Science behind setting the right temperature on the air conditioner
The Union Ministry of Power has said it is mulling restricting the temperature range of new air conditioners (ACs) in the country to between 20 degrees and 28 degrees Celsius.
In a press conference on Tuesday, Power Minister Manohar Lal Khattar told presspersons the restriction would apply to ACs in households, hotels, and cars. The option is reportedly being considered at present and no firm decision has been taken.
The idea is not new: in 2018 and then in 2021, R.K. Singh, then the Minister of State (Independent Charge) for Power had said the Ministry was speaking to AC manufacturers about labelling ACs with the optimum temperature setting from the energy efficiency and health points of view and fixing the default temperature setting at 24 degrees. At the time the Ministry had also said in a statement that it would consider instituting the default setting following an awareness campaign for four to six months and after public consultations.
'Every 1 degrees Celsius increase in the air conditioner temperature setting results in saving of 6% of electricity consumed,' Mr. Singh said. He added that the 24 degrees Celsius recommendation had come from a Bureau of Energy Efficiency study and that should all consumers adopt the setting, the country would save 20 billion units of electricity per year. The BEE had said at the time that the total connected load due to ACs would be 200 gigawatt by 2030.
Aside from calling the 18-21 degrees Celsius range 'uncomfortable', the Minister said it was 'unhealthy'. Indeed, many studies have found that the blood-pressure load rises quickly below 18 degrees Celsius, with vasoconstriction and sympathetic activation being found to drive the systolic blood pressure up by about 6-8 mm (Hg) and long-term exposure translating to higher risk of hypertension. Separate trials involving children in Japan, New Zealand, and the United Kingdom have also found they breathed easier when they slept with ACs set to more than 18 degrees Celsius. On the flip side, whole-house warming with insulation and/or heaters was found to mitigate the prevalence of respiratory infections and lower antibiotic use within a few months.
In 2018, the International Energy Agency estimated there were 2 billion ACs in use around the world and that the number of residential units tripled from 2000 to 2022, to 1.5 billion. The agency also said that as of 2022, 43% of people in the Asia Pacific region were still in need of additional cooling.
How does an AC work?
An AC works by pumping heat from one space to another. Heat naturally flows from warmer to cooler areas, which means continuously moving it in the other direction — e.g. from a room at 30 degrees Celsius to an environment at 35 degrees Celsius — requires work. This work is represented in the AC's power consumption.
The typical vapour-compression cycle of an AC uses a liquid called a refrigerant to transport the heat. A device called the evaporator holds the refrigerant at just about its boiling point. When a fan blows air in the room over the evaporator, the refrigerant boils by absorbing heat from the air. The air also becomes dehumidified as moisture in the air condenses on the evaporator and drains away. Next, it flows to the compressor as a superheated vapour. The compressor compresses it by 3-4x, in the process heating it to about 90 degrees Celsius. This is the step during which the AC consumes most of its power.
The high-pressure superheated vapour then moves to the condenser, where it naturally loses its heat to the environs while turning back into a liquid. Since its pressure is still high, it passes through an expansion device that turns it into a low-pressure liquid-vapour mix close to its boiling point, and sends it back to the evaporator.
The temperature range in which a refrigerant takes up and releases heat most efficiently is the range within which the AC is also said to be most efficient. On either side of this range the energy efficiency drops off. There is also the fact that heat transfer is more efficient at higher temperatures.
Risks of setting your AC on low temperatures
The power-cost of ACs is not the only reason to want to steer clear of lower temperatures, especially under 18 degrees Celsius, in space-cooling enterprises. Numerous studies have ascertained that save for the small fraction of people that need access to cold spaces, the general population — including infants, the elderly, people with cardiorespiratory diseases — can develop higher risks of hypertension, asthma, and respiratory infections when exposed to living spaces under 18 degrees Celsius.
Researchers have generally treated 'comfort' to be the point where a body's core temperature (around 37 degrees Celsius) and mean skin temperature can be kept constant without any sweating or shivering and when no more than about 10% of the occupants of a space say they feel too hot or too cold (called the predicted mean vote). The ASHRAE-55 and ISO 7730 standards begin from this thumb rule before adjusting 'comfort' according to the clothing, cultural sensibilities, and the prevalent types of cooling in different parts of the world.
The body at rest dissipates around 100 W of metabolic heat. Around 20 to 24 degrees Celsius, a lightly clothed person can shed that heat by radiation and convection alone without breaking a sweat or restricting skin blood flow. ASHRAE-55 allows the zone to rise roughly 0.3 degrees Celsius for every 1 degrees Celsius rise in the mean ambient temperature, up to about 30 degrees to 32 degrees Celsius.
Some sleep studies have converged on 16-19 degrees Celsius for healthy young and middle-aged adults. Cool air reportedly helps the core temperature dip by about 1 degrees Celsius, quickening sleep onset and ensuring deep sleep is stable. Infants and older adults may prefer the upper limit of around 19 degrees Celsius because their bodies' thermoregulation is less robust.
This said, the WHO's 2018 Housing and Health Guidelines recommend using 18 degrees Celsius as the minimum safe living-room temperature in temperate or cooler climates because cardiovascular and respiratory admissions were found to climb steeply below that threshold. One cross-sectional study published in 2014 reported a strong correlation between indoor temperatures under 18 degrees Celsius and 9% of the 'population attributable risk' of hypertension. Similarly, a 2016 study used data from the English Longitudinal Study of Ageing 2012-2013 to compare differences in symptoms between people exposed to living space under and over 18 degrees Celsius. It showed that those living in the colder homes had higher cholesterol and weaker grip strength.
Another longitudinal study the same year said 16% of people over the age of 50 and living in spaces cooled to under 18 degrees Celsius had higher blood pressure, lower vitamin D levels, and poorer lung function.
Respiratory and mental health
On the respiratory front: a study published in 2013 involving 309 children and more than 12,000 child-days analysed the effects of each 1 degrees Celsius drop below a mean bedroom temperature of 14-16 degrees Celsius. It revealed a drop in how quickly children could exhale air and lower lung function.
In 2022, researchers in the U.K. reported that people living in persistently 'cold homes' were at twice as much risk of new episodes of depression and anxiety, even after adjusting for incomes and baseline mental distress.
Of course, most of the studies that have helped the WHO establish the 18 degrees Celsius mark as the lower temperature threshold involved participants living in countries with temperate weather. This is partly because not many studies have been conducted in tropical or subtropical nations, where there are also fewer sub-18 degrees Celsius living spaces. Additionally, people with greater cold exposure are also likely to be more exposed to damp surfaces and/or suffer some degree of energy poverty. The latter two themselves worsen respiratory and mental outcomes.
The case for moving towards a fixed temperature range on ACs is clear — supported by public health benefits as well as energy savings.
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