Steel, aluminium or cast iron: a radiator's material choice is not just aesthetic. It determines heat-up speed, inertia, weight, price and — a point now decisive — compatibility with a heat pump and low-temperature operation. Here is how to decide according to your installation.
The three materials head to head
Steel (panel radiators) heats up fast, costs little and fits anywhere, but has little inertia: it cools as fast as it heats. Aluminium heats up even faster, is light and responsive, ideal in a room heated intermittently.
Cast iron, by contrast, is slow to heat but releases a gentle heat long after the boiler stops: it is maximum inertia, at the cost of high weight and budget.
- Steel: fast heat-up, low price, low inertia
- Aluminium: fastest heat-up, light, responsive
- Cast iron: maximum inertia, gentle heat, heavy and expensive
Inertia, responsiveness and thermal comfort
The right answer depends on your use. For a continuously occupied room (living room), the inertia of cast iron smooths variations and comfort is excellent. For a room heated occasionally (bathroom, office), the responsiveness of aluminium or steel avoids heating into the void.
Inertia has a downside: a cast-iron radiator takes time to respond to a setpoint, making it less suited to very dynamic room-by-room control.
The decisive criterion: heat-pump compatibility
A heat pump works well at low water temperature (35-45°C). At this temperature, a radiator must offer a large exchange surface to deliver enough power.
Steel panel radiators of type 22 or 33, with double or triple fin, are the most efficient at low temperature and remain affordable. Aluminium also works very well at low temperature thanks to its responsiveness. Cast iron, more inert, is less ideal on a pure heat-pump setup.
Understanding types 11, 22 and 33
On a steel panel radiator, the type describes the number of panels and convection fins. Type 11: one panel, one fin — slim and discreet, low power. Type 22: two panels, two fins — the versatile standard. Type 33: three panels, three fins — maximum power in a reduced footprint, perfect at low temperature.
At equal height and length, a type 33 delivers markedly more watts than a type 11, which is valuable when you want to keep compact emitters with a heat pump.
- Type 11: slim, low power, undemanding rooms
- Type 22: standard, good power/footprint compromise
- Type 33: max power, ideal low temperature / heat pump
Which brands to choose
In steel panels, the reliable references distributed in BE are Henrad, Stelrad, Vasco and Finimetal. For design and decorative radiators, Zehnder and Acova offer steel and aluminium with fine finishes.
Choose the brand according to the availability of the exact dimensions (height/length) you need and the desired finish. All these brands offer low-temperature compatible models in type 22 and 33.
Frequently asked questions
Which radiator for a heat pump?
Prefer a radiator with a large exchange surface, able to heat at low temperature (35 to 45 °C). Steel panels of type 22 or 33, with double or triple fin, are the most efficient and economical. Aluminium also suits thanks to its responsiveness. Cast iron, too inert, is less suited to a pure heat pump.
Steel or aluminium to heat quickly?
Aluminium heats up fastest because it is light and very responsive, ideal for a room heated intermittently. Steel also heats up quickly and costs less. Both have little inertia: they cool quickly once the boiler stops.
Is cast iron still worth it?
Yes, in a continuously occupied room where you seek gentle, stable heat. Cast iron offers maximum inertia and releases heat long after stopping. However it is heavy, expensive, slow to respond and less suited to dynamic control or a low-temperature heat pump.
How to calculate a radiator's power?
You start from the room's heat demand, which depends on its volume, insulation and climate zone. In a Belgian renovation, the heat losses are often estimated, then a radiator is chosen to cover that power at the installation's real water temperature — at 45 °C for a heat pump, not at 70 °C.
