air conditioning question
« Reply #17 on: May 28, 2020, 05:31:48 AM »
Terran and Takk:. Thanks!   Or should I say "talk"!?!  Your posts are very helpful. 

stoaX

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Re: Air conditioning question
« Reply #18 on: May 28, 2020, 07:00:31 AM »
The annoying spell check messed up my last post. I meant to say "takk", not "talk".

frugalfoothills

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Re: Air conditioning question
« Reply #19 on: May 28, 2020, 09:36:57 AM »
I have lived in SC my entire life. If you can get through July/August without having your AC on at night, even after running it all day, you are a stronger person than me. The thing about the south is that it DOESN'T cool down at night. Middle of July and August, nights can and do stay in the 80s. And then the humidity. Oh... the humidity!

As others have said, it's more energy-efficient to run the AC consistently (at a reasonable temp) than it is to turn it on and off and have it try to play catch-up. I would look into things like thermal curtains/shades for windows and keep them closed during the day, especially windows that get a lot of sun. Ceiling fans will be your best friend.

stoaX

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Re: Air conditioning question
« Reply #20 on: May 28, 2020, 02:30:58 PM »
I have lived in SC my entire life. If you can get through July/August without having your AC on at night, even after running it all day, you are a stronger person than me. The thing about the south is that it DOESN'T cool down at night. Middle of July and August, nights can and do stay in the 80s. And then the humidity. Oh... the humidity!

As others have said, it's more energy-efficient to run the AC consistently (at a reasonable temp) than it is to turn it on and off and have it try to play catch-up. I would look into things like thermal curtains/shades for windows and keep them closed during the day, especially windows that get a lot of sun. Ceiling fans will be your best friend.

Thanks. You're right about it not cooling down much at night.  I noticed that when I woke up this morning at 6am it was 70 degrees. And almost 100 percent humidity. 

clarkfan1979

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Re: Air conditioning question
« Reply #21 on: June 04, 2020, 12:12:57 AM »
I lived in Fort Myers, FL for 4 years. The lows at night in the summer are 75 and 100% humidity. You want to run your air conditioner often to dry out the air. When the air is dryer you will be able to handle a higher temp with a fan.

I would set my A/C to 78 when I got home and 82 when we left for work. It's not realistic to set the temp to 85 because the A/C unit wouldn't run often enough to dry out the air.

Paul der Krake

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Re: Air conditioning question
« Reply #22 on: June 04, 2020, 01:15:57 AM »
I noticed that when I woke up this morning at 6am it was 70 degrees. And almost 100 percent humidity. 
You will soon be longing for those brisk 70 degrees mornings!

More seriously, you'll get used to it. When I first moved to North Carolina after 20 years of living in gloriously temperate climates, I could barely stay outside for more than 5 minutes. Then I was totally fine. Then I moved to the Pacific Northwest and returned to my heat wuss ways. Now I'm in Hawaii and on 70 degrees mornings I put on a hoodie.

The human body is quite capable of adapting, just give it some time.

BTDretire

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Re: Air conditioning question
« Reply #23 on: June 10, 2020, 01:41:58 PM »
Sorry for being late, in Florida here as an HVAC engineer you do not want to keep any place warmer than 80F in general due to moisture and mold problems that cost a lot more than the energy. Removing moisture is the major issue here, which is done by cooling the air down to sub-55f, then blows that air back into the house, this causes the condensation and drain at the unit, which is a nice way you could confirm if you have a moisture problem. Mid day, run the AC, see how much condensate comes out and collect it. if your house is super tight, you may not need to run it often, if it is a leaky home (or you leave a window open) then the moisture removal will be substantial and obvious to you.

I usually only open windows when I look at the temperature outside and see it's 55F or less. But I could be a bit more selective by looking at wet bulb or dew point to go a little warmer.

Why 55F? Because that is the optimum comfort level for most humans. 70-75f dry bulb, 55f wet bulb, which translates to about 50% Relative humidity.

 Would you also agree that sizing the air conditioner to large is a problem, because it then does not run as long and does not remove as much moisture making you feel less comfortable.

