In some houses, stoves or fireplaces are lit to heat the rooms. They heat the air around them. The warm air is light and rises towards the ceiling. Cold air, Conversely, it's heavy. When hot air rises upwards it pushes cold air, descending to the ground.
Modern houses have heating. But the boiler is located in a certain part of the house. How does the heat spread through all the rooms in the house?
There are boilers that heat a mass of air. Fans blow the hot air into certain tubes that flow through openings in all rooms of the house..
Other boilers heat water inside tubes. Through these, hot water reaches the radiators. Hot water heats up radiators, and these, at the same time, they warm the rooms.
Sometimes, hot water circulates through serpentine tubes, placed under the pavement. So, the floor heats up and warms the room.