Leave the Air-Conditioner ON or OFF for Savings?

[washer111]

Is it better to leave the air-conditioning on, rather than turn it off during the day? I've heard both sides of the argument... People say it saves energy, because your house will only absorb so much heat, and won't get as hot as you think and that air-conditioners work best at 100% capacity (Correct me if I'm wrong, but most units are running at 100% power, unless they are an "inverter" type), and apparently, your house is like a "heat magnet" when it gets cool inside.

But the thing most people seem to be "forgetting" in these discussions in the interior walls, floors, ceiling and furnishings. If you let your house go from 77ºF/25ºC to 86ºF/30ºC (which it does) during your time at work, and only run the A/C at night, then you are wasting TONS of energy cooling the EVERYTHING inside the house down. For us, that is a big thing, because our house doesn't have insulation (rather some "plastered" brick walls with no insulation). Our only insulation is in the ceiling. So basically, what I'm saying is, the 15 hours our A/C is on, the first few hours will be spent with the unit trying to cool the air down, then accept the heat load from exterior walls and from the interior walls (and believe me, our interior walls are pretty thick!), then the rest is keeping the place cool. The unit cycles a fair bit during the first 5-8 hours, even in weather when its not particularly hot. The house is closed all day too. (We have a ducted A/C, by the way).
My opinion is, the short time the house is A/C'ed, we are wasting energy, since the house isn't fully cooled down (and turning the unit off basically sends the temperature back to where it was {or within a few degrees of} where it started).
Of course, the economics (research) suggests it would be more expensive to run the air (especially an older unit like ours) 24/7, but my opinion is the large amount of thermal mass contained in those inner walls and furnishings would really help keep the place cool, if they were given the proper amount of time to actually cool off in the first place. The air only cycles twice in every hour overnight too (we keep a setting of 27ºC/80ºC) - totalling around 15-20 minutes per hour running time.

What is the opinion of the member's of Automaticwasher.org? Knowing what you guys are like, I'd probably assume the attitude would be "Leave it on, who cares about the government!" So I'd people's unbiased opinions on this subject, leaving your bias out, if possible (I have the same attitude too, but, of course, telling your parents "Its more efficient," then ending up with a $1,000,000,000,000,000,000,000,000,000 power bill really doesn't do much for your trustworthiness, so thats why I'm asking here!)

Thanks for all help, in advance!
(And Sorry for the long winded post...)

 

[wayupnorth]

I keep mine off and only put it on before going to bed, as needed, as my only one is in the bedroom. But here in Maine we are running heat now. We did have a rather mild and somewhat damp summer without alot of humidity and my August power bill was $46, which is usually the warmest month running a/c. Normally I keep the windows cracked open for ventilation and shades over windows exposed to sun closed when I am gone all day.

 

[xraytech]

we run our central A/C 24/7 as our theory is that once you get the house cooled down it will cost less to maintain that temperature, since the A/C wont have to run as long to keep the temperature

 

[strongenough78]

I say if the night time temperatures where you are at are still pretty warm at night, I would leave it on. However if it's nice and cool at night, but warm inside your house, as most places get, my apartment still gets like that, I would open a few windows and place a fan or 2 in front of them to pull in the cool air. The past couple of weeks, we have had some nice night time weather here, yet when I go outside, I can hear people still running their ac units. I try to keep mine off, as long as my brother doesn't run and turn it on the moment it get's to 80 degrees inside, I then open my bedroom window with my fan a few feet in front of it and it cools my room down quite nicely. But seeing that you are in Australia, am I wrong in thinking you are going into your summer months? If you are, and the nights are going to get warmer, I would keep the ac set to where it is at least comfortable without having to run the unit as much. What I do during the summer months here is keep it at 80 to 82 during the day if no one is home. So that way when I come home or Mom or Ben, we can turn the unit to 78 and leave it there for the remainder of the day. It doesn't take as long to cool down the apartment and it doesn't cycle on and off as much. The only rooms we have trouble keeping cool are the bedrooms with west ward facing windows that bring in the afternoon heat. I keep my fan near the ac vent to accelerate and circulate the ac air better. Other wise we have to turn down the ac to 75 to keep the rooms at at least 82. THey can get up to about 90 during summer afternoons even when the living room is at 80 or even 78. So in your case, I would keep it at 80 at night with a good fan on. and 78 during the day when you are home if you so desire. And keep fans near the vent to push the air around. I hope all my babbling helps LOL. But that's what I do to keep the place comfy day and night with minimal ac usage.

 

[dustin92]

We run ours constantly when the weather is hot, because our house holds heat and will take at least two days to cool off enough for the a/c to shut off. It is better to keep it cool for a week or so at a time.

 

[sudsmaster]

Well, I can't say I run the A/C here much if at all, but that's because the climate is mild enough not to require it for more than a couple of weeks a year. During the hot spells I just run the fans all night to bring in the cooler night air, then button up the place during the day. Or I just live with the warmer temps, and dress (or undress) accordingly.