 I find humidity level very important to my comfort, if I can get the humidity below 40% I can raise the temp 2 to 4 degrees. On low humidity days, I can get cold at 78*. We have our thermostat set at 78*,
 although, my son is visiting and he was hot, so I turned it down.... to 77*.

Takk

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Re: Air conditioning question
« Reply #24 on: June 10, 2020, 02:11:56 PM »
Quote
Would you also agree that sizing the air conditioner to large is a problem, because it then does not run as long and does not remove as much moisture making you feel less comfortable.

Yes. Too large of equipment is a problem with humidity. It then conditions the space, then it stops working, warming the coil, then moisture just remains in your space.

In short, the BEST humidity control is a unit that Never turns off. it continuously runs and continuously gives you 55F or less air. Thus the BEST situation is a unit that can turn down and lower the flow of air while the compressor still runs at a minimum value.

This of course is at odds that a unit that is sized to never turn off(if it is not variable speed), means that it may be under sized and the room may be warm if there is extra load in the space.

Thus a middle ground usually is something that has 2 to 3 stages of cooling/flow rate. allowing you to dehumidify the space during low cooling requirements.

bogart

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Re: Air conditioning question
« Reply #25 on: June 10, 2020, 10:11:42 PM »
I've lived in the US SE basically my entire life and while others have posted useful info. about the science behind the humidity thing I'll just note that yes, it is a problem.  And sure, you can acclimate and there are things you can do but depending on the layout of your home and what you have in it, mildew and mold can be real and serious problems that you don't necessarily notice until they're too late to solve (clothes, books, etc. ruined).

Don't get me wrong, I struggle with running the a/c and prefer not to.  But I've also come to appreciate that the humidity (not the heat per se) really is a problem and worth not neglecting.

Those who say we've lived this way for generations, well -- yes and no.  It's not an accident, IMO, that the population in the US SE began growing rapidly around the same time central a/c became available for the single-family home.  My dad had a friend who grew up here (~1940s) who as a kid would spend all day every summer day watching matinees in the local movie theatre because it was the only place he could gain access to that was air conditioned.

RainyDay

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Re: Air conditioning question
« Reply #26 on: June 11, 2020, 12:04:08 PM »

Humid air is somewhere around 2% water vapor, so roughly speaking dehumidifying humid air is about the same energy required as lowering the temperature of dry air by 50 degrees! 


Out of curiosity, would it make sense to run a dehumidifier and set the a/c warmer?  I can't tell if the cost of running the dehumidifier would be offset by being able to keep the house warmer and thus lower the a/c bill.     

geekette

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Re: Air conditioning question
« Reply #27 on: June 11, 2020, 05:17:39 PM »
From what I understand, a dehumidifier is similar to an a/c, but it exhausts the warm air into the same room instead of outside.  So yeah, it would be dryer, but warmer.

MudPuppy

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Re: Air conditioning question
« Reply #28 on: June 11, 2020, 06:01:54 PM »
Not an expert, but a SE native.

We run the air at 76-78 and use ceiling fans to keep our occupied rooms more pleasant.

Takk

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Re: Air conditioning question
« Reply #29 on: June 12, 2020, 02:44:52 PM »
For Reference: Dehumidifiers come in two flavors.

1) Cool the air down below dew point, drains some water off the cold coils as it's at 100% humidity, air comes out with less moisture. Some of these reject the heat back into the space (Think a home unit, outside and inside, in the same box) some reject it outside (home unit, with outside connection to reject heat to) Either way, this is about as efficient as running your air conditioning, down to being less efficient if it rejects heat that you have to remove.  These dehumidification units usually find their niche around basements, as the air is already chilly, but it's moist.

2) Chemical Dehumidification: Think Damp-rid. It absorbs moisture as the material is dryer than the air at the temperature it is at, equalizes eventually, then you can sometimes remove the material, heat it up, and it releases the moisture to the atmosphere. These are usually done in emergency situations. a ton of water got where water shouldn't be.

So if you're thinking of a dehumidification unit for your (south-east) house, it may make sense for just a window unit, or a "mobile" room AC unit, instead, as you will likely get use out of the cold air it provides. and remember, a smaller unit is better for dehumidification! don't let that sucker turn off!