What temperature should radiators be, and what should the boiler be set to?

Radiator temperature, flow temperature and room temperature

Three different temperatures get mixed up in this question, and they are set in three different places.

The flow temperature is how hot the water is as it leaves the boiler for the radiators. It is set on the boiler, usually with a dial or a button marked with a radiator symbol. The radiator's own surface temperature is a little below the flow temperature at the top and cooler at the bottom, because the water gives off heat as it passes through. The room temperature is what you feel, and it is controlled by the room thermostat and the thermostatic valves on each radiator.

Turning the flow temperature up does not make a room warmer in the end. It makes the radiators hotter so the room warms faster, and then the thermostat switches the boiler off at the same room temperature as before.

Why a lower boiler setting often costs less

Modern gas boilers are condensing boilers. They recover extra heat from the flue gases, but only when the water coming back from the radiators is cool enough, which in practice means running the flow temperature lower than the old 70 to 80°C defaults. Nesta's Money Saving Boiler Challenge advises that combi boilers work best at a flow temperature of 60°C or below.

The trade-off is warm-up time. At a lower setting the rooms take longer to get warm, so you may need the heating timer to start 15 to 30 minutes earlier. If the house is well heated at 60°C in mild weather, you can leave it there and turn it up a notch only during a cold spell.

If some rooms never reach temperature at the lower setting, that usually points to an undersized radiator for the room, a radiator part-full of sludge or air, or a system that is out of balance, rather than a boiler that needs to be on full.

Setting the boiler dial

Most combi boilers have two controls on the front: one for heating (a radiator symbol) and one for hot water (a tap symbol). The heating one sets the flow temperature. Some show degrees; others show only a scale or bars, in which case the manual gives the equivalent.

Systems with a hot water cylinder are different. On many system and regular boilers the single boiler thermostat also heats the cylinder, and stored hot water needs to reach at least 60°C to control legionella bacteria. Do not turn one of these boilers down to 50°C to save gas without checking how the cylinder is heated. Our guide to hot water cylinder temperature explains the stored-water side, and the flow temperature guide goes deeper into the settings.

• Combi boiler: try a heating setting of about 60°C. Leave the hot water setting as it is.
• System or regular boiler with a cylinder: check the manual or ask an engineer before lowering the boiler thermostat, because it may also control the stored hot water.
• Heat pump: follow the installer's settings. Heat pumps run far lower flow temperatures by design and the radiators feel only warm.
• Give any change a few days of normal use before judging it, and change one thing at a time.

How hot should a radiator feel?

At a 60°C flow the top of the radiator feels hot, and you can rest a hand on it only briefly. At 70 to 80°C it is too hot to touch comfortably. If a radiator is only lukewarm across its whole face while others in the house are hot, it is not getting enough flow, which is a balancing, valve or sludge problem rather than a boiler setting.

The pattern of heat tells you more than the temperature does.

• Hot at the bottom, cold at the top: air trapped in the radiator. Bleed it, then check the boiler pressure.
• Hot at the top, cold at the bottom: sludge settled along the bottom. Flushing the radiator or the system is the usual fix.
• Cold all over while the pipes to it are hot: a closed or stuck valve, often the pin in a thermostatic valve head.
• Radiators nearest the boiler hot and the far ones barely warm: the system needs balancing.
• Upstairs cold and downstairs hot, or the other way round: often a zone valve or pump fault.

Radiators and people at risk of burns

Where very young children, older people or anyone with reduced sensation could lean against a radiator or fall against it, a hot surface is a real burn risk. The Health and Safety Executive's guidance for care settings says that where there is a risk of a vulnerable person being burned, surfaces should not exceed 43°C when the system is running at its maximum design output. Low surface temperature radiators or radiator covers are used to achieve this.

At home, a lower flow temperature reduces the risk as well as the gas bill. If someone in the household is vulnerable, a guard or low surface temperature radiator in their room is worth considering.

When a radiator will not get hot at all

If a radiator stays cold whatever the boiler is set to, bleeding and checking the valves are the first steps. If several radiators stay cold, the boiler pressure keeps dropping after bleeding, or the heating is noisy, the problem is in the system rather than the settings.

Loyal Plumbers repairs radiators, valves and heating systems across London and everywhere inside the M25, 24 hours a day, with 10 vans and Gas Safe registered engineers for any work on the boiler. Our call-out charge is £60. Call 020 4652 8585 or send an enquiry saying which radiators are affected and whether they are cold at the top, the bottom or all over.