Heat From the Cold: How a heat pump warms a house with freezing air
PHYSICS · 4 MIN
How can freezing air warm up your house?
It's −15 °C outside, yet a box in the snow is pumping heat indoors. No physics needed: we'll x-ray the house, follow one sealed loop of pipe, then dive right into the icy air itself.
8 steps · 7 quick challenges
Step 1 of 8 · The cold house
A box in the snow
Winter night, snow on the ground. Inside, the house is cosy, and there isn't a flame anywhere.
A heat pump is two boxes joined by sealed pipes. The outside box gathers heat from cold air, and the indoor box uses a fan and hot coil to blow that heat into the room.
- Outdoor unit. Outside sits a metal box with a big fan and rows of thin metal fins. The fan pulls freezing air across those fins so lots of air touches the cold pipes hidden inside. Heat can move through metal, so the box can collect energy from air that feels icy to us. The trick is making the pipe even colder than the air. Like a fishing net for heat, cast into the cold air.
The aluminium fins are thin because thin metal warms and cools quickly. They are attached tightly to the pipe, so energy from the air has an easy path into the refrigerant flowing inside.
A typical home unit moves about 2,000 to 4,000 cubic metres of air an hour. That's like emptying a small house of air every few minutes, so even a tiny bit of heat from each litre adds up.
If leaves, ice or packed snow block the fins, less air touches the metal and less heat gets in. That's why outdoor units sit raised on feet, clear of drifts and shrubs.
- Two pipes. Two copper pipes run through a small hole in the wall. They hold a special fluid called refrigerant, which can be liquid or gas. It loads up with heat outside, flows indoors, drops the heat off, then returns to load up again. Copper helps because heat moves through it quickly, and the seal keeps the same fluid cycling. Like a delivery van doing the same loop all day: load up outside, unload inside.
The pipes are sealed and never open during normal use. The same few kilograms of refrigerant travel round the loop for years, so it is a carrier, not a fuel.
One pipe is wrapped in white foam because it may be much hotter or colder than the room. The insulation stops heat leaking into the wrong place on the short trip through the wall.
If the loop leaks, refrigerant escapes and the heat pump slowly loses its carrier. Installers pressure-test the pipes before filling them because even a pinhole can spoil the cycle.
- Indoor unit. High on the wall, the indoor unit contains a hot coil and a fan. The fan blows room air across the coil, so the air picks up heat and comes out warm. Most of that heat was gathered outside and carried in through the pipes. Without the fan, the heat would sit near the coil instead of filling the room. Like the unloading bay of the delivery van.
The indoor fan is important because still air is a poor heat collector. Moving air keeps bringing cool room air to the hot metal, so heat keeps flowing off the coil.
Room air often leaves the unit about 35–45 °C. That is warm enough to heat a room without the burning-hot blast of a gas furnace.
Some homes use radiators or warm floors instead of a wall unit. The same handoff happens: heat in the refrigerant passes into air or water for the house.
- Outside
- −15 °C
- Inside
- 21 °C
Did you know? Your fridge is a heat pump too. It pumps heat out of the food and dumps it from the back, which is why the back feels warm.
Read more
Most heaters make heat. A gas boiler burns fuel and an electric heater pushes current through a wire. A heat pump does something different: it moves heat that already exists from one place to another.
In winter it moves heat from the outside air into the house. In summer many heat pumps run backwards and move heat from the house to the outside, so the same machine is also an air conditioner.
Because moving heat takes less energy than making it, a heat pump can deliver about 3 units of heat for every 1 unit of electricity. The International Energy Agency says that makes them 3–5 times more efficient than gas boilers.
The rest of this story answers the big puzzle: how can there be any heat to collect in air that's below freezing?
A heat pump doesn't make most of its heat. It collects it from the cold air, the way a water pump moves water uphill.
Step 2 of 8 · The cold house
Three times the heat
So heat is collected outside and delivered inside. Before we look inside the walls, here's why that's such a good deal.
