The Tricky Notion of Comfort
In this comment piece Professor Doug King (Special Advisor to the Chopping Company) discusses the tricky notion of comfort and why the thermostat does not provide the level of control that people desire or expect.
Comfort is a difficult notion to pin down. Thermal comfort is affected by a myriad of factors, including air temperature, air movement, humidity, radiant heat, what we have eaten or drunk recently and the average outdoor temperature over the previous 7 days. The closest that learned institutions have come to agreeing upon a definition of comfort is that it is the state in which people are not uncomfortable!
We typically control central heating systems using thermostatic devices that are supposed to be designed to maintain a steady ‘comfortable’ temperature. However, these are only able to measure the temperature of the air in the room where the thermostat is sited, they cannot account for all the other influences that affect our comfort, or what is happening in the rest of the house. A thermostat will tell us that the temperature is just right, even if the sun is shining through the window (making us feel too warm) or a draught makes it too cold.
From what we now know about comfort, any air temperature which is adjudged to be just right in the absence of other influences will be too warm if there is a radiant source of heat, such as the sun shining through the window and too cold if there is a draught. On a sunny day, a home at 19ºC may feel warm but as the sun goes down it may feel cool at 20ºC.
Given that boilers are often located out of the way in a cupboard or utility room and we don’t want to constantly reprogramme the time controller, we typically turn to the thermostat to control our heating systems. We use it as a switch to control the heating directly, but this has unintended consequences. Once we have changed the set temperature in order to switch the heating on or off, the thermostat will continue to control the system to that new temperature. When the influence that created the need for a change is removed, the draft stops or the sun comes out, our perception of comfort swings in the other direction requiring another intervention to correct the air temperature once more.
A thermostat does not provide the level of control that people desire or expect, it was never designed to do so. What we need is a simple, intuitive input device that allows us to control our heating in response to our comfort needs. A modern day equivalent of throwing another log on the fire, or allowing it to burn down a little.
The structure and contents of our homes heat up and cool down relatively slowly (over several hours) compared to air, which can change far faster. Humans are relatively insensitive to slow changes in temperature so the average output of the heating system over a period of time is more important to their comfort than its instantaneous output. A thermostat controls the average heat output over time by switching the heating system off and on in response to small changes in air temperature that we humans rarely feel. Exactly the same function could be achieved by simply switching the boiler on and off when you feel cold or hot. This would stop the boiler from operating during any period when you felt warm enough, whilst the previously built up heat dissipates.
Professor Doug King (FREng FInstP FEI FCIBSE HonFRIBA) is a Special Advisor to The Chopping Company
As a chartered environmentalist, physicist and engineer, Doug has a unique insight into building performance and energy efficiency. He has contributed to the design of numerous pioneering buildings, including: Sainsbury’s Greenwich, the first retail BREEAM ‘Excellent’; The Genzyme Centre, Massachusetts, until recently, the world’s largest LEED Platinum building and The Innovate Green Office which broke the BREEAM ‘Excellent’ ceiling. Doug is formerly Chief Science and Engineering Advisor to the BRE and presently a Visiting Professor at the Universities of Bath and Chongqing.