A radiator in a house is a heat emitter: a metal panel or column, usually fixed to a wall, that releases heat into a room. In a central heating system it is filled with hot water from a boiler or heat pump, while an electric radiator heats itself with a built-in element. Either way, the radiator does not make heat on its own terms. It is the point where heat produced somewhere else, or by electricity inside it, is handed over to the room.
That simple definition hides quite a lot of variety. There are water radiators and electric ones, steam radiators in older buildings, radiators that can run on both central heating and electricity, and fan-assisted convectors that look like radiators but work differently. They also differ a great deal in running cost, which is where most buying mistakes happen. This guide explains how radiators work, the main types, what they actually cost to run, and the real advantages and drawbacks, so you can judge whether radiators suit your home and which kind makes sense.
What Is a Radiator and How Does It Work?
A central heating radiator is part of a loop. A heat source, usually a gas boiler or increasingly a heat pump, heats water. A pump pushes that water through pipes to each radiator in the house, where it flows through the hollow inside of the metal panel. The panel warms up, passes the heat to the room, and the cooler water returns to the heat source to be heated again. Each radiator normally has a valve on each side, one to control the flow and one to balance it against the rest of the system.
Despite the name, a modern radiator gives off most of its heat by convection rather than radiation. The hot metal warms the air touching it, that air rises towards the ceiling, and cooler air is drawn in at the bottom, setting up a slow circulation around the room. A smaller part of the heat is radiated directly from the face of the panel, which is the warmth you feel on your hands when you stand in front of it. Many radiators have folded metal fins welded to the back to increase their output, and those fins work almost entirely by heating air.
The word also has a second, unrelated meaning that causes confusion in search results. A car radiator is a cooling device, not a heater. It carries hot coolant from the engine through a finned core where air passing through the grille takes the heat away, keeping the engine from overheating. The physics of heat exchange is similar, but the job is the opposite, and nothing in this article applies to cars.
Types of Radiators
Radiators are easiest to understand by what heats them, since that decides how they are installed and what they cost to run. Within each group there are many shapes and finishes, but the heat source is the decision that matters most.
Central Heating Radiators
These are the most common radiators in Europe and much of the world, and the type most people mean by the word. They are filled with water heated by a boiler or heat pump and connected to the rest of the system by flow and return pipes. Pressed steel panel radiators dominate new installations because they are light, cheap and quick to warm up. Cast iron column radiators, which were standard in the late nineteenth and early twentieth centuries, are still made for period homes and hold heat for longer after the system switches off.
Central heating radiators come in many sizes, depths and styles, from slim single panels to deep triple panels, vertical designer radiators and heated towel rails. The depth and number of panels decide how much heat a radiator can deliver from a given wall space, which becomes critical when a home switches from a boiler to a heat pump. We cover panel types and sizing in detail in our guide to hydronic heating panels.
Electric Radiators
An electric radiator contains its own heating element and needs only a power socket or a fixed electrical connection, with no pipework at all. Some are filled with a thermal fluid that the element heats and that then carries heat around the panel. Others are dry, using a ceramic, stone or aluminium core to store and spread the heat. Most modern units have a built-in thermostat and timer, so each room can be controlled on its own.
They are a practical choice where there is no central heating, such as an extension, a loft conversion, a flat with no gas supply, or a single room that is used occasionally. They are cheap and quick to install. The catch is running cost, which we come to below, and it is the reason electric radiators work best for small or occasional loads rather than heating a whole house.
Oil-Filled Radiators
Oil-filled radiators are a kind of electric radiator, usually portable, with fins that look like a traditional column radiator on wheels. The oil inside is sealed in at the factory and is never burned, used up or replaced. It simply acts as a heat store, so the radiator warms up slowly and keeps giving off heat for a while after the element switches off. That makes the heat feel gentle and steady rather than the blast of a fan heater.
Their low purchase price makes them popular as a stopgap or for one cold room. But they are electric resistance heaters like any other, so their running cost per hour is the same as any electric heater of the same wattage. A 2 kW oil-filled radiator uses 2 kWh of electricity for every hour it runs at full power, however the heat is delivered.
Dual Fuel Radiators
A dual fuel radiator is a central heating radiator, most often a towel rail, with an electric heating element added. When the central heating is on, it is heated by hot water from the boiler or heat pump in the normal way. When the central heating is off, for example in summer, the electric element can heat that one radiator on its own without running the whole system.
It does not burn gas itself, and it does not heat the rest of the house. Its value is convenience, mainly for drying towels in a bathroom out of season, rather than saving money. It needs both a pipework connection and an electrical one, and the electrical work in a bathroom must meet the wiring rules for wet locations.
Steam Radiators
In many older buildings in North America, especially in cities such as New York, radiators are heated by steam rather than hot water. A boiler produces steam that rises through the pipes to cast iron radiators, condenses as it gives up its heat, and returns to the boiler as water. The familiar hissing and clanking of these systems comes from air vents releasing air as the steam arrives and from water trapped in pipes that are not sloped properly.
