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I know exactly what they are. What I am saying is that if they are inefficient and they are taking precedence in your system, then I can't help you (except to tell you that you have a terrible system).
And it's complete nonsense to say your AC can't recover from 80 to 72. What does this even man? You just need to have it kick on sooner.
And I don't know too many people who complain at 78 degrees with a ceiling fan on. That makes it feel several degrees cooler.
It drives me nuts when I see people complain about their power bills in the summer when they have their thermostats set to 72 4 (or even lower) and aren't running the fans in the room they are in. Literally just throwing money away.
...And again, it doesn't really matter what your comfortable temperature range is. You can still set your thermostat to cut off if you are leaving for several hours and turn back on such that when you return, your home is back in the comfortable range.
TIL everyone around here likes their house really cold. I was out a while and am just catching up on this "heated" discussion.
Me, I'm good and cool with 75 or 76 degrees (and shorts). I'm less than average body fat, though, so I do well with heat. The body will adjust to new temperatures, though. When we first hit 80 in the spring, I'm dying in the heat. When we get the first cool front of the fall, and the high is 80, it seems icy cold.
Got a Nest and it payed for itself very quickly. I let the house go to 85 when I'm out, but it's nice and chilly when I get home. Utility bills dropped a lot, but that's just anecdotal evidence. Humidity was a concern, but it never gets over 55% in the heat, and I've noted no adverse effects over several years.
I'm thinking a lot of people have undersized heat pumps, which may explain the horror stories about taking hours to cool down.
Then kick it on an hour early. Or however much earlier it needs to be such that your home is a reasonable temperature when you arrive (say, 80, and on the way down). Also, lol at 95% humidity and 98 degrees. That doesn't ever happen.
Lots of untrue statements in here. If you add up the amount of time the air conditioner will run to keep your home at 75 all day compared to the amount of time it will take if you let the home heat up to 85 or 88 and then kick the AC on when you return home, you will find it will run significantly more by trying to maintain 75 all day. So no... your air conditioner will not be working "much harder" by turning it off when you aren't home.
False. The humidity could be detrimental, that I will grant you... but really, probably not. As for taking it to 75-76 during the day when you aren't home? Geez. I keep mine at 76-77 when I am home. I am consistently amazed at how narrow people keep their thermostats. In the winter, I am fine at 63-65 degrees. And in the summer, I roll in the 76-78 range.
You wrote lots of false information. And you could save money by turning it off in the summer and winter when you leave..
Where are you getting your information? There is tons of research that will show a system works much more efficiently at maintaining a temperate than trying to constantly adjust. This should be common sense. Call around to some reputable HVAC companies and then post your results back here.
As a comparison, when does your vehicle get the best fuel mileage, in the city with stop and go driving or on the highway maintaining one constant speed? It becomes more efficient the longer it runs, right? Most mechanical objects work like this. An HVAC system is no different. It's easier to manage the heat in the walls than to let it build and then try to remove it all at once.
I performed a test on our system this weekend while working on my bonus room to make the temperature manageable without installing a split unit.
Perform a test. Set up a log book. Pick a Saturday and get up early. Set the AC to a constant temp and then record how often and how long your unit has to run to maintain that temp. On Sunday, get up and raise the temp. Record how often your unit kicks on. Then about 4pm, drop the temp and record how often and long the unit runs.
Here are the results that I got from mine when doing just that over this past weekend. (7am-8pm)
Unit: Rheem 4 TON RQNL - 3 phase only package unit
Installed July 2015.
House: 2,840 sq.ft
Brick construction
Insulation - R-15 fiberglass batt insulation
Ceiling insulation - Blown in rockwool - 13" deep (R-38 value)
4 attic fans to remove heat
Saturday -
Thermostat set to 73 degrees at 7am.
9:08 am - unit powers on. Runs for 14 minutes. Powers off.
11:13 am - unit powers on. Runs for 19 minutes. Powers off.
12:45 pm - unit powers on. Runs for 22 minutes. Powers off.
2:36 pm - unit powers on. Runs for 38 minutes. Powers off.
3:04 pm - unit powers on. Runs for 33 minutes. Powers off.
4:31 pm - unit powers on. Runs for 25 minutes. Powers off.
6: 19 pm - unit powers on. Runs for 17 minutes. Powers off.
8:05 pm - unit powers on. Runs for 12 minutes. Powers off.
Total run time - 3 hrs.
Sunday-
Thermostat set to 78 degrees at 7am. Thermostat lowered at 4pm to 73 degrees.
10:02 am - unit powers on. Runs for 6 minutes. Powers off.
12:18 pm - unit powers on. Runs for 10 minutes. Powers off.
1:05 pm - unit powers on. Runs for 8 minutes. Powers off.
3:31 pm - unit powers on. Runs for 11 minutes. Powers off.
4pm - unit powers on. Runs for 2 hr, 18 minutes. Powers off.
6:15 pm - unit powers on. Runs for 34 minutes. Powers off.
