I am wondering if it is better to leave a central a/c unit running during the day when no one is home, either at the normal temperature or at a few degrees warmer -or- use a set-back thermometer when leaving for work in the morning and have the A/C unit start up again an hour or 2 before coming home later in the afternoon?
I use a set-back thermometer and it takes my unit 3 hours of continuous operation to bring the temperature down 8 degrees.
Some people have told me that it makes more sense, energy wise and cost wise, to not set the thermostat warmer in the morning and have the unit struggle for hours on end in the afternoon trying to get back to the original comfortable temperature. Instead they say to leave the A/C on as it would probably cost the same if not less to periodically cool an already cool house instead of cooling a house that is not cool at all. Con Edison says to turn off the A/C when no one is home but I think they refer to window units (as they also say to turn the AC back on again via auto-timer a half hour before returning - a half hour would do nothing for me)
Starting and stopping are not as efficient as running continuously. AC units are designed to run continuously. Also the house will gain less heat during the day as it is warmer so you will be removing less total heat. When the house is unoccupied it will have no moisture source vs. when you are home.
In your case it is a toss-up. All of my units are so oversize, it is like when you check into a motel and in an hour it is livable. So in my case a set-back works well. I have one thermostat that learns, if it runs too much to make its goal, it will start earlier the next day regardless of the set time for run up to normal. That one is a Honeywell I believe.
I have one of those "learning" thermostats that will turn on the air (or heat) ahead of time. So if I program it to be 70 degrees at 5pm, it will turn on ahead of time so that at 5 pm it is 70 degrees. However its been so humid here lately that I have been running it constantly.
While its true that starting a compressor takes more energy than running it, that surge of extra use only lasts for a second or two. Most residential compressors are sized to meet the max load and it most definitely costs more to run them continuously than to run as required.
High efficiency ACs have two small compressors. They run only one unless higher cooling is required. This is the only time running continuously or near continuously makes sense, but most people don't have these types of AC.
Again, we're not talking about whether the compressor can handle it, we're talking what is the most efficient. See above.
be removing less total heat.
Excuse me? That statement makes no sense. The issue is cooling loss from inside the house. As the interior temps rise, there's less cooling loss. By not running the AC during the day, you aren't losing cool air through windows, doors and insulation gaps which gets replaced with warm air, which has to be cooled again.
I fear your installation is one that is not easy to say. I would guess that there will not be a great deal of difference in the results either way, some days you may win and some days you may loose. If it were me, I would (and do) use a setback thermostat. Even setting it back a few degrees will help Turning it off or allowing too much of a setback could cause it to run for a long time during the hottest part of the day when it is less efficient wiping out all or some of the savings of setting it back.
Your home loses cold (admits warmth), right? If it didn't you would only have to cool it off once and you wouldn't have to run the AC again until you opened the door to leave. So when would the warmth on the outside most affect the cold on the inside? When it's the most cold on the inside, right? If it were the same temperature on the outside and inside, you wouldn't lose any cold, right, and the bigger the difference in temperature, the more you lose.
The AC does not struggle. You're anthropomorphizing the AC. It just does what it normally does, at the rate it normally does it.
For you it would be harder to somehow cool off your house if it were hot in the house when you were doing it, because you get sweaty, but it doesn't. If anything, it's easier for the evaporator to vaporize the freon when there is plenty of heat in the house (I don't really think it is easier. I think it is the same.)
OK cars sometimes struggle and they are not animals. But it's not like a car that doesn't run well struggling to get up a hill**. The AC isn't trying to go from 85 to 70 in one minute, and spinning it's compressor harder than it ever did before, saying "Yes, I can. Yes, I can." like a car might try to climb 2000 feet in one minute. The AC just does what it does at the rate it does it.
**By which I mean backfiring, engine missing, faltering. A) I don't think that happens anymore since there is computer controlled ignition, and B) when it did happen it was because the car was being told to do more than it could do. All that would be necessary is to downshift to a lower gear, or if there were no lower gear, there should have been, and the car wouldn't struggle, but it wouldn't get to the top of the hill in the one minute allowed. ACs don't have 1st,
2nd, 3rd gear because there is afaik nothing parallel to that in the AC world, and because they don't need them. They just cool as much as they are designed to cool. It's like one workman who is told to shovel the sand out of a child's sandbox, and another who is told to shovel the sand out of an Olympic size swimming pool (open at one end.) If the second guy goes at the same pace as the first guy, he won't struggle anymore than the first guy does. That's what your AC does. When the second guy feels frustrated, it's only because he can think and he knows he won't be finished for a long long time. The AC doesn't know from that.
Let's extend that theory. Say you went away for a year. Doesn't their theory say that it would cost the same if not less to periodically cool an already cool house instead of cooling a house that is not cool at all.