Steam systems are rarely installed today, since hot water systems are easier to control and more efficient. But they remain in service in a huge number of older apartment buildings, and they are maintained very differently from water radiators. Anyone working on one should treat it as its own type rather than assuming water radiator practice applies.
Convectors and Fan-Assisted Radiators
Convectors look like radiators but are built mainly to heat air, with a finned heat exchanger inside a casing that draws cool air in at the bottom and lets warm air out at the top. Trench convectors sit in the floor in front of large glazing. Fan-assisted convectors add a small, quiet fan to push more air across the heat exchanger, which can deliver far more heat from a small unit.
That extra output at low water temperature makes fan convectors a useful option with heat pumps, where a conventional radiator would need to be much larger to deliver the same heat. In commercial buildings the same idea appears at larger scale as fan coil units, which can provide cooling as well as heating when fed with chilled water, using fan coil valves to control each unit.
What Radiators Cost to Run
This is where the biggest misunderstanding sits. Electric radiators are often sold as “highly efficient”, and in one narrow sense they are, because an electric resistance heater turns essentially all of the electricity it uses into heat. But so does every other electric resistance heater. A 1.5 kW electric radiator, a 1.5 kW oil-filled radiator and a 1.5 kW fan heater all produce the same amount of heat for the same electricity. Better models differ in how evenly they deliver that heat and how precisely they control it, which can reduce waste, but none of them gets more heat out of a kilowatt-hour than another.
What changes the running cost is the heat source. A central heating radiator costs whatever it costs to make the hot water. With a gas boiler that means the price of gas, adjusted for boiler efficiency, and in many countries gas is considerably cheaper per kilowatt-hour than electricity. With a heat pump it means electricity divided by the heat pump’s efficiency. A heat pump with a seasonal coefficient of performance of 3 delivers around three units of heat for every unit of electricity, so the same radiator costs roughly a third as much to run as direct electric heating at the same electricity price. Our article on coefficient of performance explains how that efficiency is measured.
So the honest comparison is not between radiator brands but between heat sources. Electric radiators are cheap to buy and install and expensive to run, which suits small or occasional heating. Central heating radiators cost more to install and less to run, which suits heating a whole home. Heat pumps with properly sized radiators are usually the cheapest to run where electricity and installation costs allow, as covered in our guide to air-to-water heat pumps.
How Big Does a Radiator Need to Be?
A radiator has to replace the heat a room loses through its walls, windows, floor, roof and air leakage on a cold day. That heat loss depends on the size of the room, how well it is insulated, how much glazing it has, and how cold it gets outside. Radiator output is published in watts, and in some markets in BTUs per hour, with one watt equal to about 3.41 BTU per hour. A good installer works out the heat loss of each room and chooses radiators to match.
Online calculators and rules of thumb based on floor area give a rough starting point, but they can be well off for rooms with large windows, high ceilings or poor insulation. There is also a catch that many calculators ignore. Published outputs are measured with very hot water, around 75 °C going in, so a radiator delivers much less heat when it is run cooler. That matters little with an older boiler but a great deal with a heat pump, which works best at much lower water temperatures, as explained in our guide to hydronic heating panels.
Pros and Cons of Radiators
Whether radiators are a good choice depends on what you compare them with and which type you mean. The lists below focus on central heating radiators, since they are the main way homes are heated with radiators, with notes on electric types where they differ.
Advantages
- Fast response. A radiator warms a room in minutes, far faster than a screed underfloor heating system, which suits homes that are empty during the day.
- Lower installation cost in existing homes. Radiators fit onto walls with surface or under-floor pipework, so there is no need to lift floors or lay new screed.
- Easy to replace and upgrade. Swapping a radiator for a larger or more efficient one is usually a short job using the same pipe connections.
- Room-by-room control. A thermostatic radiator valve on each radiator lets individual rooms run cooler or warmer, explained in our guide to thermostatic radiator valves.
- High output from a small area. A double or triple panel radiator delivers a lot of heat from a modest amount of wall space.
- Works with almost any heat source, from gas and oil boilers to biomass and heat pumps, provided the radiators are sized for the water temperature.
Disadvantages
- Uneven vertical temperature. Because most heat moves by convection, warm air collects near the ceiling while the floor stays cooler, so people often set the thermostat higher to feel comfortable.
- Wall space. Radiators take up wall area and limit where furniture can go, and furniture in front of a radiator reduces its output.
- Hot surfaces. A radiator on a hot boiler system can reach 60 to 70 °C on the surface, which is a burn risk for young children and frail people. Care homes, schools and hospitals often use low surface temperature radiators with a cover, commonly designed to stay at or below about 43 °C.