For every 1 unit of electricity, a heat pump can deliver about 3 units of heat because the electricity moves heat from air instead of making all the heat itself.
- 1 in, 3 out. 1 kWh of electricity runs the compressor and fans for a while. The indoor unit may deliver about 3 kWh of heat, because it adds the electricity's work to heat collected outside. The extra energy is not magic: it came from the air. That's why moving heat can beat making heat from electricity alone in winter. Like paying for one bag of shopping and taking home three.
Engineers call this the coefficient of performance, or COP. A COP of 3 means 3 units of heat out for 1 unit of electricity in, while a plain electric heater has a COP of 1.
COP changes with weather. On very cold days the compressor must lift heat across a bigger temperature gap, so it uses more electricity for each unit of heat delivered.
Good cold-climate units are still tested to reach about 1.75 or more at −15 °C. That means they are still moving more heat than the electricity alone could make.
- Defrost. The outdoor fins can get below 0 °C, so water in the air freezes onto them as frost. Frost blocks airflow, like a blanket over the fins. A reversing valve briefly swaps the loop's direction, sending hot refrigerant outside to melt the ice. Then it flips back to heating the house after the fins are clear. Like clearing a frosty car windscreen before driving.
Frost forms fastest on damp, cold days. It fills the spaces between fins, so the fan moves plenty of air but less of it actually touches cold metal.
Sensors watch coil temperature and running time. When defrost is needed, the reversing valve acts like a railway switch for the refrigerant, making the outdoor coil hot for a few minutes.
You might see a white puff from the outdoor unit during defrost. That's melted frost steaming away in cold air, not smoke or a fault.
- Really cold. Modern cold-climate heat pumps keep working down to about −25 °C. There is still heat in the air, but the compressor has to work harder to gather it. On rare colder nights, a small backup heater can help. The main loop still does the work whenever the air is usable, just with more effort and power draw. Like a car that still starts in deep winter.
Cold-climate units have larger coils, smarter defrost and compressors that can speed up. Those changes keep the refrigerant colder than the air, even when the air is far below freezing.
In Norway, a very cold country, about two out of three homes have a heat pump. They're built for long, dark, freezing winters, not just mild weather.
Because they use far less energy than burning fuel, heat pumps can cut bills and pollution, especially when electricity is clean. So how does the free heat get in? Let's x-ray the wall.
- Heat per 1 kWh
- ≈3 kWh
- Works down to
- ≈−25 °C
Did you know? Norway, one of the coldest countries in Europe, has one of the highest numbers of heat pumps per home in the world.
Read more
A heat pump doesn't break any rules of physics. Energy isn't created; it's moved. The electricity pays for moving heat uphill, from cold to warm, and that costs much less than making the heat from scratch.
The International Energy Agency says heat pumps are 3–5 times more efficient than gas boilers. That's why many countries are now switching homes over to them.
Cold weather makes the job harder, but not impossible. Cold-climate units are tested to keep a good COP at −15 °C, and many keep running even colder.
So where does all that free heat come from, and how does it get through the wall? Scroll on: we'll take the covers off and look inside.
A heat pump turns freezing air into a warm home, using a third of the energy an electric heater would need.
Step 3 of 8 · The sealed loop
One sealed loop
Three times the heat sounds like magic. Let's zoom into the house wall and x-ray the two boxes and the pipes between them.
A refrigerant travels round a sealed loop. Four parts change its pressure and state so it can boil to soak up heat outside and condense to release heat inside.
- The loop. Here is the machine with its covers off. Outside is on the right, inside is on the left, and the wall sits between them. One sealed pipe loop connects the two sides, so the same refrigerant can collect heat, carry it, and return. Nothing is burned or thrown away; the loop just changes pressure and temperature. Like a racetrack with four pit stops.
The loop has four main parts: outdoor coil, compressor, indoor coil and expansion valve. The refrigerant visits them in that order again and again.
