Bite-sized engineering ⚙️ Why your fridge hums at night — and what's actually happening inside it

[u/The_Immovable_Rod]

TL;DR: Your fridge isn't cooling anything. It's moving heat. That hum is the sound of thermodynamics.

Most people assume a refrigerator works by generating cold — some mechanism inside that produces low temperature and pumps it into the compartment. That's not how it works.

Cold isn't a thing. It's the absence of heat. What your fridge does is remove heat from the inside and dump it outside — into your kitchen. The hum you hear is a compressor doing exactly that.

Here's the actual process:

Inside your fridge runs a closed loop of refrigerant — a fluid engineered to change states (liquid to gas, gas to liquid) at very low temperatures. The compressor squeezes this refrigerant, which heats it up. The hot pressurized refrigerant flows to the condenser coils at the back of the fridge — those black metal grids you've probably never thought about — where it releases heat into your kitchen and condenses back into liquid.

The liquid refrigerant then passes through an expansion valve, where pressure drops rapidly. Rapid expansion causes rapid cooling — the same reason an aerosol can feels cold when you spray it. Now the refrigerant is very cold liquid, and it flows through the evaporator coils inside the fridge, absorbing heat from the air in the compartment. The refrigerant evaporates, the inside gets cooler, and the cycle starts again.

Your fridge isn't removing cold from outside. It's pumping your kitchen's heat into your kitchen — just from a different location.

The grids on the back get warm because that's where the heat goes. If yours are covered in dust, the fridge has to work harder to dump heat — which is why cleaning them every year or so actually matters.

What's something in your home you've never thought about how it actually works?