Radiator Buying Guide: BTU, Sizes, Type 11, 21 & 22 Explained

Radiators do more than fill wall space: they need to deliver enough heat for the room while fitting the available width, height and pipe positions. Choosing by appearance alone can leave a room underheated, while buying the biggest radiator that fits can waste money and make the installation harder than it needs to be.

This UK radiator buying guide explains BTU output, radiator types, panel configurations, vertical versus horizontal models, sizing, valves and installation checks. Browse our full radiators range as you compare the options.

Start with the heat output

The most important radiator specification is heat output. In the UK this is commonly shown in BTU/h as well as watts. A radiator should be sized to match the calculated heat loss of the room, taking account of floor area, ceiling height, glazing, external walls, insulation and the temperatures at which the heating system will operate.

Online BTU calculators can provide a useful estimate, but the final figure should be checked where the heating design is important. A large open-plan room, older solid-wall property or space with extensive glazing may need substantially more output than a modern insulated room of the same floor area.

What does BTU mean?

BTU stands for British Thermal Unit. Radiator manufacturers use BTU per hour to describe how much heat a radiator can emit under stated test conditions. A higher BTU number means more potential heat output, but outputs should only be compared when they are quoted under the same temperature conditions.

For modern low-temperature heating systems, including some heat-pump installations, radiator output at lower water temperatures becomes particularly important. A radiator that is adequate at traditional boiler temperatures may need to be larger when the system flow temperature is reduced.

Single panel vs double panel radiators

Panel radiators are often described using Type 11, Type 21 and Type 22 terminology. A Type 11 single panel radiator has one panel and one convector. It is relatively slim and can suit hallways, bedrooms and spaces where projection from the wall needs to be kept down.

A Type 21 radiator normally uses two panels with one convector, giving more heat output from a similar wall area. A Type 22 double panel radiator uses two panels and two convectors and is a common choice where stronger heat output is needed without making the radiator much wider.

Type 11 radiators

Searches for Type 11 radiators are common because they are a practical choice for standard rooms with modest heating demand. Their shallower depth can also help in narrow rooms or behind furniture.

The trade-off is output. If a Type 11 radiator needs to be extremely long to meet the required BTU, a Type 21 or Type 22 may provide the same heat from a shorter section of wall.

Type 21 and Type 22 radiators

Type 21 radiators provide a middle ground between slimness and output. The Henrad Compact Type 21 radiator is one example in our range.

For higher output, Type 22 radiators are widely used in living rooms, kitchens, larger bedrooms and other spaces where wall width is limited. The extra convector surface increases depth, so always check projection as well as width and height.

Horizontal radiators

Horizontal radiators are the traditional choice for many UK homes. They fit naturally beneath windows and along low wall sections, and replacement can be straightforward where existing pipe centres are suitable.

When replacing a radiator, measure the overall width, height, depth and pipe-centre distance. A new radiator with the same nominal width may not have identical valve positions, so some pipework adjustment can still be required.

Vertical radiators

Vertical radiators are useful where wall width is limited or where a tall designer feature suits the room. They are popular in kitchens, hallways, extensions and open-plan spaces where cupboards, doors or glazing leave little room for a conventional horizontal radiator.

Products such as the Stelrad Elite K2 vertical radiator provide a practical heating-led option, while designer models such as the Nuie Sloane vertical radiator combine tall dimensions with a more decorative appearance.

Designer radiators

Designer radiators are available in flat-panel, column, wave and other decorative formats. They can become a focal point rather than something to hide, especially in kitchens, bathrooms and contemporary living spaces.

Style should come after performance. Check the BTU output of the exact size and finish because different dimensions within the same designer range can produce very different heat outputs. Dark finishes do not automatically mean higher output; rely on the published product data.

Towel radiators

Towel radiators are designed to warm towels while contributing heat to a bathroom or en-suite. They are available in straight and curved formats, traditional ladder styles and more decorative flat-panel designs.

In a small, well-insulated bathroom a towel radiator may provide the required room heat on its own. In a larger or colder bathroom, the output needed to dry towels and heat the room may require a larger model or an additional heat source. Use the room heat-loss figure rather than assuming any towel rail will be sufficient.

