Friday, 25 May 2012

What is a reasonable setting for my thermostat?

Granted this is a funny time of year to be posting on this subject (it is 24.3C outside right now and our heating is off) but it relates to some work I have been doing for DECC so it is on my mind. We are told that reducing our room thermostat setting can save lots of money: reducing it by 1 degree can save 10% on our heating bills (in my book I said 8% but it depends on where you are starting from and also on heat gains such as use of electrical appliances). So does reducing by 2 degrees save 20%? What is a reasonable setting?

Thermal comfort is not just a question of air temperature. It also depends on radiative temperature (so you will feed cold next to a cold wall even if the air temperature is warm), the speed of air flow and presence of cold or warm draughts, the gradient of temperature between floor and ceiling, the humidity and so on. For example it is quite possible to feel cold in a room which has a reasonable average temperature if it is cold near the floor so your feet are icy. This is where underfloor heating scores.

People vary in their tolerance of cold and heat. The diagram below is from a study of comfort in some high rise office buildings. There is a huge range of reported comfort values for similar temperatures. At 22C some people were hot and some people cold. There is no pleasing everyone!

A wide range of reported sensation for the same temperature [1]

Warm clothing will help you to be comfortable in lower temperature.  A woolly jumper is good and thermal underwear is even better. However, you will need to strip off if you do anything even moderately active. Your metabolic rate doubles between sitting and doing housework [2] and you need to dump that extra heat somehow.

In any case, a sweater only warms the core part of your body.  Back in 1975, some research with young volunteers subjected to a chilly 15C found that putting on a sweater did make your hands warmer but not your feet [3].

Experiments on teenagers in a test chamber with no choice of clothing aren't very realistic. I found something a bit more relevant in a paper about the impact of the Government's Warm Front initiative which funds insulation measures for households in fuel poverty [4]. Many of the households in the study included elderly people with chronic illness or disability which meant that they were likely to be inactive most of the time, so maybe this is not representative of the wider population either. However the figure below, from this paper, shows that when the room temperature is between 17 and 18C about half the occupants were comfortable (voting within the shaded band) and half a bit cold. When it was 19C most people were warm enough.

From a survey of homes upgraded by Warm front: about half of residents are comfortable between 17 and 18C. [4]
Personally, I am fine down to about 18C. Below that I complain about cold hands, even with a sweater on, and my beloved complains about my cold feet.

[1] Zhang, Y. and Altan, H., 2011, A comparison of the occupant comfort in a conventional high-rise office block and a contemporary environmentally-concerned building, Building and Environment 46 (2011) 535-545

[2] Engineering toolbox: Physiology: Activity and metabolic rate

[3] McIntyre, D. A, and Griffiths, I. D. , 1975, The Effects of Added Clothing on Warmth and Comfort in Cool Conditions, Ergonomics, 18(2), 205-211

[4] Sung H. Hong, Jan Gilbertson, Tadj Oreszczyn, Geoff Green, Ian Ridley, 2009, A field study of thermal comfort in low-income dwellings in England before and after energy efficient refurbishment