“What Does the Thermostat Say?” Why Air Temperature is Just Part of the Comfort Equation

The thermostat won't totally explain your comfort level...

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Sitting in our TV room one cold evening, my wife asked, “What does the thermostat say?” Naturally, she was implying that she wanted me to turn up the heat, but much to her chagrin, I promptly grabbed a pencil and engineering graph paper instead.


Partially because of my frustrations with design MEPs, but mostly because I didn’t want to get off the couch, I proceeded with the following explanation as to why the sole connection she makes between her discomfort and our thermostat reading is flawed:


Lots of people, including engineers involved with the built environment, have been trained from years of scrutinizing thermostat settings to assume that whether it reads 65°, 82° or any variant in between, they can directly know how comfortable they are. In reality however, air temperature is only part of what makes us comfortable.


The best true value of what we perceive on our skin and therefore what we consider comfortable is known as ‘operative temperature.’ Assuming normal indoor humidity levels, the operative temperature is a weighted average of not only the air temperature, but also mean radiant temperature – or in layman’s terms, the average temperature of all the surfaces surrounding us in a space.


Now, if we were watching TV in a closet on the interior space of our home, we could assume all surface temperatures would be the same as the air temperature and therefore the operative, air, and mean radiant temperature (MRT) would be about the same. But (looking to my right), the wall of glass exposed to the cold winter conditions outside has significantly dropped our MRT. Because this portion of the balance has shifted, the air temperature would need to be slightly higher than usual to reach the same level of comfort as during the slightly warmer day.


But operative temperature’s true usefulness isn’t really in explaining to my wife how she perceives when she’s cold. It’s much more useful in explaining radiant heating and cooling. Just like the cold weather outside created a cooled surface in my TV room, a cooled floor in a commercial structure can have the same effect.


With a radiant floor system, the air temperature can be elevated within a space to achieve the same amount of comfort. So when the radiant system is in cooling mode, rather than having to set your thermostat to, say 75°F, you can bump it up to 77-78°F and still achieve the same level of comfort.


The ramifications of this are twofold. Not only will the air side of the HVAC system operate more efficiently because it doesn’t have to lower the air temperature as much, but from the very onset of design work the load calculations can be made for a 77-78°F set point, thus saving in the initial costs of HVAC sizing.


Augmenting this shift, the radiant system will operate more effectively as its output will increase about 10% for every degree above 75°F the air temperature is elevated, shifting more of the load from the air side of the HVAC system to the more efficient radiant side.


Oh–by the way, my wife, who understands all of this fairly well, humored me. She thanked me for the explanation, and as she got up to adjust the thermostat, she said, “You could have just told me you didn’t want to get up.”


By Ryan Westlund, EIT, Systems Engineering Specialist, REHAU Construction LLC