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Cross-market term

Radiator and heat emitter

The component in a wet heating system that transfers heat from the circulating water into the room. Despite the name, most of that transfer is convection rather than radiation.
  • 4Markets
  • NoSame thing everywhere
  • 4Calculators

What it is called, by market

One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.

  • United Kingdom

    Same word, different thing

    radiator

    also rad, panel radiator, convector, towel rail

    Almost always a pressed steel panel with convector fins, on a pumped low-temperature water circuit. The name is a misnomer: a panel radiator moves most of its heat by convection.

  • Australia

    Close, not identical

    radiator

    also panel heater, hydronic radiator, column radiator

    Hydronic heating is a minority system, so radiator often means an electric panel heater in everyday speech rather than a wet emitter.

  • United Statesyours

    Same word, different thing

    radiator

    also steam radiator, cast iron radiator, baseboard, register

    The word most often means a cast iron steam or hot water unit in an older building. A modern American house is more likely to have forced-air REGISTERS or hydronic BASEBOARD, neither of which is a radiator.

  • Canada

    Same word, different thing

    radiator

    also baseboard, register, hydronic radiator

    As the United States, with hydronic systems more common, so a true wet radiator is less unusual than south of the border.

The governing standard, by market

  • United Kingdom

    BS EN 442 / Approved Document L

    Radiator output testing and the standard temperature conditions it is quoted at, and conservation of fuel and power.

  • United States

    ASHRAE Handbook — HVAC Systems and Equipment

    Hydronic and steam heating equipment, and the rating conditions for heat emitters.

Calculators for this

Each works in either measurement system, and the terminology on the page follows whichever market you have selected.

Electric Baseboard Heater Sizing Calculator →Baseboard Heater Length Calculator →HVAC Thermal Load (U-Value) Calculator →Room Heat Loss and Radiator Size Calculator (Watts and BTU) →

Frequently asked questions

Why does a radiator's stated output not match what it delivers?
Because the quoted figure belongs to a test condition, and almost no real system runs at it. Outputs are published at a standard temperature difference between the water and the room — commonly a mean water temperature 50 degrees above room temperature under the modern European convention, and older catalogues used a larger difference still. Run the same radiator at a lower flow temperature, which is exactly what a condensing boiler or a heat pump wants in order to be efficient, and its output falls steeply: a system designed around 75 degree flow and rerun at 45 delivers a fraction of the catalogue number. That is the single most important fact when converting a house to a heat pump — the emitters usually have to grow, and the radiator that was adequate becomes the reason the house is cold.
Why is it called a radiator if it mostly convects?
History, and it is worth knowing because it explains the shape of the modern product. Early cast iron units did radiate a significant share of their heat: large surface area, high surface temperature, exposed on all sides. A modern steel panel runs cooler and gets most of its output from air being warmed at its surface, rising, and drawing cooler air in behind it — which is convection, and is why the fins welded to the back of a double-panel radiator matter so much: they add surface for the air to pass over, not surface you can see. It is also why boxing a radiator in, putting a shelf immediately above it, or standing a sofa against it costs real output: those all interfere with the air path rather than with the radiation.
What is the lockshield valve for, and why is one end different?
A radiator has two valves doing two different jobs. One end is the control — a wheelhead or a thermostatic valve — which is meant to be adjusted by the occupant. The other end is the LOCKSHIELD, capped so it cannot be casually turned, and it sets how much of the system's flow this particular radiator takes. That is balancing: without it, the radiators nearest the pump take most of the flow and the far ones stay cool, regardless of how far open their control valves are. A cold radiator at the end of a run, with everything else hot, is nearly always a balancing problem rather than a fault — and it is why fitting a new pump or a new boiler to an unbalanced system does not fix it, and sometimes makes it more obvious.