AC myths 2: Thermostat at 25°C is all wrong

Thursday 15 September 2011, 12.30pm HKT

<< Back to Part 1<< || >> Forward to Part 3 >>

(Continued from Part 1)

This following piece of AC mythology is very widely believed and recommended, so it needs quite a bit of explaining to set the record straight.

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Bullshit #5
The recommended AC thermostat setting is 25°C (77°F) and best balances the economy of running the AC and providing a comfortable temperature.

Wrong! See, that 25-degree crap is just wrong on all levels.

Say the outdoor temperature is 28°C (82°F). To run the AC at 25°C is just wasting energy and not maximising the capacity of the AC unit or its refrigerant. It leads to early degradation of the refrigerant (Freon, R-400 series or whatever). It also causes unwarranted heat buildup inside the AC unit.

You get this totally groundless assertion all over the place — and from not a few AC manufacturers, retailers, refrigeration engineers, power companies and governments.

You’ll also discover that well-meaning people develop a sudden high tendency to show off their supposed knowledgeability, usually by flooding the discussion with lots of facts and stats about ACs and optimal settings. In the end, it all confuses than enlightens.

But Bullshit #4 comes with several sub-bullshits — and the most usual is this one:—

Sub-Bullshit 4A:
Set the AC thermostat to a higher temperature in summer and a lower temperature in winter.

This is a rich one. The moron who churned out this drivel (and many continue to churn out this fine disgrace) didn’t even notice he/she slipped up (and neither did we) — why are you using the AC in winter? That instantly puts this recommendation in the bullshit category.

FACT:— The actual optimal AC thermostat setting is 20°C to 23°C (68°F to 73°F) because the internationally accepted (and year-round) normal temperature and pressure (n.t.p.) is 20°C at 101.3 kilopascals (68°F at 14.7 pounds per sq. inch).

Anyone who ever did O-level (secondary/high school) physics and certainly every university engineering student knows this — so why not you?

And you need to be slapped around if you were to quibble about this. Honestly, the behaviour of some people…

ASIDE:— N.t.p. is different from standard temperature and pressure (s.t.p.), which is 0°C (32°F) at 101.3 kPa/14.69 psi — but that’s another story altogether.

If you find 20°C (68°F) too cold, then you must be one helluva effing physical wreck.

I’m serious — I’m a scrawny, skinny small fry of 120 lbs and 5 feet 8½ (55 kg, 174 cm), and 20°C is nowhere near cool-feeling. And I live in a subtropical city where three-quarters of the year is 28°C to 33°C at 90% relative humidity.

Protip:— I learnt yonks ago the rule of thumb was to set the AC at 10 degrees below the outdoor temperature (using the Celsius scale).

A friend who actually works in the HVAC (heating, ventilation and AC) business confirms that this rule of thumb IS the optimum way to run air conditioning. Here’s what my friend had to say:—

“Setting the aircon to 10 degrees [on the Celsius scale] below the outside temperature is a good rule of thumb. Aircon in the home is a ‘comfort application’ rather than a ‘process application’ such as in industrial settings.

“It’s true that the dehumidifying effect of aircons means that you will feel cooler at 25°C [77°F] inside than standing in the same 25°C outside because of the lower humidity inside.

“Sure, if you set the aircon at 10 degrees below the outside temperature, you will feel a very big difference between inside and outside. But if you run your aircon generally 10 degrees below whatever is outside, you will find it pretty cost-effective in the long run and keep your bills low.”

Alright, you can argue till the cows come home that ‘normal room temperature’ will depend on the locale and the individual. I don’t dispute that. Ask and you’ll get a range of useless answers:—

  • North Americans will say 77°F (25°C)
  • most Europeans will say 20°C (68°F)
  • 20°C to 25°C for those who like to hedge their bets (gormless!)
  • grandparents would probably say 32°C (90°F) — then again, their lives are shutting down anyway so they’ll feel cold at any temperature

I don’t claim to be an engineer, or even any good in physics. I passed my O-level physics in a resit. But I’m not exactly ignorant of physics. If a bunch of boffins around the world agreed that n.t.p. is 20°C (68°F) at 1 atmosphere, there’s reason behind it, Einstein.

What we should do

Obviously, we must set the AC to the temperature we feel most comfortable with. The temperature value itself is not that important — the temperature sensor of the AC might be far away from you, or you could be closer to a heat/cold source (window, door, kitchen, etc), or you’re in an office environment or public place and don’t have much control.

Set thermostat on high, and this becomes of your home (click image for full story)

Use common sense:—

  • if you feel a bit chilly, put on an extra shirt or jumper (the only choice in the office)
  • if you feel warmish, strip
  • shut the AC for a while if it’s getting chilly (workable only at home)

I also think what needs to be considered also is whether you’re setting the AC to cool a larger or adjacent room or just a small room (like a bedroom). If your bed is in the AC’s direct line of fire set at 24°C (75°F), it’s bound to get freezing sooner or later. If you’re trying to cool the kitchen or dining room, perhaps with several people milling around or some sunlight streaming in, setting the AC a little cooler is only common sense.

