The Illiteracy, Not the Heat

It is fifteen degrees outside. It is twenty-eight inside. A machine the size of a filing cabinet stands in the middle of a small bedroom, roaring, hosing its own exhaust out a sealed window, and it is losing. Thirteen cubic metres. Thirty minutes. Two degrees.

Outside is the cold reservoir the room needs. The hose has claimed the window, and there is no useful path for a night breeze through the building. I am running a compressor while cooler air waits on the other side of the glass.

That room is mine. The mistake in it is everyone's.

What it costs to be wrong about this

In August 2003, officials requisitioned a refrigerated warehouse at Rungis, the food market outside Paris, to hold people who had died during the heatwave. The morgues were full. The warehouse was held at five degrees Celsius and could accommodate up to two thousand bodies. France recorded about fifteen thousand excess deaths between 1 and 20 August. Older people bore the heaviest toll. The scale of the emergency gave the country its first national heatwave plan.¹²

One response was a requirement for residential care facilities to provide a cooled room. For some facilities the 2005 decree specified a fixed cooling system; mobile units were a temporary measure while they installed one. The rule created a refuge inside an institution, rather than a guarantee that every bedroom would stay safe.³

There is a story passed around about a public nursing home, somewhere in France. Could you turn on the air conditioning? It has to be over twenty-eight degrees. Perfect, it's thirty-one in the common room. The director has to authorise it himself. Well, where is he? On holiday. Back next week.

When I started writing this I believed the twenty-eight-degree rule was real. I had absorbed the folklore along with everyone else. The law was easy to look up and I had not looked it up. Article R241-30 of the French energy code says a cooling system may run only when an indoor space exceeds twenty-six degrees. The next article, R241-31, exempts homes and certain facilities, including those housing older people, as well as spaces with special temperature or air-quality needs.⁴ The nursing-home story may describe a local policy; it does not describe that law. Nor does the law govern the portable unit in my bedroom.

In a care home, a cooled room has to be reachable. In my bedroom, cool air has to get in.

What cooling actually is

An air conditioner does not make cold. It moves heat. It is a pump — a heat pump — that takes warmth from the air inside and carries it to the air outside, the way a bilge pump moves water from where you don't want it to where you don't care.

In a reversible heat pump, a valve lets the same refrigeration cycle move heat indoors in winter. It is fifteen degrees outside. The heat I want to get rid of wants, on its own, to flow to where it is cooler — outside. I do not need to pump it uphill against a gradient. The gradient is already pointing the right way.

A room needs an inlet, an outlet, and time to shed the heat stored in its walls and furniture. Swapping thirteen cubic metres of air does not cool the building's thermal mass in one pass. A cross-breeze, an exhaust fan with a deliberate inlet, or mechanical night ventilation needs time and a clear route. My machine's exhaust draws replacement air into the room through whatever gaps the building has, but I do not control where that air comes from or whether it washes the hot surfaces. The cold night is available; I have given it no route through the room.

In hot weather, the one-hose design has another cost. A single-hose model draws air from the room to cool its condenser and blows that air out. Replacement air enters through cracks and openings, carrying outdoor heat back in. The US Department of Energy includes this infiltration load when it rates portable units. A two-hose model draws condenser air from outdoors and reduces the pressure imbalance, though a better design is no guarantee that any particular two-hose unit outperforms every one-hose unit. The useful comparison is measured cooling capacity and efficiency under a test that counts the replacement air.⁵

EU Regulation 626/2011 puts single- and double-duct units on the same cooling-efficiency class thresholds, using rated efficiency at specified test conditions. It defines the single duct as a machine that takes its condenser air from the room, yet the label does not make the resulting building-infiltration load visible in the way the US portable-air-conditioner test does.⁶ A buyer cannot see that hot-air load in the efficiency class.

At fifteen degrees outside, even a two-hose unit would leave the cold outside the sealed window. It would stop pulling make-up air through the cracks. I needed a route for air, not a different number of hoses.

Build tight, then let air move

Insulation and airtightness slow heat entering from outside. They also slow unwanted heat leaving after the sun has reached the interior. Whether that helps in summer depends on shade, stored heat, and what happens when evening air cools. A tight building still needs deliberate ventilation for moisture and indoor pollutants; it also needs a way to purge heat when the outdoor temperature falls. The two jobs require different airflows.

