Is it just me or is it getting hot in here?

Are you tired of fighting with your co-workers about the thermostat settings? Are you fidgeting at night while sleeping because you’re either too hot or too cold? Are you tired of rising electricity bills in the winter or summer? At the risk of sounding like a telemarketer, I have an explanation for your problems. It involves the term “thermal comfort”.

Thermal comfort, simply stated, is a temperature at which you feel “satisfied” with your surroundings. But, you might have already identified the problem – “satisfaction” is so subjective! It’s what differentiates me – who always sets the thermostat to 70°F in the winter – from someone who sets their thermostat at 67°F in the same winter. So, getting everyone to be “satisfied” with their surroundings is a complex problem, to say the least.

In the late 1960s, as technology progressed, workers moved from outdoor to indoor work places which resulted in increased demands for a comfortable work place. This meant trying to find a temperature range that everyone could feel satisfied in. This resulted in a lot of research efforts carried out by various scientists to define what “thermal comfort” is in the first place. After all, you can only solve a problem if you know what the problem is. In 1991, J.L.M. Hensen defined thermal comfort as –

“A state in which there are no driving impulses to correct the environment by the behaviour”

Looking the extra “u”, it’s an easy guess that he was in Europe at the time, doing his PhD at Eindhoven Univeristy. Things were heating up and in 2004, The American Society of Heating, Refrigerating and Air-Conditioning (ASHRAE) also put forward a definition in the ASHRAE Standard 55 – 

“The condition of the mind in which satisfaction is expressed with the thermal environment” 

which I thought was an interesting definition, because I don’t see the word that is being defined (“thermal comfort”) appear in its own definition (“..the thermal environment”) too often. What both of them are saying though, is that you’re thermally comfortable in a room if you can sit in the room without wanting to constantly change the thermostat. To further understand this, this hilarious video from Community will definitely help – 

ASHRAE also used its Standard 55 to identify six important aspects that are important when you want to decide if someone is thermally comfortable or not, which are: 

  1. Metabolic rate:  how fast your body can change the chemicals that you obtain from your food into heat and movement
  2. Clothing insulation: how well your clothes can stop you from feeling cold, or from letting heat escape from your body
  3. Air temperature: the temperature of the air surrounding you
  4. Radiant temperature: This is a little complex. Think of yourself, sitting inside an imaginary box. Now, inside this box your body is exchanging heat with the box – because your body is constantly trying to stay at a temperature at which you feel comfortable. The box is also exchanging some heat with you, trying to come to a mid-point where you’re all at the same temperature. Radiant temperature is that temperature at which the box and your body are exchanging heat at the same rate. Now you can get out of the imaginary box
  5. Air speed: How fast the air is moving at a particular location, regardless of the direction in which it is moving 
  6. Humidity: How much water vapor there is in the air around you, compared to how much of the air is dry
+3 is also known as "Pizza that burns the roof of my mouth"
Feel free to use this PMV scale from ASHRAE Standard 55 not just for thermal comfort but also to let your waiter know how warm you really want your soup © American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org)

ASHRAE also defined two more terms (I promise these are the last two) that help in understanding how “satisfied” or “dissatisfied” you are with the temperature around you. The first is predicted mean vote (PMV) which basically asks a large group of people how they feel at different temperatures, and then tries to condense their feelings into a 7 point scale going from -3 which is “cold” to +3 which is “hot”. The second is predicted percentage of dissatisfied (PPD) which is simply what percentage of people feel dissatisfied at specific points on the 7 point PMV scale. As an exmaple, 8 out of 10 people feel quite unhappy when the PMV is at -2, because that’s really cold! Another example: 0 out of 10 people feel unhappy when the PMV is 0 because that is “neutral” temperature i.e. you don’t feel hot or cold, and so you don’t feel unhappy. 

But there isn’t just one particular temperature at which we feel comfortable. Human beings are smart, and adaptive, and found all over the world. This means that we can adapt to the temperatures around us. A range of temperatures are comfortable for us, and this is what is “adaptive comfort model”, which is based on the idea that people who are more exposed to varying temperatures in the environment – like say your friend who lives in a commune in the forest – are more receptive to and feel comfortable in a wider range of temperatures, compared to you – a person who hasn’t left the comfort of air conditioning in 20 years. So, it isn’t just your building temperature, but also the temperature outside your building, and how often you yourself expose yourself the the elements – that decide your thermal comfort. 

Adaptive_chart_-_adaptive_method
How comfortable you feel thermally, depending on how hot or cold it is outside. The dark blue region is when 90% of the people are thermally happy – neither too hot nor too cold. The lighter blue limits are when 80% of the people are thermally happy. If you follow the red dot, it means that at 26 degrees C outside (i.e. 79 degrees F), if you set your office thermostat to around 25 degrees C inside (77 degrees F) then 90% of the people in your office will be happy, except Janice from accounting of course. Image from: Center for the Built Environment, University of California Berkeley [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons

So, ultimately, all I wanted to say was this – thermal comfort is complicated, many very smart people are trying to determine how to define it, and are trying to understand it. Why? Because a third of your energy bill is just spent in heating and cooling your house! So if we have to develop an easier way of feeling thermally comfortable, we have to understand what thermal comfort is. Hopefully, this post gives you some idea about how you can understand it.

Zen is a 0 PMV

Note: Cover image from XKCD.COM made by Randall Munroe.

 

3 comments

  1. Loved this post. Always wanted to know about this very topic.

    Are you going to get more scientific in future posts? Asking earnestly.

    Like

    • Thank you for reading. I may get a little more scientific in terms of content but I’m going to try and keep it quite simple and maybe just introduce one broad scientific term.

      Like

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