Author Topic: Cooling- what's most efficient?  (Read 11284 times)

gecko10x

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Cooling- what's most efficient?
« on: June 19, 2012, 12:47:33 PM »
What's the most efficient way to take advantage of cooler nights? I'm having a hard time sorting out when to run the A/C for optimal cooling of the house...

Lets say the days are 85 and nights are 65. Is it better to:
a) Open the windows up at night and let the house cool down on it's own, then close them up when inside temp=outside temp, run A/C when inside hits 80 to keep it at 80

b) Open the windows up at night and let the house cool down on it's own, but close them up first thing in the morning to keep in the cold air, run A/C when inside hits 80 to keep it at 80

c) Open windows at night, close up in AM & turn on A/C to further cool house to 60, then turn off the A/C for the rest of the day

d) something else?

Then I'll throw another wrinkle in there - what if the outside humidity is high? If it's 65 at night, but 80% humidity, I don't think I want the house open- so would you run the A/C to keep inside=outside as it cools off at night, then turn it off in the AM? Something else?

Thanks for the thoughts...

Kriegsspiel

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Re: Cooling- what's most efficient?
« Reply #1 on: June 19, 2012, 12:50:25 PM »
I feel ya too fella.  I've been opening the windows here at night and using a fan to draw the air in, but last night the humidity was so bad I still couldn't sleep. 

gecko10x

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Re: Cooling- what's most efficient?
« Reply #2 on: June 19, 2012, 12:54:30 PM »
Haha, yeah, I'm about an hour south of you.

Forgot to mention that we have a heat pump; this may affect the answers that I receive.

grantmeaname

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Re: Cooling- what's most efficient?
« Reply #3 on: June 19, 2012, 02:05:09 PM »
It's better to cool the house when you need it than to cool the house and let it sit, cold, with the heat leaking out all day.

We leave the windows open from as soon as outside is cooler than inside at night until it's time to leave for work in the morning. We'll only run the AC in the afternoon/evening if the house is just crazy hot (like if the night only got down to 75, and so the house is at 84 in the afternoon). We don't run it strongly-- we set it at 82-- but it really, really helps with humidity.

The other thing I'll do a lot is run the AC blower but not the air conditioner. It brings all the cool air up for the basement, which is as nice as the AC running--until there's no cool air left.

tooqk4u22

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Re: Cooling- what's most efficient?
« Reply #4 on: June 19, 2012, 02:28:28 PM »
Option B is the best way if you are home.  If not don't turn on the A/C until just before you get home with programable thermostat.  My house tends to retain the coolness until about early afternoon then it starts heating up.

I recently installed a whole house fan and it has been awesome - haven't used A/C yet this year but it is supposed to get real hot over next few days so that may change but hopefully I will only need to turn the A/C on for a few hours.   

Kriegsspiel

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Re: Cooling- what's most efficient?
« Reply #5 on: June 19, 2012, 06:20:39 PM »
Haha, yeah, I'm about an hour south of you.

I grew up about 2 hours south of here.  I was definitely more acclimated prior to leaving the area for 4 years.

velocistar237

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Re: Cooling- what's most efficient?
« Reply #6 on: June 19, 2012, 06:46:03 PM »
The problem with the whole house fan or windows open at night strategy is that it only works in dry climates. Otherwise, when the temperature drops at night, the humidity shoots up to past the general recommended point of 60% RH. If you want to have an efficient system, look into installing a dehumidifying heat pipe in your A/C so you can set your thermostat at 80+ degF and still have sufficient dehumidification.

gecko10x

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Re: Cooling- what's most efficient?
« Reply #7 on: June 20, 2012, 06:24:51 AM »
The problem with the whole house fan or windows open at night strategy is that it only works in dry climates. Otherwise, when the temperature drops at night, the humidity shoots up to past the general recommended point of 60% RH. If you want to have an efficient system, look into installing a dehumidifying heat pipe in your A/C so you can set your thermostat at 80+ degF and still have sufficient dehumidification.

