An oil heater, also known as an oil-filled heater, oil-filled radiator, or column heater, is a common form of convection heater used in domestic

heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...

. Although filled with

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

, it is electrically heated and does not involve burning any oil fuel; the

is used as a heat reservoir (buffer), not as a fuel.

Function

Oil heaters consist of metal columns with cavities inside, where heat transfer oil flows freely around the heater. A

heating element A heating element converts electrical energy into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element. Unlike the Peltier effect, this process is indepen ...

at the base of the heater heats up the oil, which flows around the cavities of the heater by

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...

. The oil has a relatively high

specific heat capacity In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...

and high boiling point. The high specific heat capacity allows the oil to effectively transfer

thermal energy The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, de ...

from the heating element, while the oil's high boiling point allows it to remain in the

liquid phase A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, ...

for the purpose of heating, so that the heater does not have to be a high pressure vessel. The heating element heats the oil, which transfers heat to the metal wall through convection, through the walls via conduction, then to the surroundings via air convection and thermal radiation. The columns of oil heaters are typically constructed as thin fins, such that the surface area of the metal columns is large relative to the amount of oil and element which provides the warmth. A large surface area allows more air to be in contact with the heater at any point in time, allowing for the heat to be transferred more effectively, which results in a surface temperature which is safe enough to touch. The relatively large specific heat capacity of the oil and metal parts means this type of heater takes a few minutes to heat up and cool down, providing a short-term thermal store.

Efficiency

Although oil heaters are more expensive to run and provide far less spatial heating than gas heaters, they are still commonly used in bedrooms and other small-to-medium-sized enclosed areas. This is because gas heaters, especially without

flue A flue is a duct, pipe, or opening in a chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. Historically the term flue meant the chimney itself. In the United States, they are ...

, are not suitable for bedroom use - gas heaters cannot be used in confined spaces due to the reduced

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

, and the emissions produced. This leaves electrically powered heaters, such as oil heaters,

fan heater A fan heater, also called a blow heater, is a heater that works by using a fan to pass air over a heat source (e.g. a heating element). This heats up the air, which then leaves the heater, warming up the surrounding room. They can heat an en ...

s and

heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...

s, as the only alternative. Several efficiency metrics can be measured in regard to heaters, such as the efficiency of heating a room with a given amount of power, and the efficiency of the

electrical generator In electricity generation, a generator is a device that converts motive power (mechanical energy) or fuel-based power ( chemical energy) into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, g ...

which powers the heater and power loss from transporting the electricity over power lines. Measures may also consider how effective a heater keeps the temperature of a space above a certain point. Such measures would find inefficiencies in heating an already warm room. Many heaters (the majority of available models) are equipped with a thermostat to prevent this inefficient heating, which in turn reduces running costs. This feature was much more common in oil heaters than in cheaper fan heaters until recently; thus many older oil heaters will be cheaper and more efficient to run than their contemporary fan heaters that lack a thermostat. Typical oil heaters range in power consumption/output from 500 to 2400

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...

s, and their length and number of columns is roughly proportional to their power rating. A 2400 watt oil heater is usually approximately 1 meter (3.3 feet) in length. Operating costs are typically linearly determined by the heater's wattage and the length of operating time: a 500 watt heater will take at least twice as long to reach the same thermostat setting as a 1000 watt unit; the total consumption of electricity is the same for both. All

electric resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...

heaters are 100% efficient at converting incoming electric energy into heat. However, since most mains electricity is produced by coal/oil/gas generators with ~30% efficiency, electric heat is often less efficient and more expensive than combustion heaters (which directly convert oil/gas to heat). By contrast, an electrical

used for home heating typically has an efficiency well above 100%, expressed as its coefficient of performance, because it ''moves'' outside heat into a room rather than generating it.

Safety and features

The primary risk of oil heaters is that of fire and burns. In both regards they are generally more dangerous than

hydronics Hydronics () is the use of liquid water or gaseous water ( steam) or a water solution (usually glycol with water) as heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems ...

and

air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...

, but less dangerous than electric fan heaters or bar radiators; this is due to the surface temperature of each type of heater. Most modern small heaters have some form of tilt sensor to cut power if they are knocked over or placed on an unstable surface– which can reduce the risk of fire if a heater is knocked over. From a safety standpoint, it is best to avoid having any object within three feet of an oil heater. Using an oil heater to dry clothes is not recommended by any modern manufacturer. Even though the surface temperature of the heater in normal operation is quite low, the extra thermal resistance of the clothing on the heater can cause its surface temperature to rise to the material's autoignition temperature. Some oil heaters contain strong warnings to avoid operation in damp areas (such as bathrooms or laundry rooms) because the moisture and humidity can damage components of the heater itself. Oil heaters have been known to explode when their thermal fuses fail to trigger a shutdown, which can cause fire, thick black smoke, unpleasant odours, oil on walls and other surfaces, and disfiguring scalding. Some companies offer oil heaters with a fan, to increase the air flow over the heater. Since it is constantly bringing the colder air from the room into contact with the heater, that can improve the rate of heat flow from the heater into the room. The rate of heat flow from the heater into the air in contact with it is higher when there is a greater temperature difference between the heater and the air.

References

{{HVAC Heaters Residential heating appliances