How does a central heating radiator work?

A central heating radiator is a heat emitter connected to the hot water circuit from the boiler. Hot water (typically 70-80C flow temperature from a conventional boiler, or 45-55C from a heat pump or condensing boiler running in condensing mode) enters the radiator through the flow connection, transfers its heat to the room through the panel surface and convection fins, and exits as cooler water through the return connection. Despite the name, the majority of heat output from a standard panel radiator is by convection (warm air rising), not radiation.

How do I know what size radiator I need?

Radiator output is rated in watts (or BTUs/hour) at a standard Delta T of 50K (the temperature difference between mean water temperature and room temperature). The required output for a room is calculated from its heat loss - which depends on the room's floor area, ceiling height, construction U-values, air change rate, and the design outdoor temperature. A rough rule of thumb for a well-insulated modern room is 50-70W per m2 of floor area; for a poorly insulated older property this may rise to 100-150W per m2. A proper heat loss calculation to BS EN 12831 should be used for accurate sizing.

What is a thermostatic radiator valve?

A thermostatic radiator valve (TRV) is a self-acting valve fitted to the flow connection of a radiator that automatically modulates the water flow to maintain the set room temperature. It contains a wax capsule or bellows that expands as the room warms and restricts water flow, reducing heat output. TRVs are required by Approved Document L on all radiators except one (which must have a room thermostat or programmer instead). They allow individual room temperature control and improve system efficiency.

Why does my radiator have a cold spot at the top?

A cold spot at the top of a radiator is almost always caused by trapped air in the radiator. As air is less dense than water it collects at the highest point. The radiator needs bleeding - inserting a bleed key into the bleed valve (usually a small square-head plug at the top corner) and opening it until a steady stream of water comes out with no sputtering, then closing it. After bleeding, check the boiler system pressure has not dropped below 1 bar and top up via the filling loop if needed.

Plumbing & Heating noun

Radiator

/ ˈreɪdɪeɪtə /

Also known as: panel radiator, convector radiator, heat emitter, central heating radiator

Definition

A radiator is a heat emitter connected to the hot water circuit of a central heating system. The standard domestic radiator is a pressed steel panel with internal waterways and convection fins welded to the rear, transferring heat from circulating hot water to the room through a combination of convection (warm air rising from fins, which accounts for most of the output) and radiation (direct heat transfer from the panel surface). Radiators are controlled by thermostatic radiator valves (TRVs) and a room thermostat.

In practice

Pressed steel panel radiators come in Type 11 (single panel, single convector), Type 21 (double panel, single convector), and Type 22 (double panel, double convector - the most common for main living spaces). The heat output increases with the number of panels and convectors. Output is rated in watts at Delta T50 (the difference between the mean water temperature and the room temperature). Radiator output drops significantly at lower water temperatures - important when sizing radiators for heat pump systems, which run at 40-50°C rather than the 70-80°C of a conventional boiler.

Every radiator requires a lockshield valve on the return connection (for balancing the circuit) and a TRV or manual valve on the flow connection. A bleed valve at the top corner allows trapped air to be released. Radiators should be positioned under windows where possible to counteract cold downdraught from the glass. The pipework below a radiator must be installed with a slight fall toward the boiler to assist air venting.

Building Regulations

Relevant PartPart L - Conservation of fuel and power

TRV requirementRequired on all radiators except one (room thermostat location)

Heat loss standardBS EN 12831 - heat load calculation

CommissioningSystem must be balanced and commissioned post-installation

Approved Document L requires that heating systems in dwellings include thermostatic radiator valves on all radiators except the one in the room containing the main thermostat, allowing individual room temperature control. New heating system installations must be commissioned by the installer and a commissioning certificate provided. Radiator sizing should be based on a proper heat loss calculation to BS EN 12831 to ensure the system delivers adequate heat at the design outdoor temperature.

Full Building Regulations guidance

Radiator

Just so you know...