Thanks, I'll check it out.

grantmeaname

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Re: Cooling- what's most efficient?
« Reply #8 on: June 20, 2012, 07:13:14 AM »
Otherwise, when the temperature drops at night, the humidity shoots up to past the general recommended point of 60% RH.
This sentence pretty much exactly describes what happens every night at our place. I've heard the recommendation but not the rationale. Care to shed some light on why it's important? (I believe you that it's true, of course, but I'm always looking for the reason why...)

gecko10x

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Re: Cooling- what's most efficient?
« Reply #9 on: June 20, 2012, 07:31:40 AM »
Otherwise, when the temperature drops at night, the humidity shoots up to past the general recommended point of 60% RH.
This sentence pretty much exactly describes what happens every night at our place. I've heard the recommendation but not the rationale. Care to shed some light on why it's important? (I believe you that it's true, of course, but I'm always looking for the reason why...)

I believe it is based on mold/mildew growth.

James

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Re: Cooling- what's most efficient?
« Reply #10 on: June 20, 2012, 07:50:32 AM »
I don't know that there is one perfect way of doing things every day, often it depends on the various conditions of heat, humidity, wind, and our activities that day.  We usually keep things closed up during the day and then turn the AC on once the house temperature gets up around 80.  I usually get the house down to around 78 right before bed when the cool temp is most desired, and then turn the AC off.  We don't always open the windows at night due to high humidity.  If the humidity is low then we will open the windows at night, and sometimes we'll open the windows all day even if it's hot outside if there is a good breeze.

velocistar237

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Re: Cooling- what's most efficient?
« Reply #11 on: June 20, 2012, 08:36:37 AM »
Otherwise, when the temperature drops at night, the humidity shoots up to past the general recommended point of 60% RH.
This sentence pretty much exactly describes what happens every night at our place. I've heard the recommendation but not the rationale. Care to shed some light on why it's important? (I believe you that it's true, of course, but I'm always looking for the reason why...)

I'll let the experts answer that one:
http://www.buildingscience.com/documents/reports/rr-0203-relative-humidity

I'd be interested in your interpretation; maybe in some cases we could get away with a higher number without adverse effects. My house is consistently above 60% when it's not winter, and I assume it has been that way for 100+ years. I've read on the Building Science site that after structural issues and fire, moisture is the number one threat to building integrity, and it can be a complicated issue.

I broke out the window A/C units today, and I might have to turn them on by the end of the day. I'm tempted to get central air when we replace our furnace in the near future, mostly to handle the humidity.

tooqk4u22

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Re: Cooling- what's most efficient?
« Reply #12 on: June 20, 2012, 10:41:05 AM »
Basically if you and your house feel stick/damp then it is too humid and you need to run the air/dehumidifier. 

trammatic

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Re: Cooling- what's most efficient?
« Reply #13 on: June 20, 2012, 01:29:50 PM »
So in general, there are two competing thermodynamic principles at work here:
1. The startup for an air conditioning compressor is much less efficient than the steady-state operating efficiency.  This is why heat pump ratings matter...otherwise, we could all buy 10 ton systems and have the house cool very quickly.  So the typical size is one that will cool the house to a comfortable temperature on the 98th percentile outside temperature.  So if it only gets above 100 on 7 days out of the year (98% of days less), then the proper sized system will cool your house to 75 if the outside is 100 by running continuously.
2. The greater the temperature gradient between inside and outside, the quicker the heat migration.

So I think, based on this, the ideal is to leave the house hot during the day, if you're not home (or acclimate somewhat if you are!) and then cool the house to a few degrees below your optimum temperature and then raise the system back to the high mark.

i.e. Set it to 80 when you leave for the day.  When you get home, set it to 65, and once it reaches 65, reset it back to 80 until the next afternoon.

I would say that if there is a time where the temperature outside is significantly cooler than inside, I'd let the windows open regardless of the humidity, and then once the house has cooled off, run the A/C at a degree or two under the ambient temperature to get rid of the moisture.

happy

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Re: Cooling- what's most efficient?
« Reply #14 on: June 20, 2012, 06:18:48 PM »
The humidity where I live is just about always above 60%. Really guys, you acclimatise. The more you use air the more you feel it. In summer we can get nights that don't go below 74, and then I too find it hard to sleep, but 65 overnight is sweet. BTW my house is not full of mould or any other noticeable hazard due to not being de-humdified. House is >50 yrs old and made of wood, No rot I swear.

Each house has its own quirks. My last house heated and cooled differently to this one and it was about 1km up the road, so a little experimentation is needed.

Generally since heat moves to cool, I open up the house as soon as it is cooler outside than inside and leave it that way until it starts warming up outside. I close up the house and blinds etc a bit before the outside temp = inside temp. I leave it all closed until its cooler outside than in, then open up.