7:08 pm - unit powers on. Runs for 18 minutes. Powers off.
7:58 pm - unit powers on. Runs for 14 minutes. Powers off.
Total run time - 3.98 hrs.
My unit ran for almost an hour longer than normal yesterday. If I were to do this every day for 30 days, that's an estimated additional 30 hrs of run time on the unit.
Nlambert, not saying I don't trust your figures but...
"4pm - unit powers on. Runs for 2 hr, 18 minutes. Powers off.
6:15 pm - unit powers on. Runs for 34 minutes. Powers off."
So, let me get this straight. Your unit was running TWICE during the period between 6:15PM and 6:18PM! The 4PM start didn't end until 6:18 and the 6:15 start happened at 6:15, meaning something REALLY weird is happening at your place. Do you dance the "Time Warp" every day with Frank N. Furter?
Not only are your figures off, your logic is suspect as well.
First, the heat load can vary tremendously from day to day based upon outside temperatures and the amount of sunlight striking the building. Unless you had two days with exactly the same temperature curves and insolation, your comparison is too small a sample and suspect.
Second, your "steady state" hypothesis falls flat when you push it to extremes. Example: Say you take a one week vacation and you have a choice of leaving your AC at 73 or cranking it up to 85 for the days you are gone and then bringing it back down to 73. Which do you think will use more KWH of power?
Third, it is well known that the startup phase is the most stressful to electric and electronic equipment. The amperage draw spikes, lubrication is not yet being pumped, and capacitors are stressed. Constantly running equipment runs without failure for more hours than intermittently run equipment. Failure modes change depending on usage characteristics.
As for the industry... remember when there were no "set back" thermostats? Remember the hoopla on how those would save money by minimizing HVAC use when you were asleep or away?
I had to program some of the first Johnson Controls energy saving computers used in movie theatres, so I got first hand experience with what goes on in different situations. If an AC isn't running enough during off periods in the south, humidity will build up if there are any sources of moisture, such as mopping floors, doors open to the outside, cooking, etc. It takes time to remove that humidity, and the first few times the unit reaches the set point, the air is cooled to the proper temp, but still loaded with moisture, making it feel either still to hot and humid or clammy like a damp basement if the set point is a few degrees lower to compensate.
As long as there are no sources adding humidity, energy use by powering down and then back on when you come home will be less. That is simple physics. If the home has infiltration, that excess humidity will have to be removed either way, but the temptation may be to turn the 'stat lower than normal at first to get comfortable until the humidity is removed.
Understand that AC is properly sized to just barely be able to keep up during 100 degree temps. The real reason why HVAC companies want you to keep them running is to eliminate needless service calls and complaints.
Nlambert, not saying I don't trust your figures but...
"4pm - unit powers on. Runs for 2 hr, 18 minutes. Powers off.
6:15 pm - unit powers on. Runs for 34 minutes. Powers off."
So, let me get this straight. Your unit was running TWICE during the period between 6:15PM and 6:18PM! The 4PM start didn't end until 6:18 and the 6:15 start happened at 6:15, meaning something REALLY weird is happening at your place. Do you dance the "Time Warp" every day with Frank N. Furter?
Not only are your figures off, your logic is suspect as well.
First, the heat load can vary tremendously from day to day based upon outside temperatures and the amount of sunlight striking the building. Unless you had two days with exactly the same temperature curves and insolation, your comparison is too small a sample and suspect.
Second, your "steady state" hypothesis falls flat when you push it to extremes. Example: Say you take a one week vacation and you have a choice of leaving your AC at 73 or cranking it up to 85 for the days you are gone and then bringing it back down to 73. Which do you think will use more KWH of power?
Third, it is well known that the startup phase is the most stressful to electric and electronic equipment. The amperage draw spikes, lubrication is not yet being pumped, and capacitors are stressed. Constantly running equipment runs without failure for more hours than intermittently run equipment. Failure modes change depending on usage characteristics.
As for the industry... remember when there were no "set back" thermostats? Remember the hoopla on how those would save money by minimizing HVAC use when you were asleep or away?
I had to program some of the first Johnson Controls energy saving computers used in movie theatres, so I got first hand experience with what goes on in different situations. If an AC isn't running enough during off periods in the south, humidity will build up if there are any sources of moisture, such as mopping floors, doors open to the outside, cooking, etc. It takes time to remove that humidity, and the first few times the unit reaches the set point, the air is cooled to the proper temp, but still loaded with moisture, making it feel either still to hot and humid or clammy like a damp basement if the set point is a few degrees lower to compensate.
As long as there are no sources adding humidity, energy use by powering down and then back on when you come home will be less. That is simple physics. If the home has infiltration, that excess humidity will have to be removed either way, but the temptation may be to turn the 'stat lower than normal at first to get comfortable until the humidity is removed.
Understand that AC is properly sized to just barely be able to keep up during 100 degree temps. The real reason why HVAC companies want you to keep them running is to eliminate needless service calls and complaints.
