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July 22, 2018 4:46 AM   Subscribe

Does turning the Air Conditioning off during non-business hours save money for office buildings?

My office building turns the AC off overnight (10PM to 6AM) and on Sundays. I've been told they do this to save money. But I'm wondering if it's really worth it.

It's a 5 floor building with a Server Room on 1 floor and Switch Closets on each floor that have dedicated AC running 24/7 keeping them around 68 degrees Fahrenheit. Additionally I have a portable AC unit in my office* that's always running. It's between 80 and 85 degrees in the Mornings before the AC kicks on and takes about an hour to get down to the 72 degrees it's set at.

I know there are a lot of other factors at play here, but are they actually saving a lot of money by doing this?

*The portable unit was provided after I complained about the heat in the mornings. But after spending the last 6 hours at 78 degrees, which is all the portable unit can manage right now, I'm questioning the soundness of this strategy.
posted by zinon to Work & Money (9 answers total) 1 user marked this as a favorite
 
Yes, assuming no damage is being done by swings in humidity letting a building warm up during unoccupied periods and then cooling the building down again uses less electricity than just running the A/C 24x7 and will therefor save a significant amount of energy and presumably money.

US federal regulations mandate a maximum indoor temperature of 76 degrees which is why they gave you the portable but be aware that the savings are such that employers may just ban work during non-A/C hours rather than provide A/C 24x7. (And is why so many people have space heaters under their desks).
posted by Mitheral at 5:33 AM on July 22, 2018 [6 favorites]


Yes, turning off the AC when a building (or room) is not in use saves a lot of energy and therefore money. It is one of the largest and easiest energy-saving moves that you can make.

What they should ideally be doing though is turning it back on a little before business starts (the system should be able to handle this automatically unless it is truly ancient) so that by the time people actually show up, temperatures are back down to comfortable levels.

But yes, it's definitely quite wasteful to run AC for an entire office building when nobody is in it. The only reason to do so would be if, as Mitheral mentioned, the AC is needed in order to keep humidity levels from getting so high that they damage the building. Newer systems generally have a way of reducing humidity without using as much energy as is required to actually cool the building, but older systems may not.
posted by Anticipation Of A New Lover's Arrival, The at 6:27 AM on July 22, 2018 [4 favorites]


It is entirely possible that it takes more energy to pull a hot building back down to human temps than it would to maintain the human temps when humans aren't there. Even more likely that setting the AC to some higher temp would save more energy than turning it all the way off. ACs are more efficient when maintaining near constant temps, compared to the efficiency of being forced to do big swings. This is the entire point of the Nest smart controls and their ilk: they don't turn off things completely, or blast anything on full, they strive to make gentle changes. This is analogous to how you get best gas mileage in a car when you accelerate and decelerate slowly, every time you stomp on a pedal, you hurt efficiency, as is easy to verify in any modern car with the appropriate readouts.

What you present is a completely general thermodynamics problem, and you can't get a definitive, reliable answer from first principles without specifying lots more detail: how much insulation, how much airspace, what kind of AC units, what is the average nighttime low and daytime high, how shaded is the building, etc. etc.

Realistically, you can't perform this analysis, and you probably won't pay some team of experts to come in and do tons of measuring and mathematical modeling.

So the smart thing to do is run try it both ways and use that evidence.
posted by SaltySalticid at 7:10 AM on July 22, 2018 [9 favorites]


Sorry, before I was talking more in terms of general ideas, and wanting to push back against the notion that turning the AC completely off at night is always a good move.

But, fwiw, I just re-read your particulars and I'll go ahead and say: this is bs, a whole 13Fdegrees F down for a large space is an enormous demand, and it can't even do that because it's fighting against the rising heat of the day!

You have my encouragement to raise a stink: this is uncomfortable and almost surely using more energy on weeks with hot nights, compared to simply changing the thermostat to 78 (or so?) overnight. This also will make the server-dedicated AC systems work less. The exact temp that is best is the hard part, but you don't even need the best, you just need better than the worst, i.e. failing to ever cool the indoor space to a normal office temp. Tell them to try a dang Nest (or whatever the corporate equivalent is), it definitely knows how to sort this better than their current choices suggest they do!
posted by SaltySalticid at 12:53 PM on July 22, 2018


According to Figure 4 in this overview, a typical air conditioning system will work at its peak energy efficiency at between 35% & 60% of full load. At full load it's 10% - 20% less efficient than at those part loads. At <20% load, it's dramatically less efficient than at full load. (Direct link to diagram)

If you graph the difference between inside and outside temperatures the system's expected to maintain (x-axis is time of day, y-axis is ╬┤Temp), the area under this graph is the amount of work the air conditioner has to do.