What radiator size do I need?

Radiator size should be chosen from the required output, not from room dimensions alone. Once the BTU or watt requirement is known, compare radiators that deliver that output within the available wall space.

A short Type 22 may produce similar heat to a much longer Type 11. A vertical radiator may solve a wall-space problem but still needs to meet the same room heat requirement. If several radiator sizes meet the output, then width, depth, style and price can decide between them.

Radiator height and width

Common panel-radiator heights include 300mm, 400mm, 500mm, 600mm and 700mm, with many widths available. Vertical radiators reverse the proportions and often reach 1600mm to 1800mm or more in height.

Measure clear wall space and remember to allow for valves, curtains, skirting, furniture and the installation clearances required by the manufacturer. Positioning a radiator tightly behind a large sofa or cabinet can reduce effective heat circulation.

Radiator valves

Radiator valves connect the radiator to the heating circuit and control flow. Manual valves provide basic adjustment, while thermostatic radiator valves can regulate room temperature automatically by reducing flow as the room warms.

Valve orientation matters. Angled valves are common where pipes rise from the floor or come from the wall close to the radiator. Straight valves suit other pipe arrangements. Check the radiator connection size, pipe size and whether valves are included before ordering.

Do new radiators come with valves?

Many radiators are supplied without valves because the required style depends on the pipe layout and customer preference. Designer valves can also be chosen to match chrome, black, brass or anthracite finishes.

Always check the product listing rather than assuming valves are included. Ordering the radiator, valves and any tails or reducers together can prevent installation delays.

Radiator materials

Steel panel radiators remain a common choice because they offer a strong balance of price, output and availability. Aluminium radiators are lighter and respond quickly to changes in water temperature, while traditional column designs may use steel or cast-iron-inspired construction depending on the model.

Material affects weight, water content, appearance and response time, but heat output should still be the primary comparison. Use the manufacturer's stated output for the exact product.

Replacing an existing radiator

If you are replacing like for like, record the current radiator dimensions, valve arrangement and pipe centres before removing anything. The new radiator may have different brackets or connections even if the overall size is similar.

It is also worth checking whether the old radiator was correctly sized. Home improvements such as new windows or insulation may have reduced the room heat loss, while extensions or layout changes may have increased it.

Radiators for heat pumps

Heat pumps typically operate most efficiently at lower flow temperatures than many traditional boiler systems. Lower water temperatures reduce radiator output, so existing radiators may need to be larger or higher-output models to deliver the same room heat.

If the heating system is being designed for a heat pump, compare radiator performance at the design flow and return temperatures rather than using only the standard catalogue output. A heating designer can calculate the required emitter size for each room.

Radiator buying checklist

• Calculate the room heat loss or required BTU output.
• Compare Type 11, Type 21 and Type 22 panel configurations.
• Choose horizontal or vertical based on wall space and layout.
• Measure width, height, depth and existing pipe centres.
• Check output at the intended heating-system temperature.
• Select compatible radiator valves.
• Allow enough clearance for airflow and cleaning.
• Check brackets, plugs and bleed fittings included with the product.
• Use a competent installer for alterations to the heating system.

Frequently asked questions

Is a Type 22 radiator better than Type 11?

A Type 22 generally gives more heat output from a similar width and height, but it also projects further from the wall. Type 11 can be better where a slim profile is more important and the heat requirement is lower.

Are vertical radiators less efficient?

Not simply because they are vertical. Compare the published output of the exact model and size. A correctly sized vertical radiator can heat a room effectively.

How do I know how many BTUs I need?

Use a room heat-loss calculation that considers dimensions, glazing, insulation and external surfaces. Online calculators can help with an estimate, but major heating projects benefit from a proper room-by-room calculation.

Can I replace a radiator with a smaller one?

Yes, if the smaller radiator still provides enough output for the room. A higher-output panel configuration can sometimes reduce the width needed.

Shop radiators online

Browse radiators online from leading heating brands, including single-panel, double-panel, vertical, horizontal and designer models. Start with the heat output, then choose the dimensions and style that work with your room and existing pipe layout.