- Must be sized for heat pumps. Radiators chosen for a boiler may be too small when the same home switches to a heat pump.
- Maintenance. Water radiators need occasional bleeding to remove air, and the system needs inhibitor and filtration to stop internal corrosion and sludge.
- Running cost for electric types. Electric radiators avoid pipework but cost much more per hour to run than central heating in most homes.
Most of these drawbacks are relative to underfloor heating, which spreads heat across the whole floor at a lower temperature and leaves walls free, but costs more to install in an existing home and responds slowly. Our comparison between radiator vs underfloor heating sets out the trade-off in more detail, and many homes end up using both.
Installing and Looking After Radiators
Adding radiators to a home that already has central heating is usually straightforward. A new radiator needs flow and return pipes taken from the existing circuit, valves, a suitable wall fixing and a check that the heat source and pump can handle the extra load. Replacing an existing radiator on the same pipe centres is simpler still. Installing central heating in a home that has none is a bigger project, since it needs a heat source, a hot water strategy, pipework to every room, and controls.
Most problems with water radiators come from two things inside the system: air and sludge. A radiator that is cold at the top and warm at the bottom usually has air trapped in it and needs bleeding with a radiator key. A radiator that is cold at the bottom and warm at the top usually has sludge, a black deposit of iron oxide that forms when oxygen gets into the water and reacts with the steel. Cleaning the system, adding a corrosion inhibitor and fitting a magnetic filter prevent most of it.
Controls make a large difference to both comfort and cost. A room thermostat switches the heat source on and off according to the temperature in one reference room, while thermostatic radiator valves fine-tune the other rooms. Timers or programmable thermostats avoid heating an empty house. The role of the main thermostat is covered in our article on the central heating room thermostat.
Frequently Asked Questions
What is a radiator in a house?
A heat emitter, usually fixed to a wall, that releases heat into a room. A central heating radiator is filled with hot water from a boiler or heat pump, while an electric radiator has its own built-in heating element.
Do radiators heat by radiation?
Only partly. Most modern radiators deliver the larger share of their heat by convection, warming the air that then circulates around the room. A smaller share is radiated directly from the surface.
Is an oil-filled radiator cheaper to run?
No. It is an electric heater, and it uses the same amount of electricity as any other electric heater of the same wattage. Its oil is a sealed heat store and is never burned or replaced.
What is a dual fuel radiator?
A central heating radiator, often a towel rail, with an added electric element. It runs on hot water from the central heating when that is on, and on electricity alone when the central heating is off.
Are electric radiators more efficient than central heating?
At the point of use, all electric resistance heaters convert almost all their electricity into heat. But electricity usually costs more per kilowatt-hour than gas, and a heat pump delivers several units of heat per unit of electricity, so electric radiators generally cost more to run for whole-home heating.
Can I use my existing radiators with a heat pump?
Sometimes. Radiators are rated with hot water at around 75 °C, and they deliver much less heat at heat pump temperatures. A room-by-room heat loss calculation shows which radiators are large enough and which need replacing.
Why is my radiator cold at the top?
Usually air is trapped inside. Bleeding the radiator with a radiator key releases it. A radiator that is cold at the bottom more often has sludge.
Radiator Components from Legom
Jiaxing Legom Technology Co., Ltd. manufactures components for hydronic heating systems in Jiaxing, Zhejiang, supplying brands, distributors and wholesalers in more than 90 countries. We do not make the radiators themselves. We make many of the parts that decide how well a radiator system is controlled and how efficiently it is supplied with heat.
That includes our own wax thermostatic elements, the temperature-sensing component used in thermostatic valves, along with HVAC valves including mixing and control valves, room thermostats, and manifolds for homes that combine radiators with underfloor heating. For systems where radiators and floor heating share one heat source, our article on underfloor heating mixing valves explains how the two temperatures are kept apart.
On the heat source side, our air-to-water heat pumps are designed for low water temperatures and work with correctly sized radiators, underfloor heating or both. For distributors and brands, our OEM and ODM service covers branding, specification and market documentation.
“People often ask us which electric radiator is the most efficient, and the honest answer surprises them: at the same wattage, they all turn electricity into the same amount of heat. What changes your bill is where the heat comes from. A radiator on a properly sized heat pump system can cost a fraction of the same room heated by direct electricity. So before choosing a radiator, choose the heat source, and then make sure the radiator is sized for the water temperature that heat source will actually deliver.”
— Maggie Shen, Director of Legom
Reviewed by Maggie Shen, Director at Legom, on November 3, 2026. This article was reviewed for technical accuracy, including how central heating radiators work, the convective and radiant share of radiator output, the construction and running cost of electric, oil-filled and dual fuel radiators, steam radiator systems, fan-assisted convectors, the conversion between watts and BTU per hour, radiator rating temperatures and their effect on heat pump sizing, low surface temperature radiators, and the causes of air locks and sludge.