The colours show temperature and state. Blue is icy and low pressure, orange is hot and high pressure, and yellow is warm liquid returning to the valve.
Because the pipe is sealed, the refrigerant does not get used up like fuel. The machine keeps changing its pressure so the same fluid can keep moving heat.
- Refrigerant. The pipe is filled with refrigerant, a fluid chosen because its boiling point can be pushed very low. Boiling means changing from liquid to gas. That change soaks up heat outside, and changing back gives the heat out indoors. Pressure is the knob that moves the boiling point up or down on demand here. Like water in a kettle, but it boils way below freezing instead of at 100 °C.
Many new heat pumps use a refrigerant called R-32. At normal air pressure it boils at about −52 °C, and changing the pressure moves that boiling point up or down.
Turning liquid into gas takes energy because molecules must pull away from each other. Turning gas back into liquid releases that energy as heat.
The loop is sealed for life. A few kilograms of refrigerant go round and round, never used up, unless there's a leak.
- Four jobs. First the refrigerant gas is squeezed until it is hot. Indoors it gives heat to room air and becomes liquid. Then a tiny valve drops its pressure so it turns icy, and outdoors it boils while soaking up fresh heat. Each part sets up the next one, so the loop never needs to pause between heat loads again. Like a sponge: soak up outside, squeeze out inside, repeat.
The compressor and the valve are the pressure controllers. High pressure makes the refrigerant able to condense while hot; low pressure makes it able to boil while icy.
The two coils are the heat exchangers. Their fins give air lots of metal to touch, so heat can cross between air and refrigerant quickly.
If any one part fails, the loop stops working. A stuck valve or broken compressor means no heat comes in, even though the fans still spin. Let's start with the compressor.
- Main parts
- 4
- Refrigerant
- R-32
Did you know? Lord Kelvin described the heat pump idea back in 1852, and the first working one was built in Austria in 1856, to dry salt.
Read more
Every heat pump, from a fridge to a whole-house system, uses the same four-part loop. Engineers call it the vapour-compression cycle.
The refrigerant changes between liquid and gas as it goes round. It's a liquid entering the outdoor coil, boils into a gas there, gets squeezed, turns back into a liquid in the indoor coil, then passes the valve and starts again.
Pressure is the master control. Low pressure makes the refrigerant boil at a very low temperature, so it can soak up heat from cold air. High pressure makes it turn back into a liquid at a high temperature, so it can heat a room.
The electricity mainly runs the compressor and the fans. It isn't turned into heat directly. It's used to push heat uphill, from cold outside to warm inside.
The refrigerant in your home heat pump might travel round its loop tens of thousands of times every winter, and never wear out.
Step 4 of 8 · The sealed loop
Squeeze it hot
First stop: the compressor, in the box outside. Gas arrives full of heat from outdoors, but it's still below freezing. Too cold to warm anything. So we squeeze it.
The compressor squeezes refrigerant gas into a smaller space. The moving scroll or piston gives the molecules energy, so the gas gets hot enough to heat the house.
- Squeeze = heat. Gas is made of tiny bouncing particles called molecules. Inside the compressor, a moving scroll or piston pushes those molecules into a smaller space. The moving metal does work on them, so they bounce faster and the gas gets much hotter. Faster bouncing is exactly what temperature means in a gas sample. Like a bike pump: pump hard and the end gets warm.
Temperature is molecule speed. When the compressor wall moves inward, molecules bounce off a moving surface and leave faster than they arrived, a little like a tennis ball hit by a racket.
The same energy is also packed into a smaller volume, so each litre of gas carries more energy. That is why a bike pump warms when you push hard.
Home compressors are usually scroll or rotary types, not pistons, but the job is the same. They spin tens of times a second and use most electricity; if they stop, the indoor coil stays cold.
