Why does the radiator get hot when you turn it on?

Answer

Dear Wieke,

When you open the valve on the radiator, hot water will flow into the radiator. This water usually has a temperature of 60 °C to 80 °C and ensures that the radiator becomes so hot that you cannot hold your hand against it (for long). The warm radiator will now radiate the heat (radiator = radiator) to all surfaces that can “see” the radiator. If you can see the radiator, you will get heat from the radiator. But also your bed, your closet and all other things that can “see” the radiator (that is, there is nothing between, for example, your bed and the radiator), will receive heat and warm up.

Where does this hot water come from? Well, somewhere in your house is the central heating boiler. It’s probably in the basement or garage somewhere. Fuel oil or gas is burned there (just as wood burns in a stove) and the heat released will heat water. This water is in a pipe system that will flow through every radiator throughout your house as soon as the tap on the radiator is open.

How exactly does this hot water reach your radiator? Well, that takes a pump. Near the central heating boiler is a pump that ensures that the water is moving all the time when heat is required.

There are also electric radiators. It contains electrical resistors. These have the property that they heat up when you pass current through them. Just feel a light bulb (but not for too long!): it also heats up because current passes through it. Because heating via electricity is more expensive than heating oil or gas, these are less common. The advantage, however, is that you can move them as long as you have an outlet nearby.

With warm regards,
Pieter Schevenels