 

But thinking in terms of physics, I don't think you're going to save any energy dollars by running the A/C 7x24. The walls are going to absorb the same amount of heat during the day whether or not the A/C is running. The A/C will just help absorb that heat from the walls and put it back outside. If it takes hours for the A/C to make the place comfortable when you get home, then set the timer to bring it on a couple hours earlier.

 

As far as insulation goes... the walls here aren't insulated either, but they are mostly wood and plaster. I did find a major improvement in summer comfort during hot days by adding maximum insulation to the attic. It's about R38 up there now. I also added more ventilation to the roof: ventilated the enclosed soffits. Before I did all this, I could feel the heat radiating down from the attic on a hot day. Now, not so much if at all. Next I'll be finishing off insulating under the floor over the crawl space. This also can help keep outside heat out on hot days. And of course it keeps the place more comfy on cold nights.

 

So... you didn't say how much insulation is in your ceiling, but I suggest that you max it out, and make sure your attic has good ventilation. Roofs can absorb a LOT of heat and radiate it down to the living space. I'm guessing they absorb way more heat than the walls, due to the fact that they are facing the sun. Lots of attic insulation combined with good attic ventilation will help keep the living quarters cooler.

 

 

[frigilux]

My house (built in 1963) isn't well insulated, nor are the windows energy efficient; and if I let it get hot and humid during the day, it takes a long time to cool it down again. I set the thermostat at 73 degrees and leave it there 24/7 during the summer.

In southern Minnesota, we get maybe 2-3 weeks in the fall and another 2-3 in the spring where no heating or a/c is required. Otherwise, it's full-time furnace or full-time a/c. It's not a place to live if energy bills scare you.

We get the heat and humidity of the deep-ish south one season and the deep-freeze/snow of the arctic the next. Why we haven't the brains to move elsewhere is beyond me. Plain stubbornness, having been born and raised here, I guess.

 

[whirlcool]

Down here in SE Texas we leave the A/C running 24/7. The humidity here is a killer.
But the house is so well insulated that it cools down overnight and even on the hottest days the heat in the house doesn't build up until about 10:30-11:00am. We keep the thermostat set at 76F. Out biggest bill this summer was $110.00.

When I was growing up in the Chicago area my parents had a Vornado reversible fan.
After sunset they would open all the windows in the house and have the Vornado in a window blowing outwards from the house to pull all the hot air out and pull the cool air in from the open windows. It worked really well. In fact by 11:00pm or so you'd better have a top sheet and lightweight blanket to stay warm with.

If we tried that down here everything in the house would feel "damp". When you walk around the house your feet stick to hard flooring, the furniture would feel "wet".
Yuck.

 

[washer111]

Thanks for all the replies.

In regards to heat from the walls, the interior walls should stay cool once cooled down. The exterior walls, of course, shall never cool until the weather actually starts to cool down.

We can't add any more insulation to the ceiling, since the roof and ceiling practically hug one-another, there is no room for anyone up there (Low ceilings/roof) and there isn't an access point. Full-Stop!

The insulation in the ceiling is a 2-3" thick, but its the stuff of the 80's (Or 60's, depending on whether the old owners decided to fix that up whilst refurbishing this house back then).

I've done some more maths, and based on how much the unit will run, it will still cost more (even after I corrected my mistakes). So I think, effectively, this issue isn't going to get any better unless we bite the bullet and get a new ducted system (or replace the unit). The 28 year old has completely lost all of its performance since the 2010-2011 season (So yes, Summer is around Christmas in this country), and barely blows air colder than around 65º. Yes, we've had it checked. A new Charging point cap, fan motor and more R-22 later, the unit still isn't performing... Time to hit the bank (lol!)

 

[daveamkrayoguy]

I've heard if I leave the A/C totally off that it would cause my refrigerator to run more, consuming even more power, and of course likely to wear it out more...

(Should really look at my electric bills, but I have my own "standard practice" based on my above theory, that I try not to let anything else matter...)

So I found a reasonable temp. to leave it on & just turn it up (temp DOWN, actually) when I get home... I never leave windows open...

As for heat, I would have it on just so my pipes won't freeze & just turn it up when I get home...

I find having the heat up when we sleep & down when we are awake (we can always wear sweaters & create our own heat & we usually get out of the house then anyway) also makes for low gas bills!

-- Dave

 

[sudsmaster]

Well it sounds like your home construction is very energy inefficient so you're going to be running the A/C a lot no matter what.

 

There may be some things you could do to improve the situation, such as ventilating the limited attic space if it's not already adequately ventilated. Installing heat reflective roof covering. Heat reflective paint on the outer walls, as well as heat reflective coatings on the windows. 

 

It's also possible that more attic insulation could be blown in to reduce heat transfer to the living space. But it's difficult to diagnose sight unseen. Have you considered bringing in a consultant to assess where the heat is coming and going?

 

 

[washer111]

We haven't considered any consultants. To be honest, I don't think any would be in our area (remote location, low population).

We cant ventilate the roof either. Basically, there is ceiling --> no gap --> insulation --> no gap --> roof (Sorry!)

We were going to get the proper paint, but found it was too expensive (yeah, it would pay off, but we didn't have the funds).

