Liquid Filled Radiators Coursework Definition

 The Science and Technology of Oil-Filled Heaters

Oil-filled radiator heaters are some of the most popular space heaters in the market today, thanks to their effectiveness, energy efficiency, and safety.

So what is it that makes these small heaters so desirable? This article will discuss the technology and science that goes into heating your room with an oil-filled heater, therefore you can decide if one is right for you.

What is an Oil Filled Heater?

An oil-filled radiator typically looks a lot like the old-fashioned radiator (the kind that you usually see someone beating with a wrench on TV) except they are a lot smaller. They consist of a series of joined columns or fins, sometimes with open spaces between them which are attached at the base and fronted with a control panel. A few oil-filled heaters are constructed as a single flat panel.

Unlike a traditional radiator, oil-filled radiators are quite mobile. They connect to your household electricity with an ordinary wall plug, so they can be moved around almost everywhere. Most have carrying handles, while the larger models have wheels for easy repositioning.

Inside the body and fin of the heater is diathermic oil. The most common question asked about oil-filled heaters is, “Do I have to refill the oil?” The answer is, “No, you don’t have to refill the oil in an oil-filled radiator.” The oil is not used as fuel, but instead serves as a heat reservoir therefore it never gets used up. It just continues circulating through the heater for as long as you use it.

Here’s a brief overview of how it works:

  1. Electricity is channeled into a resistor inside the heater, which turns the energy into heat.
  2. That heat is absorbed by the diathermic oil that is enclosed in the heater.
  3. As the oil in the heater warms up, it begins to circulate through the fins and columns.
  4. As the oil moves, it transfers heat into the metal of the heater’s fins, creating an even surface temperature.
  5. As the metal fins heat up, they begin to radiate heat into the room. This heat is circulated throughout the room by natural convection in the air.

Now let’s take a deeper look at some of the individual key components.

Diathermic Oil as a Heat Reservoir

The diathermic oil used in an oil-filled radiator has two properties that make it a superb heat reservoir.

  • High specific heat capacity – The amount of heat a material can hold before its temperature rises.
  • High boiling point – The temperature at which a liquid turns into a vapor. The boiling point for diathermic oil is three times higher than water.

Together, these two factors mean that the oil inside your heater can store a lot of heat without getting hot enough to boil. When a liquid boils it becomes a vapor (like water turning into steam), and requires high pressure systems to contain the extra volume. Because diathermic oil won’t reach boiling temperatures, an oil-filled heater doesn’t have this extra engineering complication.

The high heat capacity of the oil also means that the heater will continue radiating heat even after the electricity has been turned off. This means the heater won’t have to run as much, and you’ll save money on your energy costs.

Heater Fins and Surface Area

The body of an oil filled heater is made up of a series of stacked metal fins or columns, or sometimes a single flat panel. Warm diathermic oil circulates through channels built inside the fins and panels, heating the metal as it goes. The warm metal surface then radiates heat into the air surrounding the heater.

The larger the surface area of the heater, the more surface area that’s in direct contact with the air, and the faster the air will warm up. Oil-filled heaters are often criticized for being slow to warm up, so manufacturers try to improve this by stacking multiple fins together, or using larger thin panels to radiate heat.

Don’t confuse surface area or heater size with the amount of heat produced. However heat output is determined entirely by the amount of electricity consumed, which is measured in watts. Surface area only affects how quickly that heat can be spread into the room.

Natural Convection and Air Circulation

One of the favorite features of an oil-filled heater is that they operate so silently. This is because most models don’t have built-in fans to distribute hot air. Instead, once the air around the heater has warmed up, physics takes care of circulating warmth throughout the room through convection. Convection is the natural process described in the familiar phrase, “heat rises.”

Scientifically speaking, when the temperature of a liquid or gas goes up, it becomes less dense and rises upwards. This means that when the heater by your feet warms the air around it, that air rises up to towards the ceiling and pushes the cooler air back towards the ground. The cooler air is now warmed by the heater, creating a continuous convection current throughout the room.

Over time, this convection current will create an even temperature through the whole room (depending on the size of the room and the power of the heater). In the meantime, anyone in proximity with the heater itself will feel the direct effects of the heat radiating from it.

Advantages of Oil-Filled Heaters

Now that you understand the mechanics behind oil-filled heaters, here are all the advantages to using one in your home or office.

  • No noisy fan. A traditional space heater with an exposed heating element uses a fan to blow hot air out into the room. Oil-filled heaters are almost entirely silent. The most noise it makes is some clicking as the thermostat adjusts itself.
  • Won’t dry the air. The lack of a fan means the air in your room won’t dry out either.
  • Energy efficient. Electric heaters are almost 100% efficient, which means that all the energy used is converted directly to heat. An oil-filled heater doesn’t even have to provide power for a fan motor.
  • Slow to cool. They keep radiating heat even after the power has been turned off.
  • Thermostat controlled. Once the area or room reaches a comfortable temperature, oil-filled heaters will automatically cycle on and off to maintain that warmth, instead of running continuously, saving you money while preventing overheated rooms.
  • Oil never needs replenishing. The oil isn’t used as fuel, so there’s no need to replace it, ever.
  • Compact and portable. Oil-filled heaters are lightweight and easily move from place to place. Many are small enough to fit right under a desk.