I don't think these people can explain why they are right. I think they are just speaking intuitively and their intuition is wrong.
They are not just talking about window units.
And why would window units be different from central AC?
Do you live in a house or apartment. How much insulation do you have in your ceiling and elsewhere? These have maybe nothing to do with your question, but they are 3 of my thoughts.
I believe you and Griswold got it right; so many don't.
The only thing I would add, which complicates things, is a consideration of what time you turn the AC back on when you come home. If you turn it on during the blazing hottest part of the day, the AC has lots of work to do when it is least efficient--working against the outdoor heat.
Well, sort of. A/C compressors are nothing more than heat transfer devices. As someone else posted, they aren't working harder of easier based on what the outdoor temps are. They consume the same amount of electricity.
A little refresher course: I'm making up the numbers, but the principles hold.
An AC compressor takes a freon like gas and compresses it. Basic thermodynamics states that when you compress a gas it heats up. Lets say its 95 outside. By compressing the gas it goes up 15 degress to 110. If it were 70 outside, it would go up to 85. The 15 degrees from compressing doesn't change - that's basic thermo again.
We run the compressed gas through an outdoor radiator and blow a fan across the radiator. That cools the compressed gas back down to outdoor temp - say 95. Bring the gas inside and through a heat exchanger, only this time we let the gas expand.
Basic thermo says the gas gets colder, lets say by the same amount, so the heat exchanger temp is now 75. By blowing the 90 degree house air through a 75 degree exchanger, the house gets cooler and the cycle repeats.
Will the house cool faster if the heat exchanger is 50 degrees (ie outdoor temp/compressed gas is 65)? Sure. Is the compressor running less? Probably. Is the AC working easier? No. Its working the same.
Will you run the compressor more by leaving it on all day compared to turning the thermo up while you are gone? Absolutely, because the house isn't perfectly insulated.
Will the house take a while to cool down? Sure - there's a certain amount of thermal mass involved besides the air that has to be cooled. But that same mass also acts as a brake to keep the house from heating up.
I forgot one thing I meant to mention. I used to feel that it must be hard to get an electronic oscillator to start oscillating, and that eventually it would get tired doing so. Yet I've had tube and transistor radios and tv that run for hours (or in one case, weeks, on end, and they never seemed to get tired.
I think I thought it was hard to start them, because I wouldn't want to oscilate. And that when they started it was sort of like a fluke (even though, if the device wasn't broken, it always happened. Later I learned how the circuit works and it didnt' seem amazing anymore.
I THINK that he really means TWO speed compressors. Heat pumps have this feature too.
The only two compressor things I am aware of are ultra high end refrigerator/freezers. Viking and SubZero have a separate compressor for Refrigeration and for Freezing
When you leave the AC on, it has to work harder to keep the house cool (versus keep the house warm). This uses more electricity.
However, if the recovery time is very long, it might be more important to keep cool, compared to the energy saving. Since you describe it "struggling", I'd leave it on. I'd also call a HVAC company to clean and service the unit. Might be running very inefficently.
Dear Jeff, Your answer is confusing. "less efficient to stop and start". Well, most AC I've seen stop and start, unless you crank em down to freezer temps. "house will gain less heat during the day when it's warmer". Well, when it's warmer out, the house will gain more heat, compared to night when it's cooler. "no moisture source". Such as through the cellar floor, or around doors and windows? Also you didn't say if you suggested leave on, or setback.
I had a unit that ran almost constantly even with a setback thermo. It got so bad that if more than two people were in the house (i.e. grandkids came over), we had to go escape to the mall to get cool. AC bill more than doubled. Time for a new unit.
Now in spite of at least 50% increase in electricity rates, house stays at a reasonable temp and AC bill is staying close to the peak before the old unit got REALLY bad.
I agree that starting/stopping a central AC is NOT a problem, unless the interval between a stop and a restart is too short, i.e. hard start conditions exist.
On balance, a house with adequate insulation and adequate air leaks plugged will need little more than a bit of cooling and a bit of dehumidification, depending on where the house is located. The rub is 'adequate' insulation and 'adequate' plugging of air leaks. Adequate in one climate is inadequate in another. The concerns of someone in Calgary Canada are quite different from those who live in Mobile Alabama. With adequate insulation and sealing even on the Gulf Coast, peak summertime AC expense for a 2000 sq ft home CAN be less than $100 a month.
Another issue is that with 'adequate' sealing of air leaks, odors can accumulate in the home, so a controlled air exchange with the outside air then becomes necessary. AprilAire's Energy Recovery Ventilator is an example of such a product.
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shows one of these in operation with minute by minute readings of temperatures and power consumption.
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