- Hot gas. The gas leaves the compressor at about 70–80 °C, hotter than the room by a long way. It contains heat picked up outside plus the work added by squeezing. Because it is now hotter than indoor air, heat can flow out of it. The compressor has made outside heat useful for a warm room, not just a cold coil. Like packing a whole suitcase of warm clothes into a tiny bag.
The pressure in the hot pipe is high, around 30 times normal air pressure. That's why the pipes are thick copper with brazed joints rather than push-fit plumbing.
The compressor does not create most of the heat from nothing. It raises the temperature of heat already collected outdoors, then adds its own electrical work as extra heat.
If the discharge gas got too hot, oil in the compressor could break down. Sensors and electronics slow or stop the machine before that happens.
- Squeeze harder. The colder it gets outside, the bigger the temperature jump the compressor must create. To make indoor heat, it has to squeeze the gas to a high enough pressure and temperature. On very cold nights it often speeds up. That extra work is why efficiency drops in deep cold, but heat still moves into the house. Like pedalling harder to climb a steeper hill.
Modern inverter compressors can change speed smoothly instead of only switching on and off. On mild days they hum slowly; on colder days they spin faster.
Squeezing harder costs more electricity, so the COP drops in deep cold. The heat pump still works, but each unit of heat takes more effort.
Cold-climate heat pumps use stronger compressors and special controls to keep the gas hot when it's −25 °C outside. Up next: the hot gas gives its heat indoors.
- Gas out
- 70–80 °C
- Pressure
- ≈30× air
Did you know? A diesel engine has no spark plugs. It squeezes air so hard that the air gets hot enough to light the fuel by itself.
Read more
Squeezing a gas heats it up. Pumping air into a bike tyre warms the pump, and air in a diesel engine gets hot enough to light fuel. Scientists call this adiabatic compression.
The compressor is the heart of the heat pump. It's driven by an electric motor and is where most of the electricity goes.
It also keeps the loop moving. By sucking gas from the outdoor coil and pushing it into the indoor coil, it sets the low pressure on one side and the high pressure on the other.
The bigger the temperature jump from outside to inside, the more work the compressor has to do. That's why heat pumps are most efficient on mild days and in homes with warm floors or big radiators that only need 35–45 °C.
The same few grams of gas that were colder than the snow are now hotter than a cup of fresh coffee.
Step 5 of 8 · The sealed loop
Heat comes home
The hot gas flows along the pipe, through the wall and into the indoor coil. This is where the heat finally comes out.
The hot gas flows through the indoor coil. A fan blows room air across that hot metal, heat flows into the room, and the gas condenses back into liquid.
- Hot coil. Inside the indoor unit, 70–80 °C gas flows through another coil of pipes and fins. The room air is much cooler, so energy naturally flows from the hot refrigerant into the cooler metal and air. Heat always moves that downhill way. The coil just gives it a wide path into the room air and across the fins. Like a hot water bottle warming your bed.
The copper pipe and aluminium fins spread the heat out into a large surface. A larger surface gives air more chances to touch warm metal and pick up energy.
Nothing needs to burn here. The refrigerant is simply hotter than the room, so its faster-jiggling molecules pass energy to the cooler metal and air.
Outside, the same downhill rule will work the other way round: the refrigerant will be colder than the air, so heat will flow in.
- Warm air out. A quiet fan in the indoor unit blows room air across the hot coil. The air touches warm fins, gains energy, and comes out about 35–45 °C. That moving air spreads the heat around the room instead of leaving it trapped near the coil. You can see it spinning just behind the coil. Like a hair dryer on a gentle setting.
Fans do not make the heat; they move air to the heat. If the fan stopped, the coil would get hot locally while the room warmed slowly.
Heat pumps work best with gentle, steady warmth instead of short blasts. Many run almost all day in winter, using little power.
Filters matter too. A dusty filter blocks airflow, so less cool room air reaches the coil and the heat pump has to work harder.
