Radiators: A Complete Guide to Types, Valves & Bleeding

Early versions of the modern radiator first appeared in the middle of the 19th century.

Before that, Ancient Roman elites had used a system that sent heated air from furnaces beneath floors called a hypocaust.

However, it wasn’t until the 1970s that most UK homes had radiators. And now many of us take them for granted.

But choosing the right radiator involves more than just picking a design that looks nice.

How radiators work

Despite the name, a radiator radiates about 20% of its heat. The other 80% is emitted through a process called convection, when air is heated by the radiator’s surface, rises, and circulates around the room.

The process starts with hot water from your boiler (or heat pump) circulating through the radiator panels. 

Cold air is drawn in at the bottom of the radiator, warms up as it passes over the metal, and rises into the room.

As this warm air cools, it sinks back down to the floor to be drawn into the radiator again.

To speed up this heat exchange, many modern radiators increase the surface area with convector fins. These are zig-zagging metal sheets welded to the radiator panels.

British Thermal Unit (BTU)

BTU is the standard measure of heat output used for radiators in the UK. 

One BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. But in practice it simply tells you how much heat a radiator can produce per hour.

To make sure your radiator is powerful enough for your room, use our radiator sizing guide to calculate the BTU output you need.

Calculating your BTU output requirements

First, calculate the volume of the room with the following formula:

Length of the room (m) x Width of the room (m) x Height of the room (m) = Volume of the room (m3)

Next, you multiply the volume of the room with 153. The result is the BTU output required to heat the room.

However, as mentioned before, there are a few factors that play a role in increasing or decreasing the required BTU output.

For instance, if your home is not insulated, you will require a radiator with a slightly higher BTU output. See the table below for details and adjust the BTU output accordingly.

Factor	Adjust by
For solid floor	-10%
For non-insulated cavity walls	+10%
For foam-filled cavity walls	-20%
For upstairs bedrooms	-25%
For double glazing	-5%
For two outside walls	+15%
For three outside walls	+40%
For northern aspects	+10%
For missing loft insulation	+15%
For high ceilings – 3m	+20%

Types of radiator by panel configuration

Radiators have codes describse the number of water panels (tanks) and convector fins the model has. The more panels and fins, the more heat it will emit for a given physical size.

Here is a quick overview of the most common panel configurations:

Name	Alternative Name	Type	Radiator Panels	Convection Fins
P1	Single panel	Type 10	1	0
K1	Single panel one convector	Type 11	1	0
P2	Double panel	Type 20	2	0
P+	Double panel one convector	Type 21	2	1
K2	Double panel double convector	Type 22	2	2
K3	Double panel triple convector	Type 33	3	3

Choosing the right configuration depends heavily on your room’s size and wall space.

Single-panel radiators 

These include such as P1 and K1 (Type 10 and 11), which have a very slim depth.

It makes them ideal for hallways, tight landings, or small downstairs toilets, where wall space and projection (i.e., how far the radiator sticks out from the wall) into the room are restricted.

Double-panel units

This includes K2 (Type 22). This type offers the best balance of heat output and wall footprint.

It is the most common radiator configuration in UK homes because it can efficiently heat large and drafty spaces

Which radiator gives out the most heat?

The K3 (Type 33) configuration features three water panels and three sets of convector fins.

It provides massive thermal output. However, it is very deep and bulky, so isn’t as popular as most a K2.

Types of radiator by material

Cast iron

Cast iron was the standard material before modern insulation. It takes a long time to heat up, but it retains heat for a long time after your heating switches off. 

It is bulky and expensive, but it also offers a beautiful traditional aesthetic.

Mild steel

Mild steel is the most common material used for radiators in the UK. It’s low cost and available in a range of shapes, sizes, and colours. 

Mild steel heats up and cools down at a steady, average pace, making it a middle ground between cast iron and stainless steel.

Stainless steel

Stainless steel is a high-quality material that doesn’t rust. It heats up slowly and retains its warmth for a long time after a system is turned off. 

Whilst more expensive than mild steel, stainless steel radiators require little maintenance and are durable.

Aluminium

Aluminium is a thermal conductor It heats up efficiently and is very lightweight, which makes installation quick and cheap. 

However, it also cools down quickly when the heating is turned of.

Types of radiators in homes by style

Modern radiators come in a wide range of designs to fit different room layouts and personal tastes. 

The most popular styles include:

• Compact panel radiators: The classic standard with a grid grille on top and closed side panels, offering high efficiency at an affordable price.
• Column radiators: Traditional radiators constructed from vertical steel tubes, giving a classic Victorian appearance combined with modern heating efficiency.
• Vertical radiators: Tall, slim units that make use of vertical wall space. They are perfect for modern kitchens, hallways, or rooms with large bifold doors where horizontal wall space is limited.
• Towel rails: Specifically designed for bathrooms and wet rooms. While excellent for drying towels, chrome-plated towel rails are often less efficient than painted ones because the shiny chrome coating acts as an insulator, radiating less heat.

Radiator components

To understand how to maintain your heating, it helps to know the key parts of a radiator:

• Radiator body (panels): The hollow metal panels that fill with hot water to warm your room.
• Convector fins: The zig-zagging metal sheets inside or behind the panels that generate convection currents.
• Flow and return connections: The inlet and outlet openings where your central heating pipes connect to the radiator (see below).
• Bleed valve: The small metal valve located at the top corner of the radiator, used to vent trapped air.
• Valve ports: The bottom openings where your radiator control valves are screwed in.

Why flow and return matters on a radiator

Hot water must enter the radiator through the designated flow connection (where your thermostatic valve is usually fitted). It then exits through the return connection (the lockshield side). 

So, if your radiator is piped backwards, it will operate far less efficiently and can make loud, rattling noises.

Radiator valves

Manual valves

A manual valve is the simplest option. It works like a basic tap. You twist the dial to manually increase or decrease the flow of water into the radiator, turning the heat on or off.

Thermostatic radiator valves (TRVs)

A TRV features a built-in temperature sensor containing liquid or wax. 

As the room warms up, this sensor expands and physically restricts the flow of water into the radiator to prevent the room from overheating. 

These valves  regulate water flow to control the room temperature, as opposed to regulating the boiler’s heating output.

Lockshield valves

A lockshield valve regulates the flow of water leaving the radiator. 

It’s covered by a plastic cap and is adjusted using a spanner. 

Lockshield valves can balance your central heating system, i.e., ensure all radiators heat up at the same rate regardless of their distance from the boiler.

Where to position a radiator

The best place to install a radiator is usually under a window. This might seem counter-intuitive, but it helps to counteract the cold air falling off the glass. 

As this air falls, the radiator’s rising warm air meets it, circulating the heat further and more evenly.

This under-window guidance still applies to homes with double glazing. Whilst warmer than old single-glazed panes, they still create a minor cold downdraft.

Radiator maintenance

To keep your heating system running well, you should maintain your radiators. 

Over time, air bubbles can become trapped at the top of your system, creating cold spots. You can fix this by bleeding your radiators.

And if your radiators are cold at the bottom, your system likely has metallic sludge. Bleeding will not solve this issue. Instead, you will need to organise a power flush.

Installing magnetic filters & limescale inhibitors is also extremely effective for increasing your radiators lifespans (and sometimes necessary for a warranty).

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