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Type 11 vs Type 21 vs Type 22 Radiators: Which One Do You Need?
If you've been shopping for a new radiator, you've almost certainly run into the labels Type 11, Type 21 and Type 22 — and unless you're a heating engineer, they probably mean very little. They aren't brand names or model numbers. They're a standardised code that tells you exactly how a convector radiator is built, how much heat it puts out, and how far it will stick out from your wall.
Get the type right and your room heats quickly, evenly and efficiently. Get it wrong and you'll either be left shivering or paying to heat a space you've oversized. This guide breaks down the three most popular types in plain English so you can buy with confidence.
What is a convector radiator?
A convector radiator heats a room primarily through convection rather than radiated heat alone. Behind (or between) the flat front panels sit rows of zig-zag metal fins called convectors. Hot water flows through the panels, the panels heat the fins, and the fins rapidly warm the air passing over them. That warm air rises, cooler air is drawn in to replace it, and a steady circulation heats the whole room.
The more panels and convectors a radiator has, the greater its surface area — and the more heat it can produce from the same wall width and height. That is the entire logic behind the Type numbering system.
Cracking the code: what the numbers actually mean
The two-digit code is simple once you know the rule. The first digit is the number of panels. The second digit is the number of convector (fin) layers.
| Type | Also known as | Panels | Convectors | Depth from wall | Heat output | Best for |
|---|---|---|---|---|---|---|
| Type 11 | K1 | 1 | 1 | Slimmest | Lowest | Small rooms, tight spaces, en-suites |
| Type 21 | P+ / PC | 2 | 1 | Medium | Medium | Most general-purpose rooms |
| Type 22 | K2 | 2 | 2 | Deepest | Highest | Larger or harder-to-heat rooms |
You may also come across Type 33 (K3) — three panels and three convectors — for the highest output where wall space is limited. For most homes, though, the choice comes down to 11, 21 or 22.
Type 11 radiators (single panel, single convector)
A Type 11 radiator has one front panel backed by a single layer of convector fins. It's the slimmest convector radiator available, projecting the least from the wall, with a clean, minimalist profile.
Because it has the smallest surface area, it also has the lowest heat output of the three. That isn't a weakness — it's exactly what you want in a small bathroom, en-suite, cloakroom or hallway, or any space where a beefier radiator would overheat the room and waste energy. If you're tight on space and the room is small, Type 11 is often the smart, efficient choice.
Choose Type 11 if: the room is small, you want the slimmest possible profile, or you simply don't need a high heat output.
👉 Browse our Type 11 Radiators collection
Type 21 radiators (double panel, single convector)
A Type 21 radiator — sometimes labelled P+ — has two panels with a single layer of convector fins sandwiched between them. That extra panel boosts heat output noticeably over a Type 11, while keeping the radiator slimmer than a Type 22.
This makes Type 21 the all-rounder. It strikes the best balance between heat output and depth, which is why it suits the majority of standard-sized rooms. If you want more warmth than a Type 11 can offer but don't want a radiator that protrudes as far as a Type 22, this is usually your sweet spot.
Choose Type 21 if: you have a medium-sized room, want a strong balance of performance and a slim footprint, or you're simply not sure — it's the safe default for most spaces.
👉 Browse our Type 21 Radiators collection
Type 22 radiators (double panel, double convector)
A Type 22 radiator — also known as K2 — has two panels and two full layers of convector fins. This is the highest-output radiator of the three, and the deepest, projecting furthest from the wall.
All that extra surface area means it can heat larger rooms, rooms with high ceilings, or spaces that are harder to keep warm (such as those with large windows or external walls) far more effectively. If raw heating power is your priority and you have the wall depth to accommodate it, Type 22 delivers.
Choose Type 22 if: the room is large or hard to heat, you need maximum output from a given wall width and height, and depth isn't a concern.
👉 Browse our Type 22 Radiators collection
How to choose the right radiator type for your room
The Type tells you the build — but the right size depends on your room. Before you buy, work through these four steps:
- Calculate your heat requirement (BTU). Every room needs a certain heat output to reach a comfortable temperature, measured in BTUs (or watts). This depends on room dimensions, window size, insulation, external walls and the room's use. A quick online BTU calculator will give you a target figure.
- Match the output to the type. Once you know your target BTU, you can see which type achieves it within the wall space you have. A small bathroom might hit its target with a slim Type 11; a large family bathroom may need a Type 22.
- Measure your available space. Check both the wall width and height, and crucially the depth — Type 22 radiators project furthest, so make sure there's clearance for doors, towel rails and walkways.
- Factor in the room's purpose. Bathrooms benefit from quick, responsive heat and a warm surface for towels, which is why convector radiators are so popular in them.
Quick rule of thumb: Small room → Type 11. Average room → Type 21. Large or hard-to-heat room → Type 22. Then confirm with a BTU calculation before you buy.
Why the right radiator matters in a bathroom
Bathrooms are unique — they need to heat up quickly, deal with moisture and damp, and keep towels warm and dry. The fast convection heating these radiators provide is ideal for that. Choosing the correct type and output means a room that's warm when you need it, without wasting energy heating an oversized unit.
At Essential Bathrooms, we stock high-quality Type 11, Type 21 and Type 22 radiators — including the trusted Kartell K-RAD range, known for excellent heat distribution, robust construction and a sleek, contemporary finish that fits seamlessly into a modern bathroom.
Frequently asked questions
What is the difference between Type 21 and Type 22 radiators?
Both have two panels. The difference is the number of convector fin layers: a Type 21 has a single convector layer, while a Type 22 has two. This gives the Type 22 a higher heat output but also makes it deeper, projecting further from the wall.
Is a Type 11 radiator enough to heat a room?
For small rooms — en-suites, cloakrooms, compact bathrooms — a Type 11 is often perfectly sufficient and more energy-efficient than a larger radiator. For bigger or harder-to-heat rooms, a Type 21 or Type 22 will usually be a better fit. Always check the BTU output against your room's requirement.
Which radiator type gives off the most heat?
Of the three covered here, the Type 22 produces the most heat for a given width and height, thanks to its two panels and two convector layers. (A Type 33 produces even more, where available.)
How do I know which radiator type I need?
Calculate your room's BTU requirement, then choose the type that meets it within your available wall space. As a starting point: Type 11 for small rooms, Type 21 for average rooms, and Type 22 for large or hard-to-heat rooms.
Are Type 11, 21 and 22 radiators all the same width and height?
No — each type is available in a wide range of widths and heights. The Type only describes the panel-and-convector construction (and therefore the depth and output band). You then choose the dimensions that suit your wall and heat requirement.
Find your radiator at Essential Bathrooms
Whether you need a slim Type 11 for an en-suite, a versatile Type 21 for the family bathroom, or a powerful Type 22 for a larger space, Essential Bathrooms has a radiator to match — combining efficient, reliable heating with a design that elevates the look of your bathroom.
Still unsure which type is right for your space? Get in touch with our team — we're happy to help you choose.