- Back out. As the gas gives up heat, it condenses, which means it turns back into liquid. The refrigerant leaves as a warm, high-pressure liquid. It flows back through the wall to the valve, where it will be made icy again. The heat is indoors now, but the carrier is ready for another lap outside through the coil. Like the delivery van heading back out empty for its next load.
Condensing releases hidden heat because gas molecules stick back together as a liquid. That is the mirror image of boiling, which soaks heat up.
Some systems heat water instead of air. The hot coil warms water for radiators, underfloor pipes or a hot water tank.
Once the heat is handed over, the liquid must be reset to a colder temperature. Next stop: the valve, the tiny part that makes that happen.
- Gas in
- 70–80 °C
- Air out
- 35–45 °C
Did you know? The heat warming the room was floating around outside in the snow just a minute or so earlier.
Read more
In the indoor coil, the refrigerant does the opposite of what it does outside. Outside it boils and soaks up heat; inside it condenses and gives that heat out.
Because it's at high pressure, the refrigerant condenses at a high temperature, around 40–50 °C. That's warm enough to heat the room, even though the heat came from freezing air.
The heat given out indoors is the heat collected outside plus the energy the compressor added. That's why a heat pump gives out more heat than the electricity it uses.
In summer, a reversing valve swaps the roles of the two coils. Now the indoor coil is the cold one, collecting heat from the room, and the outdoor coil gives it away outside.
The same machine heats your home in winter and cools it in summer, just by running the loop backwards.
Step 6 of 8 · Colder than cold
Through the tiny gap
Its heat given to the room, the refrigerant flows back out through the wall as a warm liquid. To collect more heat outside, it has to get colder than the freezing air.
The expansion valve is a tiny adjustable gap. High-pressure liquid squirts into a low-pressure pipe, its boiling point drops, and boiling steals heat so it turns icy.
- Tiny gap. The warm liquid reaches a gap much narrower than the pipe. The compressor keeps the upstream side squeezed at high pressure, while it sucks the downstream side toward low pressure. Liquid is forced through the tiny opening into that emptier, lower-pressure pipe. The valve is where the pressure cliff happens. Like squeezing a garden hose so the water sprays out.
Pressure is just molecules pushing on the walls around them. Before the valve they are crowded together; after the valve the pipe has more room because the compressor is pulling gas away.
The valve is a small brass part with a needle that can move in and out. A computer or a sensor adjusts the gap every few seconds to feed just enough refrigerant.
If the gap is too wide, liquid floods the outdoor coil without boiling fully. Too narrow, and too little refrigerant flows to collect heat.
- Pressure drop. Boiling point depends on pressure: lower pressure lets molecules escape the liquid more easily. After the valve, the refrigerant's boiling point falls far below room temperature, so some liquid flashes into gas. Boiling steals heat from the rest, chilling it to about −25 °C. That cold is made by the pressure drop. Like a spray can turning icy cold as it sprays.
A liquid boils when its molecules can escape as fast as the air or gas above pushes them back. Lower the pressure above it, and they can escape at a lower temperature.
The instant boiling is called flashing. The energy for that flash has to come from nearby, so it comes from the remaining liquid and metal, making them suddenly cold.
This is the step that makes the outdoor fins colder than the air. Without the pressure drop, the loop would never get cold enough to collect heat in winter.
- Round again. The refrigerant leaving the valve is a cold misty mix of liquid and gas. It flows into the outdoor coil at low pressure, so the remaining liquid is ready to boil. As long as the compressor keeps the pressure split, the loop repeats. The cold side is now prepared to collect heat again from outdoor air. Like a reset button at the end of every lap.
The whole trip round the loop can take about a minute in a home system, depending on speed and pipe length. Each lap carries another load of heat inside.
The compressor and valve must work as a pair. The compressor creates the low-pressure side by sucking gas away, and the valve meters liquid into it.
Next stop: the outdoor coil, where the icy liquid meets the freezing air and starts boiling again.