My last house had a large skylight that could be opened right up  in the high point of the roof in a mezzanine study upstairs. Opening this was great, the hot air inside gushed upwards and out, drawing in cooler air downstairs. This is a good strategy to consider. In a 2 storey house opening the top floor windows also has the same effect to some extent.

My current house is 2 storey and there is a big temp difference between top and bottom floors ie in summer spend more time downstairs and in winter spend more time upstairs.

I recently retrofitted ducted air con and it has a fan setting and a dry setting and is zoned. The fan setting can be used to shift air from one part of the house to another to equalise temps eg in summer I can send the cool air downstairs to the upstairs. This costs much less than cooling and also introduces movement of the air which is cooling. In winter I use our wood fire, which is down one end of the house, and if needed move the hot air in that room around the house to rooms we are using. (the rest are closed off).

If the humidity really is way too high I use the dry setting (I'm guessing but >80-90%).

I only turn the cooling part on in the afternoon/evenings once we are home if the temp has been/is >85F and turn it off when I go to bed but will run it overnight if it looks like the temp is not going down below 74. Once you turn the aircon on, you have to close up the house and loose the free passive cooling.

Don't set an automatic turn on, turn it on manually, you'll save. Does you house really have to be precooled before you get home? It will only take an hour or two to cool down. You can stand a little sauna just for a short time.







kudy

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Re: Cooling- what's most efficient?
« Reply #15 on: June 20, 2012, 09:47:34 PM »
I've been opening windows at night since I moved into this house is '07 - I close them up in the morning, and the temp holds below 80 till later afternoon/evening.

Last year I went and bought a window A/C unit when we had a run of days where evenings didn't cool down until 1 AM; the problem with the window unit is, I can't have it in my bedroom because it's in the way of good airflow on nights where the temp does drop nicely.  I have it in a spare bedroom across the hall, but even with a series of box fans, only a fraction of the cool air seems to make it to my bedroom while it's running.

I've been looking at ductless mini pump systems, but their cost is insane! If I want cooling *in* my bedroom, I will have to either get the ductless system, or cut a hole and install a through-wall unit (much cheaper, but much bigger hole, and seems somehow less permanent and professional).

Mr Mark

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Re: Cooling- what's most efficient?
« Reply #16 on: June 20, 2012, 10:06:28 PM »
When i was a teenager I lived in Singapore for a while. We slept with no AC, but had ceiling fans on full blast and it was fine.

velocistar237

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Re: Cooling- what's most efficient?
« Reply #17 on: June 26, 2012, 07:12:34 AM »
BTW my house is not full of mould or any other noticeable hazard due to not being de-humdified. House is >50 yrs old and made of wood, No rot I swear.

I've had mold on the windows. Our house is old, and the structure itself is able to handle the humidity changes. Since we don't have central A/C, most of the structure doesn't get down to the dew point.

I'm more concerned with respiratory issues than comfort. I sneeze miserably, and humidity below 60% is supposed to keep the dust mites at bay.

gecko10x

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Re: Cooling- what's most efficient?
« Reply #18 on: June 26, 2012, 08:56:56 AM »
velocistar237, thanks for the great link!

So in general, there are two competing thermodynamic principles at work here:
1. The startup for an air conditioning compressor is much less efficient than the steady-state operating efficiency.  This is why heat pump ratings matter...otherwise, we could all buy 10 ton systems and have the house cool very quickly.  So the typical size is one that will cool the house to a comfortable temperature on the 98th percentile outside temperature.  So if it only gets above 100 on 7 days out of the year (98% of days less), then the proper sized system will cool your house to 75 if the outside is 100 by running continuously.
2. The greater the temperature gradient between inside and outside, the quicker the heat migration.

So I think, based on this, the ideal is to leave the house hot during the day, if you're not home (or acclimate somewhat if you are!) and then cool the house to a few degrees below your optimum temperature and then raise the system back to the high mark.

i.e. Set it to 80 when you leave for the day.  When you get home, set it to 65, and once it reaches 65, reset it back to 80 until the next afternoon.

I would say that if there is a time where the temperature outside is significantly cooler than inside, I'd let the windows open regardless of the humidity, and then once the house has cooled off, run the A/C at a degree or two under the ambient temperature to get rid of the moisture.

