Keeping warm in cold conditions - field studies of personal heating devices

My last post discussed how much we need to heat our homes to keep them healthy. The short answer was, about 8°C warmer than outside. This could mean temperatures as low as 12°C or even less. So how do we keep ourselves comfortable? Wearable devices, warm cushions, portable electric heaters? This post reviews some field studies described in academic literature. It seems to be a hot topic in China.

Studies often involve students who are fit and healthy - but similar principles apply to all.

There have been several studies of thermal comfort in offices connected with universities. They test personal heating devices such as heated cushions and radiators under the desk. Unfortunately all these studies seem to be conducted with young, fit adults such as post graduate students. It is not entirely clear how to extrapolate from them to people like me, still less my husband with his dodgy heart. However, there are things to learn from their results

With warmth for legs, seat, back and arms, students can be comfortable in temperatures as low as 11°C.

Yang et al [1] asked their test subjects to work at a desk in an unheated area of the office with indoor temperatures as low as 11°C. They were allowed to use any combination of: a heated cushion, heated backrest, a heated mat on the desk (presumably for their arms) and a radiant heater for their legs under the desk. The most popular combination was all of them. In moderate temperatures (about 15°C) the next most popular combination was the radiant heater with the heated table mat. Possibly the cushions were not sufficiently warm as they used only 20W each (1.4p/hour for both at the current price cap) but the radiant heater used up to 170W (6.8p/hour). With this support, 80% of the participants said they were comfortable even at 11°C. 

The experiment was conducted over 45 days and the average room temperature was about 15°C, with average power for personal heating at 145 W (about 5p/hour at current prices). The maximum power used for personal heating, corresponding to 11°C was 300 W. 

For tests in [1] test subjects were allowed to use any combination of: a heated backrest, heated cushion, heated desk pad and a radiant heater under the desk. Note the latter has warms the sides of the leg as as well as the front.

In another study, a heated cushion was most popular.

A similar study [2] asked students about their preferences between three devices: another heated cushion but more powerful this time, at 40 W; a radiant electric heater; and a warm air blower. This time the cushion was most popular - the warm air blower was least liked by a long way. However, in this experiment conditions were not nearly so cold as in the first study. The ‘cold’ conditions were actually 18°C.

A Huotong is a traditional heated stool/brazier for market traders.

Sometimes the old ideas are the best. Another journal article [3] tested a ‘Huotong’ which is a device traditionally used by market traders in China to keep warm. It consists of a charcoal brazier inside a wooden barrel which you sit on. It also has a foot rest to keep your feet off the cold ground. In some ways this is like a heated seat but it warms the legs by radiation and the back by convection too. The device was tested in semi-outdoor conditions by 326 traders in age ranging from under 20 to over 60 years old. 96% of the traders found the barrel provided acceptable warmth in conditions down to 9.2°C.

The Huotong is a wooden barrel-like stool containing a charcoal brazier. It heats the seat, legs and back and keeps the feet off the cold ground.

The researchers did not record the amount of charcoal used but it did say that the brazier would be loaded with about 100g at a time and I should imagine this would be reloaded several times a day. 100g charcoal has 0.8 kWh energy. Using charcoal in the home would be very dangerous and probably poison you with carbon monoxide. If the Huotong could be powered by electricity instead it would cost about 28p for each 'charcoal load'.

Radiant versus wearable heat for factory workers.

One more another nice study [4] looked at wearable heating devices for people doing manual work requiring dexterity. This compared four devices: a portable radiant heater (450 W) and three heated wearables: wrist bands (16 W), ankle bands (20 W) and a vest (60 W). There were twenty participants, of which half were female. During the test they had to perform two manual tasks: assembling screws and building shelves. I have no idea what assembling screws means; clearly something was lost in translating from the Chinese. However the article clearly says the task involves handling metal things whereas the shelf task was mainly handling wood. The ambient temperature was either 13°C or 15°C.

The best heating combination for dexterity was a vest and heated wrist pads.

The radiant heater was not popular as it only heated one side of the body leaving the other cold. The best performance came from combining the heated vest with wrist or ankle bands. (The combination of all three was not tested.) Performance, especially for the screw task at the lower temperature, was closely linked to how the hands were feeling: cold hands were clumsy and slowed the work. The wrist bands made a big difference. High performance did not necessarily mean thermal comfort or vice versa. However thermal satisfaction was also best with the vest and heated ankles or wrists. A vest and wrist bands together would use 76 W which would cost 2.5p/hour.

In January and December you can save half your gas bill by lowering the thermostat.

To put these costs into context, I used my model of a semi detached house with filled cavity walls with Cambridge weather. I compared three heating patterns: normal (3 hours in the morning and 4.5 hours in the evening), brief (lower thermostat and only 1 hour in the morning, 2 in the evening) and constant (14- 15°C all the time). 

Living room temperature on one December day using the three heating patterns from my modelling. In the brief pattern the room temperature drops below 13°C overnight.

Either of the two miserly patterns save about 25% of the annual gas bill (£400 in the model). During January/ December, the coldest months, gas savings were about half. With two of you to keep warm for 12 hours each day, if you used 50 W each, that would mean 1.2 kWh total for the day, costing about 48p. Using the semi-detached model, you would still save £1.80/day on the brief pattern, a little less with the constant pattern.

Daily cost of gas used by month with different heating patterns from my modelling. The summer time gas use is for hot water. Cooking is electric.

For these estimates I assumed less energy consumption for appliances than usual. Further savings on appliances would increase gas use but reduce overall costs because gas is so much cheaper than electricity.

Conclusions

There are some principles that we can draw from these examples.

• Forced air devices are least preferred for keeping warm
• Radiant heaters that apply to only one side of the body are not very comfortable
• The closer to the body the less power needed. So heated cushions or wrist bands use far less than radiant heaters (e.g. 15-40 W versus 150-500 W).
• Legs and arms are most likely to get cold and need the most support. 
• If you need to work with your hands, keeping your wrists warm helps
• Savings from personal heating devices can be high with only one or two of you at home but each additional person warmed reduces the savings.

Although these tests were for active heating devices, some of the lessons would apply to warm clothing, such as wrist warmers helping for manual tasks. I often use fingerless gloves for typing and this makes a huge difference. Currently my feet are cold but that does not affect my typing. 

[1] Bin Yang, Mengchun Wu, Zhe Li, Huangcheng Yao, Faming Wang (2022) Thermal comfort and energy savings of personal comfort systems in low temperature office: A field study. Energy and Buildings https://doi.org/10.1016/j.enbuild.2022.112276

[2] Yin Tang , Hang Yu, Kege Zhang , Kexin Niu , Huice Mao , Maohui Luo (2022) Thermal comfort performance and energy-efficiency evaluation of six personal heating/cooling devices. Building and Environment https://doi.org/10.1016/j.buildenv.2022.109069 

[3] Yingdong He, Nianping Li, Wenjie Zhang, Linxuan Zhou (2016) Thermal comfort of sellers with a kind of traditional personal heating device (Huotong) in marketplace in winter. Building and Environment. https://doi.org/10.1016/j.buildenv.2016.06.035

[4] Haiying Wang , Manshu Xu, Chunxiao Bian (2020) Experimental comparison of local direct heating to improve thermal comfort of workers. Building and Environment. https://doi.org/10.1016/j.buildenv.2020.106884