Some people are so sensitive to temperature that more heroic efforts are needed:—

“Personally, I prefer to set the aircon to a degree or two more than thermal comfort during summer, and a degree or two less during winter — just the opposite of what others do to make up for inadequate thermal shielding [i.e. insulation] of the building — in order to achieve reduced thermal shock while leaving the building, and reduced mental shock while receiving the electricity bill.”

But the real question is, why the hell are you using the AC in winter? Throw open the bloody windows in winter, sport.

(BTW, I happen to feel that particular individual quoted just now was a liar and was trying just a bit too hard to lend believability to Bullshit 4A.)

Ultimately, for the sake of your AC lifespan and electricity bill, it would have been better just to do this:

Protip:— Keep the AC thermostat setting and room temperature constant at one fixed point all year round.

Just keep a thin cardigan around in the office or switch the unit off at home if things get a bit chilly. The more you mess with the AC, the quicker it effing breaks down.

Unless you bought a stupid Fortess AC like I did.

What about propaganda?

After a while of searching and asking around, you begin to notice like I did that virtually all governments around the world give out the same two recommendations:—

  • the optimum AC room temperature is 23°C to 26°C (73°F to 77°F)
  • for every 1° increase in indoor AC temperature, you can save 10% off your energy costs (e.g. Energy Rating website of the Australian government)

The U.S. and Canadian governments recommend 25°C (77°F). The Aussies recommend 23°C to 26°C. Our Hong Kong government recommends 25.5°C (78°F).

Every country has their own unique characteristics and develop standards most relevant to their situation. So how is it that almost all of the major AC-using countries could have almost identical recommendations for AC settings but physically different climate profiles?

That fact alone is a telling sign that there’s been quite a bit of statistical recycling going on — one country reduping the stats of another country — most probably because the civil servants in charge were bone idle to check or test things out for themselves, which is what I often find in my long experience of dealing with the bureaucracy.

Even at face value, both recommendations are lousy logic. You could just as easily argue that you would achieve zero energy cost if you set your room temperature to 30°C when the outside is also 30°C. See what I mean?

The upshot? Literally decades’ worth of collaboration by thousands of scientists to draw up international standards just go out the window when governments go it alone and decide to use completely different and variable sets of standards for making recommendations. Way to go.

Do your research properly

Let’s look at things:—

FACT:— Power companies have been promoting this 25°C (77°F) AC thermostat setting because they will have minimal brownouts (drops in voltage in the electricity supply) on their ageing grids — a plus for their bottom line. (Source: my HVAC friend)

I promise you, that’s why you’ll find nothing in the scientific literature on air conditioning that directly backs up the 25°C/77°F.

But look under the heading of ‘control engineering’ for prevention of power dips in legacy power grids and you’ll find plenty of references. Don’t take my word for it — check things out yourself.

FACT:— The human body operates most comfortably and with the least amount of physical stress at temperatures of 19°C to 22°C (66°F to 71°F).

You can look this up in any university human physiology textbook or lab manual. See later instalments of this article for details.

FACT:— All high-rises, shopping centres and public indoor spaces are set at 22°C (71°F) — because that is what building structures are designed to operate at.

Most likely you are an architect if you know this. I happen to know something about this because Dad was an architect — and I don’t think yours is one. I read Dad’s books too, y’know. And Dad passed his physics, qualified in engineering and surveying, qualified in architecture at the Bauhau (no less), passed his German and French, failed his Chinese, and not so hot in law. Unlike me, Dad never took Latin or Greek. So there. So screw you if you don’t think I know what I’m talking about for this bit.

FACT:— All climate control systems (for houses, cars, etc) have factory preset temperatures set at 22°C (71°F).

And you never thought to question why this is so.

Normal room temperature and pressure is 20°C (68°F) at 1 atmosphere, internationally agreed

Go and ask a power company what it sets ACs to in its own buildings. The ACs will be controlled by an environmental control system that sets ACs to 22°C (71°F) and 50% humidity — while they tell you to set yours at 25°C or 26°C. A bit contradictory, don’t you think? No, you didn’t think.

FACT:— The more times the AC compressor is running, the more moisture is removed from the air, therefore providing greater comfort. Conversely, the higher the set point temperature, the less moisture is removed from the air, and the more unpleasant or uncomfortable the air quality is.

Couple of protips:—

Protip:— If you have a remote for the AC, removing the batteries and reinstalling them should give you back the factory preset temperatures in each mode that the product was originally made for. For example, the Mitsubishi preset is 21°C (71°F). Think about that.

