Why does it seem hotter when you step outside your air-conditioned home or office?
Because it is!
I do yoga in my treehouse in the evenings, and ever since late spring (in the South Bay, CA) when the daytime temperature first went above 78F, the peaceful meditative sounds of my quiet neighbourhood — rushhour traffic, lawn-mowers, leaf-blowers and water fountains — have been drowned out by my neighbours’ air-conditioning units kicking on. It is enough to startle my cat.
Some of my neighbours complain about it being hotter when they step outside, “It is not global warming, but it feels hotter!”. Of course it couldn’t be global warming, if it were you would actually have to do something about it — like drive less, consume less, drink less stupid bottled water …
So I decided to see if it is true, as famous teen conspiracy theorist Greta Thunberg has alleged, that “turning on your air conditioner causes the outside air temperature to rise”.
We want to be able to estimate the rise in temperature of the outside air, the “hot” heat sink, relative to the decrease in temperature of the inside air, the “cold” heat sink.
The heat or energy required to increase the temperature is the product of the increase in temperature, the specific heat and the mass of the heat sink
Wending your way through a couple of wikipedia articles or a thermo textbook
where EER = Energy Efficiency Ratio. The best EERs are just less than 14, for a drop in temperature between 95F and 80F. So
- the entire building stock is being cooled
- Only the air inside is being cooled
- The heat from the HVAC or AC is being shed into the same air layer that the building occupies — so if the building is 1-storey the “hot heat sink” is also only 1-storey.
1 and 3 allow us to estimate the ratio of the masses as the building coverage. Here s a Google Earth image of a residential part of San Jose
Since the building coverage is about 0.5, the ratio of the masses is about 1:1. Assumption 2 means the specific heats are the same.
But probably the exterior effective thermal mass is easily 10 times as great, due to convection, breeze and radiation.
With these assumptions, the “AC effect” is that for every 8 degrees of cooling of inside air the outside air temperature rises by 1 degree.
So cooling your house to 80F when the outside is 95F means you are raising the outside temperature by about 2F! You just made it hotter for me in my shaded tree house!
Compare this to the “urban heat island effect,” in which average annual temperatures are 1.8–5.4°F higher in cities than surrounding suburban and rural areas, from http://css.umich.edu/factsheets/us-cities-factsheet.
- Building coverage ratios vary from 0.1 (suburban) to 0.7 (urban). So 0.5 for an “average” city seems reasonable. (Data provided by Dr. Elek Pafka personally; see this article for related urban density stuff.)
- Perhaps only a fraction of the entire building stock is being cooled at any given time — not all rooms in all buildings. This would reduce the impact of the AC effect.
- If there is a breeze, or convection is more powerful, the mass ratio could be easily greater than 10, this would decrease the AC effect.
- Of course one doesn’t just cool the inside air, one cools all the higher specific heat material inside. Furniture, toys, books, electronics (which is also shedding heat and this contributes extra to the heat that has to be removed from the inside, and, I love this, all the high specific heat drinking water in plastic water bottles in refrigeration units which are also shedding heat inside the building! This will increase the AC effect.
- The South bay has increasingly many 100F days. And what warm-blooded American cools the air to just 80F? None. The women want it warmer, men want it colder, guess who wins in spite of the “increased productivity due to warmer temperatures”. So realistically, 65–70F for the inside room temperature. This has TWO effects. First, for the same AC effect, the double temperature difference will cause a doubled temperature increase in the outside air, to 4 F. Second, cooling is less efficient the larger the difference in heat sink temperatures. So the EER will be less than 14. I know the Carnot Efficiency will be halved, it is reasonable to assume that so will the EER.
Thus the effective increase in outside temperature due to the “AC effect” is 8F. Of course, this applies to hot summer days when the AC is running, and I hope people turn their ACs off when they go to work and that places of employment turn theirs off when they are closed. (See 2 above.)
On average, the impact of the AC effect is probably a couple of degrees F increase in outside air temperature.
Hot enough for you?