- Liquid in
- ≈35 °C
- Liquid out
- ≈−25 °C
Did you know? A can of compressed air turns frosty when you spray it for a while, for the same reason the refrigerant goes icy after the valve.
Read more
The expansion valve is the opposite of the compressor. The compressor raises the pressure and makes the refrigerant hot; the valve lowers the pressure and makes it cold.
As the high-pressure liquid passes through the narrow gap, its pressure drops sharply. At the lower pressure it wants to boil at a much lower temperature, so part of it flashes into gas, soaking up heat from the rest.
Modern valves are electronic. They read sensors on the coils and open or close the gap a tiny bit, so the outdoor coil gets just enough refrigerant to boil completely.
Together, the compressor and the valve set two pressures: high and hot indoors, low and icy outdoors. That's the whole trick of the heat pump.
In a fraction of a second, the refrigerant falls from warm-bath temperature to colder than a freezer.
Step 7 of 8 · Colder than cold
Boiling in the snow
Now the icy liquid, at about −25 °C, reaches the last stop: the coil in the box outside, behind its fan and thin metal fins.
The outdoor coil holds refrigerant at about −25 °C. Since −15 °C air is warmer than that, heat flows from the air into the refrigerant and makes it boil.
- Colder than air. Liquid refrigerant arrives around −25 °C, while the outside air is about −15 °C. Warmer always means more molecular jiggle, so energy flows from the air into the colder metal and pipe. To the refrigerant, that freezing air is a heat source. The colder thing wins the heat flow until temperatures start to match. Like a cold glass of water pulling heat out of a warm hand.
The direction is simple: heat moves from higher temperature to lower temperature. −15 °C is warmer than −25 °C, so the air loses a little energy and the refrigerant gains it.
How much colder the refrigerant is decides how fast heat flows. A gap of about 10 °C is typical; on a colder night, the system lowers pressure to make the refrigerant colder still.
The thin fins are pressed onto these pipes. They give the air far more metal to touch than bare pipes would, so the heat path is short.
- Boiling soaks heat. As heat flows in, the refrigerant boils into gas. Boiling soaks up a lot of energy because molecules must pull apart from the liquid, not just jiggle faster. That lets the coil absorb plenty of heat while staying cold. The gas then carries that hidden heat onward to the compressor inlet pipe for another squeeze. Like sweat cooling you: as it dries, it carries your heat away.
This heat is called latent heat, meaning hidden heat. The temperature can stay almost the same while energy goes into separating molecules.
Boiling one kilogram of R-32 soaks up roughly 300 kilojoules of heat, enough to warm a large mug of tea by about 70 °C.
By the end of the coil, all the liquid has boiled into a cool gas. It's still below freezing, but now it is carrying the heat it collected.
- Fan and fins. The outdoor fan keeps pulling fresh air through the fins. Air that already gave up heat is swept away, and new −15 °C air touches the metal. The air leaving the unit is a few degrees colder because some of its energy moved into the refrigerant. Moving air keeps the heat supply fresh for the whole coil. Like a breeze drying washing faster by bringing fresh air past it.
Without a fan, a pocket of extra-cold air would sit around the coil and slow everything down. Moving air keeps the temperature difference fresh.
Chilled air may leave the unit about 5 °C colder than it went in. Then it mixes with the huge outdoors, which contains far more heat than the house needs.
Below freezing, water in the air can freeze onto the cold fins as frost, which is why the reversing valve defrosts now and then. But how can −15 °C air have heat at all?
- Refrigerant in
- ≈−25 °C
- R-32 boils at
- −52 °C
Did you know? Some refrigerants boil at around −50 °C at normal air pressure, colder than the coldest night in most places people live.
Read more
Liquids soak up a surprisingly large amount of heat when they boil. That's called latent heat, meaning hidden heat: the energy goes into pulling molecules apart, not into making them hotter.
