The short answer
The numbers count the panels and the sets of convector fins. A Type 11 has one panel and one set of fins, a Type 21 two panels with one set of fins between them, and a Type 22 two panels with two sets of fins. More panels and fins give more heat from the same height and length, but make the radiator deeper. As a rough guide, a 600 × 1000mm Type 22 gives nearly twice the heat of a Type 11.
What the numbers mean
Panel radiators are made of flat steel water panels with zig-zag convector fins welded to the back. The first digit is the number of panels; the second is the number of fin sets. The fins heat the air passing up through them, which is where most of a radiator's heat comes from.
| Type | Also called | Panels | Fin sets | Output at the same size |
|---|---|---|---|---|
| Type 11 | Single panel, single convector (K1) | 1 | 1 | The baseline |
| Type 21 | Double panel, single convector (P+) | 2 | 1 | About 35–40% more |
| Type 22 | Double panel, double convector (K2) | 2 | 2 | About 75–80% more |
Some makers use their own names — Stelrad's K1 and K2, for example, are Type 11 and Type 22. Type 10 (one panel, no fins) and Type 33 (three panels, three fin sets) exist too, but are less common.
How much more heat does each give?
As a rough guide, a 600mm high by 1000mm long radiator gives about 1,000 W (3,400 BTU) as a Type 11, about 1,400 W (4,700 BTU) as a Type 21 and about 1,800 W (6,200 BTU) as a Type 22, at the usual ΔT50 rating. Exact figures vary by maker, so always check the output for the model you're buying.
That's why the type matters more than the size when wall space is tight: you can often replace an old single-panel radiator with a Type 22 of the same length and nearly double the heat, without moving the pipes.
How deep are they?
Every extra panel and fin set adds depth. A Type 11 is the slimmest and a Type 22 the deepest, roughly twice the depth of a Type 11 once the brackets are on. Check the maker's depth if the radiator goes in a narrow hall, behind a door or under a window sill, and allow for the valves.
Which type should you choose?
- Type 11 for small rooms, halls, landings and cloakrooms, or wherever depth matters more than heat.
- Type 21 as a middle step, when a Type 11 is a little short and a Type 22 more than you need.
- Type 22 for most living rooms and bedrooms, rooms with little wall space, older houses with solid walls, and heat pump systems.
Use the radiator size calculator to work out the heat each room needs first, then pick the type that gets there in the space you have.
Swapping a radiator like for like
- Measure the old radiator's height and length, and the distance between the centres of the two pipes coming up from the floor.
- Panel radiators of the same length from different makers don't always have the same pipe centres. A small difference can be taken up at the valves; a bigger one means moving the pipe tails.
- Turn the valves off, drain the radiator into a tray, and have new valves or tails ready — old ones often won't seal again.
- Bleed the new radiator, top up the system pressure if it's a sealed system, and check the joints once it's hot.
Questions
- Is a Type 22 radiator always better?
- It gives the most heat for its size, but it's deeper and costs more. In a small room or hall a Type 11 is often all you need.
- Does a Type 22 cost more to run?
- No. The room needs the same amount of heat whatever the radiator. A Type 22 just delivers it from less wall, and lets the boiler run cooler, which can make a condensing boiler more efficient.
- What's the difference between Type 21 and Type 22?
- Both have two panels; a Type 22 has a second set of fins, giving roughly 30% more heat than a Type 21 of the same size, for a little more depth.
- Can I replace a Type 11 with a Type 22 of the same size?
- Usually, yes — check the pipe centres and that the extra depth fits. It's an easy way to get much more heat into a cold room.
Related guides
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- Heating & boilersHow to Bleed and Balance Radiators
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