AC myths 5: Lower temperatures cost more?

Thursday 15 September 2011, 2.05pm HKT

<< Part 4 << || >> Part 6 >>

(Continued from Part 4)

Now the fifth part of the series, we’ll examine the myth that lower AC temperature settings will jack up your electricity bills.

* * *

Bullshit #8
Higher AC thermostat settings costs less and lower settings more in summer.

If you look around the Internet long enough, eventually you’ll encounter this bizarre statistic:

“Research on Florida homes showed that summer AC cost increases
12% for each degree the thermostat is lowered below 80°F [26.7°C].”

If this sounds idiotic to you, then you must already understand basic logic.

This sentence is found almost word for word all over the Internet. Origins unknown. Recyclability, very high. Usually found in any type of online discussion about air conditioning.

It’s crap — and here are the two dead giveaways:

  • the 80°F — the figure is too rounded and too high, even for Florida
  • the 12% — a wee high (and I live in one of the most energy-costly places on earth)

Everything you’ve read so far from me, have I ever given out temperatures in such completely rounded figures? No, you haven’t (not yet anyway). The percentage? Already explained. So someone, somewhere, is into crap invention and/or distribution.


Srsly, this is the idiocy that some of us still subscribe to — to equate lower temperature settings with more power use and higher bills. Or, conversely, less power use and cheaper bills with higher settings.

To restate from Part 2:

FACT: The AC runs at its rated power consumption level regardless of the thermostat setting.

To be 100% honest and objective in absolute terms, there is a difference in power consumption between different temperature settings. In relative terms, however, the difference is just under the threshold of significance in the context of billing.

A friend has this setup and cost bugger to run:

  • two 8½-kilowatt Mitsubishi splits (living and dining rooms, kitchen, study)
  • four 3½-kilowatt Mitsubishi inverter splits (3 bedrooms, office)
  • total load is (2 × 8.5) + (4 × 3.5) = 17 + 14 = 31 kilowatts an hour
  • basic domestic rate is 90¢ Hong Kong (11¢ US or 7 pence) per unit (1 kW/h)
  • therefore, billing at 31 kW/h × $0.90 = $27.9 × 8 hours = HK$223 a day

That’s roughly US$28.60 or £18.16 a day if everything was on full blast.

But of course the friend doesn’t have all six ACs on all the time. It’s mostly just one or two ACs at any given moment. And friend sets the ACs at 20°C (68°F), which basically cools the entire place well enough to around 24°C.

From that, you can see setting the AC to 20°C doesn’t cool the place down to 20°C — you have to take into account that cooled air gets heated up by the ambient temperature of the premises. And concrete jungles like Hong Kong retain a lot of heat.

As if by the laws of physics, for every vision there is an equal and opposite ‘revision’:

“I hate running the AC. Every way I look at it is a waste of energy. I lived in Nigeria [or Djibouti, Tanzania, the Amazon or whatever] for two years and never once used the AC. I was totally comfortable in the daytime when it’s never below 36°C with 100% humidity and the nights never lower than 32°C with 95% humidity. Your body just adapts.(Invented example)

Yeah, right. And some people find swimming with sharks totally comfortable too. I love swimming in leech-infested waters; your body just adapts. Gee, were you previously violated by a barnyard animal or something?

Most of us find living in a humid, semitropical country (like Hong Kong or Singapore) with almost year-round temperatures in the mid-30s centigrade and 90+% relative humidity annoyingly uncomfortable — just as you, prat, find ACs annoyingly cold. Your body adapts (true) but there’s a limit.

You obviously never lived in the Middle East. Summers there average over 50°C (112°F) with 100% humidity. (I once lived in Beirut, whose climate is a shagging lot nicer than, say, Cairo or Dubai.) If you don’t turn on the AC below 22°C, everything goes mouldy or otherwise disintegrate under the heat. Ever wondered why archaeological artefacts from the Middle East are generally in a distressed state?

These are the same livestock people who have no concept of even a test tube unless it’s got the words “environmentally friendly” written on it. They are also the kind who can’t tell you the difference between ‘element,’ ‘molecule,’ ‘compound,’ ‘alloy’ or even ‘substance’ — stuff that middleschoolers are expected to know from General Studies classes.

FACT: In practical terms, running the AC set at higher temperatures produces roughly the same amount of pollution as set at lower temperatures.

Listen, dimwit, if we’re deadset on polluting our planet, allow us to do it with some degree of comfort.

FACT: Human body temperature is 37°C (98.6°F). As the ambient temperature rises to our body temperature or more, the less adaptable we become. The reason is that, as temperature rises, the protein molecules in our cells become increasingly denatured, thereby inhibiting our overall ability to produce somatic cells properly.

Don’t you remember anything from school?

* * *


Metabolic differences between individuals counts for shite
— unless you’re comparing yourself to a gerbil.

<< Part 4 << || >> Part 6 >>

* * *

© The Naked Listener’s Weblog, 2011.

Images: ‘Trust The Lies’ via irational ♦ Test tubes via Yancheng Rongkang Glassware Co. Ltd.

3 Responses to “AC myths 5: Lower temperatures cost more?”

  1. If you are going to be rude and condescending, at least be right.

    Setting your AC thermostat to higher temperatures saves energy because the AC unit will be on for less time. Your FACT that an AC unit runs at the same power level no matter what the temperature setting is true, but you apply it incorrectly.

    AC units regulate temperature by turning on and off. If you set your unit to 80°F, it will output the coldest air it can, until that ambient temperature is reached. This may only take a few minutes out of each our. If you set that same unit to 60°F, it will have to work for a longer period of time, thus using more energy.


    • @ James Marshall: True, you are correct insofar as the AC’s thermostat will control the output of the coldest air until the ambient air reaches the set temperature. Broadly speaking, the AC will still continue to run beyond reaching that point.

      From a strictly physics point of view, even a jerk like me knows that producing cold air will require more energy than for ‘non-cold’ air (maintenance air?). How much more energy? I’ve stated only this in the 9th/10th paragraph:-

      >To be 100% honest and objective in absolute terms, there is a difference in power consumption between different temperature settings. In relative terms, however, the difference is just under the threshold of significance in the context of billing.

      Like I said, the AC runs on its rated power regardless of the temperature setting (the first fact given in the post) – and I’ve especially confirmed this with my heating-and-ventilation technician at the office for the purpose of this reply.

      The fact that we’ve set the thermostat to 16°C/60°F or 26°C/80°F is irrelevant because the AC will still be running at the same COST. This is borne out repeatedly in the BILLS for my home, the office and the factory – and I have YEARS’ worth of them retained for our Hong Kong tax purposes.

      The same COST picture had also been borne out when I was living in the Middle East.

      Your statements are entirely true – I’ve studied physics too. But the point I was making was from a billing point of view as against the physics of the whole thing.


    • The HVAC technician says both of us are right, but from two different points of view.

      Your point is correct insofar as energy is concerned. More energy is used up to produce cold air. Unless the AC shuts off altogether after reaching set temperature, the AC is still running but this time in a kind of ‘maintenance mode.’

      Without getting longwinded, the technician says AC are basically designed for constant/continual cooling operation, such as in environments where the stream of cold air is continually heated up by surroundings – such as uniformly hot places like the tropics, subtropics and hot deserts.

      My point is also correct insofar as billing is concerned. Since the AC will be kept running (whether producing cold air or on maintenance mode), the fact that the AC thermostat is set higher or lower is largely irrelevant. The machine is still being run and it runs on the manufactured power rating.

      So there we have it.


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