When a liquid's pressure is low, it boils at a low temperature. Water boils at 100 °C at sea level but only about 70 °C on top of Mount Everest. The heat pump uses the same rule, but turned way down.
In the outdoor coil the pressure is kept low, so the refrigerant boils at about −25 °C. Anything warmer than that, including freezing air, can make it boil.
The fan and the fins make sure plenty of air touches the coil. Without them, a thin layer of chilled air would sit around the pipes and stop the heat flowing in.
The refrigerant is boiling while surrounded by snow. To it, −15 °C air is a heatwave.
Step 8 of 8 · Inside the air
Cold air still has heat
But how can air at −15 °C have any heat to give? Let's dive between two of those fins and keep zooming until we can see the air itself.
Air is made of molecules that never stop moving. That motion is heat, so −15 °C air still has energy to give to something even colder.
- Heat is jiggling. Every bit of air is made of tiny molecules, mostly nitrogen and oxygen. They zip around, spin and bump into each other all the time. Temperature is the average speed of that motion, so faster jiggling feels hotter and slower jiggling feels colder. Heat is motion at tiny scale, not a substance you can see. Like a crowd: a calm crowd is cool, a dancing crowd is hot.
In a warm room, an average air molecule zooms at about 500 metres per second, faster than a passenger jet. It changes direction constantly because other molecules keep hitting it.
We can't see molecules: each one is less than a millionth of a millimetre across. This scene is zoomed in millions of times and slowed down so the invisible motion makes sense.
Heat is not a glowing substance hiding in the air. It is the energy of all those tiny moving molecules.
- Still moving. At −15 °C, air molecules are slower than in a warm room, but they are nowhere near stopped. They still carry most of their motion energy. Molecules would only stop at −273 °C, called absolute zero, far colder than any winter on Earth. So cold air is not empty of energy; it is just colder than us and the room. Like a crowd that's tired but still dancing.
Scientists count temperature from absolute zero when comparing molecular energy. A 20 °C room is 293 degrees above absolute zero, while −15 °C air is 258 above it.
That means the winter air still has about 88% of the molecular motion energy of the room air. It feels cold to us because our bodies are warmer and lose heat to it.
That's the secret. Freezing air is not empty of heat; it just has less than we like, and there is an enormous amount of air outside.
- Heat flows downhill. When a jiggling molecule hits a colder fin, it gives some motion energy to the metal atoms. So heat flows from warmer −15 °C air into colder −25 °C refrigerant. The air cools a little, the refrigerant boils, and the compressor carries that heat home. The direction is warmer to colder, always, until balanced. Like a ball rolling downhill: heat always rolls from warm to cold.
Watch the molecules near the fins: they slow down and turn blue as they hand energy to the colder metal. The metal passes that energy into the refrigerant inside the pipe.
This does not break the rule that heat moves hot to cold. −15 °C air is colder than your hand, but it is warmer than −25 °C refrigerant.
That's the whole trick: the valve keeps the fins colder than the air, the air's molecular motion flows in, and the loop carries that energy home.
- Absolute zero
- −273 °C
- Fin temperature
- ≈−25 °C
Did you know? Air at −15 °C still has about 88% of the jiggle energy that air in a warm 20 °C room has.
Read more
Heat is the energy of moving particles. Every gas, liquid and solid is made of atoms or molecules that wiggle, spin and bump. Faster jiggling means a higher temperature.
Scientists measure temperature from absolute zero, −273 °C, where the jiggling would stop. On that scale, a 20 °C room is 293 and a −15 °C winter night is 258. The winter air is only about 12% less jiggly.
Heat moves by bumping. When fast molecules hit slower ones, the slow ones speed up and the fast ones slow down. That's why heat always flows from hot to cold, never the other way on its own.
So to collect heat from cold air, you need something even colder. If the fins are at −25 °C, heat flows from the −15 °C air into them, just like heat flows from a hot drink into your cold hands.
In the air around you right now, each molecule gets hit by others about 5 billion times every second.