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 & how to use?

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.

How Oil-Filled Heaters Work

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.

How Oil-Filled Heaters Work

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.

How Oil-Filled Heaters Work

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.
How Oil-Filled Heaters Work

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!

Browse Oil-Filled Heaters

How Oil-Filled Heaters Work


  • Nice blog. It will surely help beginners update their knowledge. The efforts you have put in to create the posts are quite interesting. Looking forward to seeing you soon with a new post.

    Aaron Lal on

  • I had one electric radiator heater from Wal Mart and my problem is that there is a piece inside that burnt out and I just connect the red wire to red wire to make it run. The little piece is like there’s a ball inside when you shake it.. Is that safe to do that. Please help.


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