Oil-filled heaters are some of the safest space heaters available, too

  • Non-scorching surfaces. The metal surfaces get warm to the touch, but never hot enough to burn if brushed against accidentally.
  • No exposed heating element. The heating element is sealed inside the heater, so there’s no chance it will encounter anything flammable.
  • No grilles or vents. An oil-filled heater is entirely enclosed, so there’s no need to worry about dust or debris getting inside or fingers poking through an exposed grille. Also, there’s no concern about blocking air flow through intake and outtake vents.
  • No gas or fumes. Since they don’t burn oil or gas, they are safe to use indoors.
  • Built-in safety features. Most models include overheat protection and tilt-switches that turn the heater off in case of a problem.

Of course, as with any product, there are a few disadvantages as well, primarily that they take a little longer to heat up than a fan-forced heater. That’s because oil-filled heaters have to first heat the oil, then warm the air around them. Fan-forced heaters deliver an almost immediate blast of hot air.

Troubleshooting Oil-Filled Heaters

Oil-filled space heaters require very little in the way of maintenance or service. The most common problems are electrical – faulty wiring or a bad power switch – and can be repaired by an authorized service technician. If you’re heater isn’t providing heat, and the problem is not with the circuit, have it checked out professionally.

Sometimes an oil-filled heater will develop a leak. If you notice a viscous liquid coming from your unit, disconnect it from the power and cease using it immediately. The oil is sealed inside the heater at the factory, and leaks cannot be repaired or replenished. If your heater is still under warranty, contact the manufacturer for replacement options.

Sometimes, people hear noises coming from these otherwise silent heaters. Popping and crackling noises are normal – this happens as the oil and metal warms up. If your heater has been turned upside down, you might hear a gurgling noise when you set it upright as the oil settles into the channels again. Wait till the noise stops to use the heater.

For any other questions or concerns about oil-filled radiator heaters, please contact our Customer Service representatives. We’ll be happy to help!



NewAir AH-450B Electric Oil-filled Space Heater

  • • 7 Fin radiator design quickly warms any small room
  • • Fan-free air circulation means whisper quiet operation
  • • Easy to move from room to room 




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. Although filled with oil, it is electrically heated and does not involve burning any oil fuel; the oil is used as a heat reservoir (buffer), not as a fuel.

How it works[edit]

Oil heaters consist of metal columns with cavities, inside which silicone oil (mostly siloxanes like polydimethylsiloxane) flows freely around the heater. A heating element at the base of the heater heats up the oil, which then flows around the cavities of the heater by convection. The oil acts as a heat reservoir, with a relatively high specific heat capacity (approximately 2 kJ·kg−1·K−1)[1] and high boiling point (approximately 150–300 degrees Celsius).[2][3] The high specific heat capacity allows the oil to store a large amount of thermal energy in a small volume, while the high boiling point allows it to remain in the liquid phase for the purpose of heating, so that the heater does not have to be a high pressure vessel.

Using the oil as a heat reservoir, the heating element heats the oil, which remains warm for a long period of time while the heat is transferred to the metal wall through convection, through the walls via conduction, then to the surroundings via convection and 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 volume of the oil heat reservoir. 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 rapidly from the heater into the room.


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 when unflued, are not suitable for bedroom use - gas heaters cannot be used in confined spaces due to the reduced oxygen, and the emissions produced. This leaves electrically powered heaters, such as oil heaters and fan heaters, 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 which powers the heater and power loss from transporting the electricity over power lines. Measures may also consider how well a heater keeps the temperature of a space above a certain point. Such a measure 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 the 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 the thermostat.

Typical oil heaters range in power consumption/output from 300 to 2400 watts, 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.

All electric resistance heaters are 100% efficient,[4] with operating costs determined by their wattage and the length of operating time.[5] A 500 watt heater will take twice as long to reach the same thermostat setting as a 1000 watt unit; the total consumption of electricity is the same for both.

By contrast, an electrical heat pump used for home heating typically has an efficiency well above 100%, expressed as its coefficient of performance.[6]

Safety and features[edit]

The primary risk of oil heaters is that of fire and burns. In both regards they are generally more dangerous than hydronics and air conditioning, 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. This 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. There is a substantial fire risk if flammable materials such as clothing or bedding are left near a heater, especially synthetic fabrics such as polyester, which can melt or burn. 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. It should also be noted that 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 shut them off.[7] This can cause fire, thick black smoke, unpleasant odors, 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.


The Electricaire Economaster 2400 electric oil heater

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