This is precisely the kind of answer I was hoping for!

So far, we have not done this, but I may try it and see how it the usage works out. I'm thinking it may be a good option, because so far, the usage looks like this on hot days:
https://www.dropbox.com/s/3v2l14b4moodn65/Photo%20Jun%2026%2C%2010%2047%2014%20AM.png

Kriegsspiel

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Re: Cooling- what's most efficient?
« Reply #19 on: June 27, 2012, 10:53:05 AM »
http://www.ted.com/talks/wolfgang_kessling_how_to_air_condition_outdoor_spaces.html

Don't know how much this talk will help anyone, but it was interesting nonetheless.

igthebold

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Re: Cooling- what's most efficient?
« Reply #20 on: June 27, 2012, 11:27:40 AM »
The problem with the whole house fan or windows open at night strategy is that it only works in dry climates. Otherwise, when the temperature drops at night, the humidity shoots up to past the general recommended point of 60% RH. If you want to have an efficient system, look into installing a dehumidifying heat pipe in your A/C so you can set your thermostat at 80+ degF and still have sufficient dehumidification.

I'm interested in this, but am finding precious little about how to actually do it. The article you linked to just talks about how good it is. Any ideas how to DIY?

mechanic baird

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Re: Cooling- what's most efficient?
« Reply #21 on: June 27, 2012, 12:58:01 PM »
Here is another alternative for you guys...Learned from my childhood from 3rd world country where AC didn't exist

bamboo or grass sheets for sleeping.

Instead of sleeping in your normal cotton sheets, get bamboo sheets and put it on top of your bed. It feels very cool to the skin and helps you to sleep comfortably. Humidity is still a bitch to deal with if you are not in dry climate. But for anyone who is lucky enough to be in the dry climate, this trick basically eliminates the needs for AC at night.

Jamesqf

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Re: Cooling- what's most efficient?
« Reply #22 on: June 27, 2012, 01:16:18 PM »
How about a real low-tech energy-saving option?  Spend more time outdoors in the evening.

crunchy_mama

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Re: Cooling- what's most efficient?
« Reply #23 on: June 27, 2012, 01:19:43 PM »
We open the windows at night and put box fans in the windows.  I shut the windows, curtains and blinds in the am- around 8-9 am, depending on the day.  I have the thermostat set at 82 during the day and 80 at 4pm when my husband gets home.  Even on the hot days (it has gotten up to 101 so far) so far it generally doesn't kick on until 2 pm at the earliest and often not until 3:30 or so.  It seems a lot depends on how cool it gets at night moreso then how hot it gets during the day.  So, our air runs for about 6-7 hrs or so and then not continuously.  I'm sure come later July and August when the humidity shoots high and the night temps don't drop as much there might come a time where it wouldn't be worth it to open the windows at night but for the time being it is well worth it for us with it getting down to 70 or lower at night(historically even in July our average low is still 69 but of course that varies from day to day) .  My husband has on his to-do list something to block our fan from bringing upstairs in to help w/ bringing in the basement air.  I'm hopeful if we are able to do that we can knock our ac use down even more.

gecko10x

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Re: Cooling- what's most efficient?
« Reply #24 on: June 27, 2012, 01:42:47 PM »
http://www.ted.com/talks/wolfgang_kessling_how_to_air_condition_outdoor_spaces.html

Don't know how much this talk will help anyone, but it was interesting nonetheless.

Cool, thanks!

velocistar237

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Re: Cooling- what's most efficient?
« Reply #25 on: June 28, 2012, 11:33:45 AM »
dehumidifying heat pipe

I'm interested in this, but am finding precious little about how to actually do it. The article you linked to just talks about how good it is. Any ideas how to DIY?

I didn't find much, either. Online, I mostly saw discussions on an HVAC forum, and they mentioned that the commercial solutions were pretty expensive.

igthebold

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Re: Cooling- what's most efficient?
« Reply #26 on: June 28, 2012, 12:01:20 PM »
dehumidifying heat pipe

I'm interested in this, but am finding precious little

I didn't find much, either.

Other discussions seemed to indicate that the performance would be marginal, but that in theory someone could design one for residential. Dead end for now, I guess.

Anybody here use a whole-house dehumidifier to make your place feel cooler? Looks like I'll be out $200 retail if I go that route, but I'd probably be able to raise the temperature a bit.