“Cold” isn’t a thing you can add — it’s just the absence of heat. So a fridge can’t make cold; instead it grabs heat from inside and carries it out to the room. It uses a special liquid that loops round and round. A pump squeezes it until it’s hot, the coils at the back let that heat escape into your kitchen, then the liquid is allowed to expand suddenly and turns very cold — cold enough to soak up heat from your food. Round and round, scooping heat out and dropping it behind the fridge. Feel the back: it’s warm, because that’s where your food’s heat ends up.
Most people think a fridge makes cold. In fact there is no cold to make, cold is just the absence of heat, so a fridge pumps heat out of the box and dumps it into your kitchen, which is why the back is warm.
What's actually happening
Start with the idea that trips everyone up: there is no “cold” to manufacture. Cold is simply less heat — slower-jiggling molecules. So a fridge has nothing to add to your milk; it has something to remove. Its whole job is to be a heat-mover, lifting warmth out of an already-cool box and carrying it somewhere warmer. That sounds like it should be impossible (heat flows from hot to cold on its own, never the other way) and it is impossible for free. The price is the electricity that runs the pump.
The carrier is a refrigerant, a fluid chosen to boil and condense at convenient pressures, and it runs a four-stop loop. First the compressor squeezes it into a hot, high-pressure gas — squeezing anything heats it. That hot gas flows to the condenser, the black coils on the back, where it’s now hotter than your kitchen, so heat pours out of it into the room and the refrigerant condenses to a liquid. Next it slips through a narrow expansion valve and the pressure suddenly drops; the fluid expands and flashes shockingly cold, colder than the inside of the fridge. Finally that cold refrigerant runs through the evaporator coils inside, where it’s now the coldest thing around, so heat from your food flows into it and it boils back to gas. Then it returns to the compressor and goes round again.
Follow one parcel of refrigerant and the trick is obvious: it picks up heat where it’s cold (inside) and lets go of heat where it’s hot (the back), because we keep changing its pressure to flip which side of the temperature gap it’s on. An air conditioner is the same machine pointed at a room; a heat pump for warming a house is the very same machine run the other way, harvesting heat from cold outdoor air and dumping it indoors. None of them make hot or cold — they relocate heat, and the law they obey demands you pay in work to push heat uphill.
A fridge relocates heat rather than making cold, paid for in electricity — and a home heat pump is the very same machine run the other way.
- 1Reach behind or beneath a running fridge (carefully) and feel the coils or the breeze — it’s warm. That’s your food’s heat leaving.
- 2Now recall a different pump: spray some aerosol or let air rush out of a bike tyre and feel it go cold — expanding gas chills, exactly like the fridge’s expansion valve.
- 3Squeeze (warm) and expand (cold) are the two halves of the loop — the fridge just does both, endlessly.
Common questions
The compressor squeezes the refrigerant into a hot gas; the back coils (condenser) shed that heat to the room; an expansion valve drops the pressure so it flashes cold; and the inside coils (evaporator) absorb heat from your food. Then it repeats.
No — it warms the room. An open fridge dumps more heat out of its back than it removes from the air, plus the motor's own heat, so the net result is warmer.
Essentially yes, run the other way. A home heat pump harvests heat from cold outdoor air and dumps it indoors, delivering several units of warmth per unit of electricity because it pumps heat rather than making it.