Among French new-build projects inspected by the state, Cerema reported a ventilation-system defect in 70 percent of operations. In a separate British study of 85 dwellings with mechanical heat-recovery ventilation, commissioning data were available for 51; only 16 percent of those systems had been commissioned correctly for airflow and balance.⁷⁸ A duct on a drawing is no promise of air in a room.

A French simple-flux system usually extracts air from wet rooms and draws replacement air through inlets elsewhere. A double-flux system brings supply and exhaust through a heat exchanger, saving warmth in winter. Many double-flux units can bypass that exchanger when outdoor air is cooler than indoor air. Bypass prevents the outgoing warm air from reheating the incoming night air. It does not, by itself, turn a normal fresh-air flow into the high-volume purge that cools walls. Night cooling may need a boost mode or openable windows on opposite sides of the building. ADEME recommends that kind of cross-ventilation when conditions allow it.⁹

A working inlet and outlet might have been enough for my room.

The ventilation system must also remove moisture. In an exploratory study of 72 energy-efficient French homes, the indoor-air observatory found active mould growth in 47 percent, compared with 37 percent in its national reference sample. The buildings and residents differed in more ways than airtightness, so those percentages do not show that insulation caused mould. They are a reason to inspect how well moisture is removed when an energy-efficient home is built or renovated.¹⁰

The person a comfort model misses

Thermal-comfort research distinguishes a standard model of heat balance from an adaptive one: in buildings where occupants can adjust windows, clothing, and other parts of their environment, acceptable temperatures shift with outdoor conditions. ASHRAE's adaptive method has specific conditions for when it applies. It is not a license to take away control and tell people to adapt.¹¹

Air movement extends comfort without lowering the room's temperature. A fan helps sweat evaporate, and a warmer air-conditioned room with a fan can feel cooler than the thermostat alone suggests. WHO suggests setting air conditioning to twenty-seven degrees and using a fan where that remains comfortable. It also warns that above thirty-five degrees, a fan may not prevent heat-related illness.¹² The effect depends on humidity, air temperature, and the person in front of it.

My roaring box is my own illiteracy, not Europe’s alone: I am the healthy adult the fan was invented for, fighting physics in the dark because I, too, never learned what my building was supposed to do. An older person with limited mobility or impaired thermoregulation may need mechanically cooled space and help reaching it. When the night itself stays hot, the outdoor heat sink disappears.

Shade, night air, and a fan can reduce compressor hours when they work. In a care home or a run of hot nights, a right-sized machine may carry the load.

A river you cannot touch

Paris has built a much larger version of the heat-moving machine. Fraîcheur de Paris circulates chilled water through an underground network that serves buildings such as the Louvre. Government data list 903 delivery points and 401 gigawatt-hours of cooling delivered in 2024; the published network length is 103 kilometres.¹³ The Seine can provide direct cooling when its water is cold enough. When that route is unavailable, it absorbs heat from chillers.¹⁴

A flat above the pipe cannot subscribe on its own. The building needs a connection and a way to distribute cooling indoors. The network's recorded deliveries in 2024 were all classified as tertiary; its reach into ordinary housing remains limited. Paris plans to extend it, and residential buildings can be considered for connection.¹⁵

France's electricity is unusually low in carbon. RTE reports an average of 19.6 grams of CO₂-equivalent per kilowatt-hour of electricity produced in 2025, against 178 grams for the wider European comparison it uses.¹⁶ That is an average, not the emissions caused by switching on one more air conditioner at a particular hour. Exports do not, by themselves, prove the marginal unit is carbon-free. The low-carbon supply makes efficient electric cooling a serious part of French heat protection.

Some gases used in cooling equipment have high warming potential if they leak, and the EU's 2024 F-gas regulation phases in tighter limits for new equipment.¹⁷ The climate cost of a given machine depends on its charge, leakage, electricity use, and the electricity available when it runs. A low-leak, efficient machine with a lower-impact refrigerant cuts both costs.

A shaded window slows the next day's heat. When the outdoor air cools, an inlet and outlet let the room shed what it stored. A fan helps the person; a maintained cooling system takes over when those measures cannot keep them safe.

The hose seals my bedroom window. Outside, it is fifteen degrees.