What do type 11, type 21 and type 22 mean on a radiator?
The two-digit code on every UK panel radiator, what each digit actually describes, and why it is the number that decides whether a room will ever be warm enough.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 4 min
Short answer
The first digit is how many water panels the radiator has; the second is how many sets of convector fins are welded to them. Type 11 is one panel and one fin set — the slimmest. Type 21 has two panels and one fin set. Type 22 has two of each and gives roughly double the output of a type 11 in the same length.
It looks like an arbitrary code and it is actually a very direct physical description. Understanding it means you can look at a room that never quite warms up, look at the radiator in it, and know immediately whether the answer is a bigger radiator, a deeper one, or something else entirely. It also means you can replace a radiator without accidentally halving the heat in the room.
Step by step
- Read the code.First digit: the number of flat water panels. Second digit: the number of convector fin sets — the zigzag steel welded behind or between the panels. Type 11 is one panel, one fin set. Type 21 is two panels, one fin set. Type 22 is two panels, two fin sets. Type 33 is three of each.
- Translate the trade names.K1 or 'single panel single convector' is a type 11. P+ is a type 21. K2 or 'double panel double convector' is a type 22. K3 is a type 33. Different manufacturers use different names for exactly the same construction.
- Understand where the heat actually comes from.Most of a panel radiator's output is convection, not radiation. Cool air is drawn in at the bottom, heated as it passes over the fins, and leaves the top grille. That is why the fins matter far more to output than the visible front face — and why a type 22 gives so much more than a type 11 of identical size.
- Compare the depths.A type 11 is the slimmest, projecting least into a room — useful in a hallway or behind a door. A type 21 adds a second panel with little extra depth and a flat front. A type 22 is roughly twice as deep as a type 11 and a type 33 deeper still.
- Use it to solve a cold room.If a room is never quite warm and everything else in the system checks out, look at the radiator type. Swapping a type 11 for a type 22 in exactly the same wall space, with the same valves and the same pipe centres, is often the whole fix.
- Use it to avoid a mistake.Replacing an old radiator with a modern one of the same physical size but a lower type is how people accidentally make a room colder. Match or exceed the output, not just the dimensions.
- Note the response-time trade-off.A deeper radiator holds more water, so it takes slightly longer to warm from cold and stays warm longer after the heating stops. In an intermittently heated room a thinner radiator responds faster; in a continuously heated house that difference is irrelevant.
- Find the type on the radiator itself.Stamped or labelled on the back or the top, or on the manufacturer's sticker inside the top grille. If it has been painted over, count the panels by looking down through the top grille — that tells you the first digit, and you can usually see the fins.
Tips
- Two radiators of the same height and length can differ in output by a factor of two or more purely because of type. Never compare on size alone.
- Fins clog with dust and pet hair, which measurably reduces output. A long thin brush or a vacuum crevice tool down through the top grille is a genuine heating improvement.
- In a room with a low flow temperature — a heat pump, or a boiler run in compensation mode — radiator type matters even more, because each radiator gives out less at lower temperatures and needs to be bigger to compensate.
- Fitting a type 22 where a type 11 was will need deeper brackets and may affect skirting, door swings and furniture placement. Measure the depth before ordering.
Questions people ask
Which type should I buy?
Whichever meets the room's calculated heat requirement in the wall space available. Start from the required output, not from the type. Where space is tight, a deeper type gets more heat into less wall length.
Do the fins really make that much difference?
Yes. Convection over the fins carries most of the output. It is why a type 22 with the same face area as a type 11 gives close to double the heat, and why blocking the bottom or top of a radiator with a fitted cover or a deep shelf hurts it so much.
Is a designer or column radiator equivalent?
Not directly — designer, column and towel radiators use their own output figures and often give considerably less heat per unit of wall than a comparable panel radiator. Always compare quoted outputs at the same temperature conditions.