Protip:— The correct way to measure the AC air output temperature is use a thermometer 1 metre (3 feet 3 inches) from the air outlet. Don’t stick it right into the blowhole, like those two Laurel and Hardy repairmen did when they tried to fix my Fortress ACs.

* * *


Why running a fan with the AC on is counterproductive.

<< Part 1 << || >> Part 3 >>

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© The Naked Listener’s Weblog, 2011.

Images: Hand setting a thermostat by Tom Grundy via 123RF ♦ Home AC on fire via ♦ Obey Our Recommendations by thenakedlistener ♦ Gridline brownout servicing via Little i-cay ♦ Office workspaces via Fashion Trends.

14 Responses to “AC myths 2: Thermostat at 25°C is all wrong”

  1. Pingback from: […] Paleo Village – Effect of Environmental Temperature on Exercise – Exercise, Iowa Hospitals Clinics, Nutrition, Paleo Diet […] at


  2. Angry at Open Abuse said

    “If you find 20°C (68°F) too cold, then you must be one helluva effing physical wreck.”
    I live in a city where it is “normal” for temperatures to go upto 40 deg Cel.
    You’re the physical wreck who disregards regional influences.
    How about you tell Eskimos 0 deg is cold?


  3. dontcare said

    Either intentionally or by mistake, I think you missed the point. The sentence “Set the AC thermostat to a higher temperature in summer and a lower temperature in winter.” actually means “Set the thermostat to a higher temperature in summer and a lower temperature in winter.” Obviously the thermostat will be in heating mode in the winter (controlling a furnace for example) and in cooling mode in the summer (controlling the AC).


    • That’s how the rest of us normal people like you and me would read that sentence (“Sub-Bullshit 4A”). It just stands to reason (and common sense) that we cool in summer and warm in winter.

      Unfortunately (and unbelievably), that was NOT the meaning meant by the person who gave out that incredible advice (and by the people who recycled it). My HVAC engineer and I were totally floored by it. I suppose it takes all sorts in this world.

      Thank you for your comment and interest in this satirical post.


  4. Jonny said

    Your pseudo-science is interesting. Your so-called “sources” are quite obviously fake. Anyone can say their “sources” told them the grass is purple, but it doesn’t mean people with eyes and a brain need to accept it. Very very funny “research.” When you turn 14, this post will make sense to you.


  5. Howard said

    In international standard atmosphere (ISA), the temperature is 15°C at the sea level.


  6. Ilay said

    That is very disrespectful to call people as physically wreck because they feel cold at 20 C. Everybody is uniqe and so is their metabolysm. I am a mediterrian woman and I feel cold at that temprature. I think wanting to be ourstanding should not mean offending other people. Thanks


    • They are, if they feel 20C is “too” cold. That doesn’t mean (and shouldn’t mean) they don’t feel cold at that temperature. I mean, I grew up in COLD CLIMATES, yet I feel cold at 20C myself. You assume that I don’t know everyone has their own temperature range because of metabolism, but I do know, having grown up in 13 different countries around the world.

      Thank you for your comment, but you are taking this much too personally.

      Liked by 1 person

  7. Not sure I agree completely with your assertions on the air conditioning stuff.

    I believe, by experience, a lot of it depends on where you live, humidity (perceived temperatures), and just how well insulated your house is. While I was living in Alaska in a poorly insulated house, to keep the heat from running 24/7, it was set lower. No need for A/C in the summer. In a well-insulated house, the one I built myself, I set the A/C unit, combined heating and cooling, at 72F with a three-degree variant, 1.5-degree swing. It cost less to heat and cool that house than my first by better than half.

    Living in Las Vegas, it was the same experience. Pushing the A/C down to 68 or so in the winter made it almost never run. If I pushed it up to 72-74, it ran a lot. Summers… well… lost cause. It’s going to run 24/7 in the summer no matter what.

    It was just a matter of picking a comfort level and leaving it. Now that I am in Adelaide, with crappy insulation brick house, no insulation in the roof, I am about to give up. Nothing works well here. I guess I need to build again and super insulate.

    Keep in mind, with the modern A/C systems, as is the one I am using now, they are designed to run ‘forever’. (not literally, of course). So you need only judge two things. How much do you want to spend in power? And how comfortable do you want to be? Find your own happy place. Oh, and having a central air system makes the whole house feel better… no cold spots or warm spots… IF it was installed properly.

    Mechanically, listen to your unit as it runs. There is such a thing as a ‘short cycle’. That means it kicks on and kicks off within minutes. That IS rough on the equipment. To fix that, adjust the ‘swing’ to a higher number. Most factory units are set to something like a degree to two degrees. Long cycling is no big deal, not to me anyway. Short cycling… just unnecessary and indeed a bit wasteful, and particularly hard on gas operated heaters.

    In our hedonistic society, it’s all about the comfort anyway, isn’t it?


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