And setting the temperature up/down by 2-3º (C), that is extremely wasteful, if you leave the system on, setting up and down means the system has to play catchup, and will waste power. Setting it to your "breakpoint" or just below means the house is cool, but not so much you have to set it warmer each time you leave.

And Re: Inefficient design: This house was built in the late 60s. The people who built it wanted a good, long-lasting and sturdy home to live in. So it is pretty basic when it comes to "efficiency." So thats not our fault, by any means. We favoured this design for its better construction over other houses.

 

[norgeway]

We set our heat pump on...

70, summer and winter, as hot as this summer was, the highest bill was 140.00, I was pleased with that.I have allergies so I NEVER open a window!

 

[volsboy1]

Every thing I have read say to leave it running because of heat soak but if you have a programmable T-stat then you let it go up about 2 degrees when your at work and back to normal before you get home but I am talking about my system.We leave are's set and 72 and we have 18 Seer. American Standard Heat pump and a 20 Seer Mitsubishi Opti-cool heat pump.The power bill here when we had that week of 105+ in late May or June was 210 bucks.I have been turning off the big heat pump and just using the Mitsubishi and leaving the main unit on

circulate and it keeps the house pretty even.The motor in my air handler  uses 60 watts on circulate mode, not much air comes

out but enough to keep from using the the main unit.When it gets real cold or snows like it did two years ago here our bill

was 350+ but I fixed that buy putting a relay on the E.M. heat to only come on when I hit the button.Your best bet would get a new

unit.Mitsubishi Opti-cool Heat pumps are great if your short on space for duct work and they will heat well below 0F and cool just

as well.My neighbor had a working 1981 York package heat pump and it never stopped running and her power bills were always

around 300 bucks and it's just her and her husband.When we replaced our unit she did the same and her bill dropped about 150 bucks a month.Also,I see a lot of people here when they have older homes that lack insulation in the walls use that foam that

expands and fills every gap.They put it in by just drilling a few holes in the wall my best friend did it to her house but she is

a typical lesbian LOL and does all her own work.It did help alot keeping the house warmer and cooler we took temp measurements before and after and on that side that faced the sun and it dropped over ten degrees in the summer.

 

[DADoES]

 
Always, less power is consumed when an electrical device is off.

 

[DirectDriveDave]

House Siding

When this was changed on the house the A/C has run a lot less than it used to. The first floor thermostat, in particular, is set to 74 during the day. With our old siding (cedar) if got up to about 90s or higher outside, even with the A/C running the thermostat would work it's way up to 77F ambient temperature, and it would run constantly.

After the siding was replaced and had new insulation outside in 2010, there was a huge difference. There were those few summer days where it got to the triple digits outside. This time during the A/C months the Thermostat, still set at 74 would go no higher than 75 and it was still good enough to make it actually cycle on and off as needed instead of just running constantly.(though, I know not all digital thermostats work this way)

 

[washer111]

New Unit

I wouldn't touch Mitsubishi with a 10ft pole. I'd rather Daikin or "Actron-Air."

To let you all in on a secret, I'd import a Trane from the US, since they are the only company that wouldn't force some 100% generic thermostat and 3 square mile air-handler onto you. At least their Hyperion air-handlers would fit nicely in at least 2 of our cupboards (located near where our current package system inputs into the main duct).

In fact, our system is equivalent to 8 SEER. Ouch. Apparently, two 20x20 returns are not enough for a 5-ton system, coupled with the fact the fan is running WAY too fast (we "balanced" the air vents, now the big lounge one whistles loudly, whereas other houses with a similar design do not, even when set up the same). Those two small returns would explain why the Fluro lights in the Hallway lift-up when the system is on (THe returns empty air into a large cavity. The hallway ceiling is low. The lights live in that cavity, and the main duct is also there. The unit sucks air from here - so it would be VERY easy to implement a new system).

 

[Iowegian]

Check out a psychrometric chart.

Dry air is easily cooled; moisture-laden air isn't.

Most utility companies will recommend that you adjust your cooling setpoint upward while you're away from home.

People that live in dry areas can cool their homes with a swamp cooler - introducing water vapor to cool the dry air. That's inexpensive, but it doesn't work in a lot of places.

 

[arbilab]

There's air heat, and there's infrared heat. Air heat is what a thermometer reads but you feel infrared heat.

To take infrared heat out, air temp has to be below infrared temp over time. Infrared heat is what poorly-insulated ceilings do. I've had several.

See also above, dry air is easier to change temp than wet air. Is Australia dry? What's your dewpoint? Before the air can do much about temp it has to pull the dewpoint down. So don't turn it OFF, that lets indoor dewpoint rise. Unless it's already low, then that doesn't matter.

Infrared builds itself into walls and fittings over time. Takes time again to remove it. You're pretty much stuck either way.

The system is somewhat more efficient--lower head pressure, less current--when it's cooler outside. So pulling down SOME built-up heat after sundown doesn't really cost more.

There are so many variables it goes well over my math head. I'd let it warm some daytime, specially if you're not home, but not extremely. Then pull it down to comfort after sundown.

Even though it's typically 100-105F here in summer, I never leave the air on overnight. Once you pull out the infrared you can turn the thing off and there's nothing cheaper than 'off'.

Simple, innit?