The extra work done maintaining a larger temperature difference between outside and inside temperatures for the 12 hours a day the building's not occupied dwarfs any variation in system efficiency between running at full load and running at part load.
posted by ambrosen at 4:51 PM on July 22, 2018 [1 favorite]


SaltySalticid, I understand your thoughts, but based on laws of thermodynamics it will ALWAYS take less energy to turn off the AC at night and heat it up when needed, unless there is some huge mechanical problem (like a fuse shorting) from the AC being on for a long continuous period (which would be a defect not usual for AC units).

The principle of thermodynamics that is crucial here is the speed to which two things come to equilibrium. Temperature energy is transferred faster the greater the difference is between the two interacting systems. Imagine a example:

In space, there are two cubes. One is 1 foot cube, the other is a 1000 foot cube. They are separated until you move and connect them to each other with magnets. Because it's a vacuum, the temperature of the two cubes doesn't change until they are touching each other.

Example A: the 1 foot cube is 80 degrees, and the 1000 foot cube is 82 degrees. You connect the cubes. Over the course of two hours, the insulation provided by the glass separating the cubes Slowly brings the temperature of the 1 foot cube to 82 degrees, and the 1000 foot cube to 81.999 degrees.

Example B: The 1 foot cube is 70 degrees, and the 1000 foot cube is 82 degrees. You connect the cubes. In the first 5 minutes, the 70 degree cube is quickly warmed to 75. In the next five minutes, it's warmed to 79. In the next 20 minutes, it's warmed to 80, then for two hours or so, it eventually reaches equillibrium at 82 degrees.

Example C: The 1 foot cube is 33 degrees, and the 1000 foot cube is 82 degrees. You connect the cubes. The 1 foot cube rapidly warms to 50, then 60, degrees, in the first minute. Then it warms following the declining-over-time pattern above.

These examples are relatively realistic (without doing the math on the time or the volume). This is the principle of temperature setbacks: The amount of difference between two temperatures dictates how quickly they reach equilibrium. Thus - the greater difference in temperature means more energy is lost between the controlled system and uncontrolled system.

If, over the weekend, the AC was off, and the temperature went from 73 to 77 and it was 85 outside, the energy lost at 77 would be much less than the energy lost at 73. Thus, the amount of work the system must do to lower the temperature back to 77 to 73 is less than if it had to maintain the temperature at 73 the whole time.

Imagine trying to keep the temperature at 33 in example 3. You would be fighting the steep slope all weekend to keep it at that low temperature - every 2 minutes, the AC would turn on to bring it back from 50 to 33, requiring it to cycle all weekend every two minutes. But, to bring it from 77 to 50 isn't so bad, then from 50 to 33 would be work, but it would only have to do it one time instead of every two minutes of being turned off.

I hope these examples help illustrate the work side of the equation.

On preview - ambrosen posits that AC heat pumps may vary their effectiveness based on the amount of load they are on. However, over the time period of the weekend, even 1 degree of difference will offset any load-issue variences in the AC unit (Looks like it's only 10-20% less efficient at "full load" - whatever that study means by that in terms of that part of the AC compressor).
posted by bbqturtle at 5:29 AM on July 23, 2018 [1 favorite]


A Nest is just a smart thermostat controller - it is dependent upon your actual A/C unit if it can run at 'part load' or 'full load', meaning it's a multi-stage unit. Most cannot. Most central A/C units for the home only run at full, unless your system is relatively new.

A Nest also doesn't necessarily make the home run on low A/C even if you have a 2 stage system - it is designed to run your A/C as little as possible, meaning turning it off when you are not at home. It also tells you how long it thinks it's going to take your A/C to get to the temperature you have set.
posted by The_Vegetables at 9:27 AM on July 23, 2018


This question is asked often in many places. Yes, it is more efficient. If your office is unusually warm in the morning, you may have a lot of sun. Since you like it cool, get blinds and use them to reduce the sun coming in. There are window treatments that will help. There are lots of passive responses to heating & cooling that enhance efficiency and comfort. Get a fan, point it at yourself and you will likely be more comfortable.

Space heaters are hella inefficient, have a high power draw, are dangerous. People forget and leave them on. They get banned for a good reason. Wear a sweater, use a fleece throw, etc. unless you're like me and really hate AC, in which case Maintenance will close the vent directly above your desk, thank you so much. I am a big wussy-baby about being cold, but I also hate using too much oil, so I develop pretty good cold tolerance during winter, then bask in the warmth in summer.
posted by theora55 at 10:38 AM on July 23, 2018


theora55: "Space heaters are hella inefficient"

Space heaters, of the electric variety, are 100% efficient. They often aren't cost effective compared to 95% gas furnace but efficiency isn't the problem.
posted by Mitheral at 12:37 PM on July 23, 2018


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