My apologies. Admittedly, that was my mistake on my times. I was manually typing in the times from my notebook and made a clerical error.
4pm run time was *1 hr, 48 minutes* (I should have checked this twice instead of punching from my number pad without looking).
That brings the total run time to 3 hrs, 48 minutes. Which is 48 minutes longer than a constant temp. Over the course of a month that's 24 hrs total additional run time. Thanks for catching that.
I have a reasonable understanding of how the systems work (but no, I'm not a HVAC tech). I've got a few friends/relatives in the business that perform service calls daily which are some of the people I have discussed this with. They get no benefit from me, as they don't live here nor do they get my service calls. This was their recommendations along with the research I've done (both personal research as noted above plus reading other articles).
The sample is small because this requires a lot more time than I can devote to it. That being said, conditions were very similar both days. The high for Saturday was 97, with the high yesterday reaching 95. Both days saw about an hour of rain around 3pm-4pm. We didn't cook inside this weekend (at all) so no humidity from cooking.
So if I understand correctly, your statement is that it takes less energy to remove stored heat from the insulation in the house if you change the temperatures during the day than it would require to keep the heat from ever soaking into the house?
Neighbor keeps selling me on the nest. She says her electric is about 20 bucks cheaper compared to last year without. Just not sure it's worth it. I know Friday when I got home at 5 it seemed my AC struggled just to maintain 76.
I probably run my air less than 99% of people in Alabama who have central air. I often keep my air set at close to 80 even when I am home. I do plenty of good.
Good for you. Some of us would prefer not burning up. I more than make up for any extra energy usage from about October until April because I don't run the heat much at all.
Quote:
Originally Posted by steveklein
If you call knowing science and using that to draw conclusions arrogance, then so be it. What I am belittling is people giving poor advice. Some people actually believe that turning the AC off for 8 hours and then "having to run it full blast" uses more energy (and thus costs more money) than keeping it steady all day. I am just dispelling that myth.
Except you aren't dispelling anything. I've actually tested that theory before and turning the AC off during the day while I wasn't home actually made my bill higher than just setting it to a couple of degrees higher than my preferred temperature (which means I set it no higher than 74). I was also much more comfortable as I didn't have to wait 6-8+ hours for the AC to catch up and cool the place down.
Here are the results that I got from mine when doing just that over this past weekend.
What was the overnight set temp? 73?
Did you start at 73 degrees for test 1 and 77 for test 2? If so, your run times don't take into account the energy required to lower the house from 77 to 73.
I know exactly what they are. What I am saying is that if they are inefficient and they are taking precedence in your system, then I can't help you (except to tell you that you have a terrible system).
And it's complete nonsense to say your AC can't recover from 80 to 72. What does this even man? You just need to have it kick on sooner.
And I don't know too many people who complain at 78 degrees with a ceiling fan on. That makes it feel several degrees cooler.
It drives me nuts when I see people complain about their power bills in the summer when they have their thermostats set to 72 4 (or even lower) and aren't running the fans in the room they are in. Literally just throwing money away.
...And again, it doesn't really matter what your comfortable temperature range is. You can still set your thermostat to cut off if you are leaving for several hours and turn back on such that when you return, your home is back in the comfortable range.
We leave ours at 76 until bedtime and then we set it to 74. I sleep better when it's cold. Preferably 70 or less.
Except you aren't dispelling anything. I've actually tested that theory before and turning the AC off during the day while I wasn't home actually made my bill higher than just setting it to a couple of degrees higher than my preferred temperature (which means I set it no higher than 74). I was also much more comfortable as I didn't have to wait 6-8+ hours for the AC to catch up and cool the place down.
It is easier for heat to enter your home when there is a big delta T (difference in temperature) between inside and outside. If more heat is entering your home, then your air conditioner will have to do more work.
For instance, if it 90 degrees outside and the air conditioner inside your home is not running, your home will warm from 68 to 73 degrees quicker than it would warm from 83 to 88 degrees.
This is settled science and has been for decades.
This also means that when you are cooling your home back down, going from 85 to 80 degrees is going to be faster than 77 to 72 because when the home is in the 80s, the delta T is lower so less heat is coming into your home.
I am sorry that the laws of physics aren't enough evidence for you.
If you are waiting 6-8 hours for your AC to catch up and cool the place down, then your system is not working as intended for the size of your home. My home cools from 88 to 76 in just over an hour, even when the sun still out and temps in the upper 80s to low 90s.
As for the person who ran his test... you may think the variables were about the same, but do you really know what percentage of the day had cloud cover, how much the temperature dropped with the rain, when that drop occurred, etc... All of these factors would significantly change the results of your test.
I am sorry to tell you, but you will need to rely on science. You don't have to jump off two differemt buildings to determine how fast you will be going when you hit the ground on each one. It is simple physics. Heat transfer is a little more complicated, but when you work everything out... you will find my statements hold water.
Also 2 hours to cool from 78 to 73 is not that good, even in the afternoon heat.
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