do oil heaters need to be refilled

Do Oil Heaters need to be refilled?

If you’re just getting into oil-filled radiators, you might wonder if oil heaters need to be refilled? Does an electric oil radiator run out of oil? Luckily, this question is easy to answer.

Quick answer: No, oil heaters do not need to be filled. An oil heater should never run out of oil. The metal heating body has no openings. Since it is a closed system, the oil cannot leak. At least as long as the heater is undamaged and in a good condition. However, yes a damaged heater can run out of oil. In this case, you should replace the heater. An oil heater that runs out of oil is not a reliable and safe source of heat.

This was the quick answer. But let’s have a closer look, in case you are still a little confused.

Do you have to refill an oil heater?

I am assuming we are talking about electric oil heaters. The mobile kind of space heaters that you can move around the house and plug in wherever you like. With oil heater, in this article, we are not referring to the big central oil heating systems which are oftentimes the primary heat sources of houses. At least they are where I live.

In a mobile oil heater, the heat is not produced from the oil. This is a common misunderstanding! The heat is generated by an electric heating element. The oil does not burn to produce heat. The oil must just be present to propagate and distribute the heat from the electric heating element to the metal surface of the heater. The electric heating element gets very hot. The oil distributes the heat from this small, dangerous, and very hot heating element to the big metal surface. Because of the big surface area of the metal body, oil heaters are safe to touch. This is the reason I consider an oil heater the safest kind of heater. It does not expose any dangerous heating elements.

Why is oil used inside an oil-filled radiator?

Some people then wonder: Why do oil heaters need to use oil? If it does not serve as a fuel, then we could replace the oil with water and build water-filled heaters. Let me walk you through why exactly oil-filled heaters need oil instead of any other fluid.

Imagine you are an engineer and your task is to develop a device that can heat a room. At hand, you have an electric heating element. One requirement for your heater is to heat safely. The heater should not introduce any safety hazards into a household.

An engineer knows that when you have a single hot source of very high temperature, that is very dangerous. Therefore you can’t just plug the heating element into your wall outlet and be fine.

Oil is a very safe fluid to distribute heat

To produce safe heat, the heat has to be transferred from the hot heating element to a bigger surface. Increasing the surface means reducing the temperature, even though the provided heating energy stays the same. Big warm areas are better and safer than small, hot points.

Now, the first idea would be to use water. You can build a metal body that contains water. Inside the water-filled body, you put the heating element. The heating element heats the water and the water, in turn, heats the metal surface. Sounds reasonable, right?
Yet, there is a big downside to this water-based setup. The problem is that water boils very fast. In physic’s terms one could say that the volume of water increases rapidly with rising temperature. This means the hotter the water is, the more space it wants to occupy.

This would put the water body under very high pressure. 

If the smallest thing goes wrong, for example the space heater tipping over and hitting a sharp edge, or the metal body having small imperfections in the metal surface, there is a high risk of explosion. The space heater will pop just as a balloon does when too much air is inside. This is a severe safety hazard and that’s the reason, water is not suitable for use inside heating bodies.

Oil is safer and lighter than water and therefore a better filling for heaters

Instead, we use oil. But why? The answer is simple. Oil is basically a safer version of water. While both water and oil are fluid and good conductors for heat, oil has one advantage:

“The boiling point of diathermic oil is three times higher than water, so it can absorb more heat without subjecting the unit to the same amount of internal pressure. This allows oil-filled space heaters to be lightweight and portable.”

In simple terms, oil does not boil as fast as water does. If you’ve ever observed oil boiling in a frying pan, you know that the pan is way too hot. At these temperatures, the water would already have evaporated.

Therefore, oil does not turn into an explosive when it is heated inside an enclosed metal body. Yes, the pressure increases with temperature also when you use oil. But it is not comparable to water at all.

Additionally, remember the saying “Fat swims”? Or have you ever seen the experiment of oil swimming on top of water? That’s because oil is lighter than water. So, oil is also from a logistics standpoint the better choice. Oil heaters are heavy, no question. But if you filled those heaters with water instead, they would be really hard to move around. Water is just so impracticably heavy.

Oil heaters are more convenient in all areas.

Can oil heaters run out of oil?

Since oil heaters have an oil-filled metal body that is fully enclosed, the heater can’t run out of oil. Therefore, oil heaters never need to be refilled. Remember: The oil inside the metal body is not the source of heat. And also the oil is not burned. Oil is just a transport medium for the heat generated by the electric heating element.

An oil heater can only run out of oil if it is damaged and the metal heating body starts leaking oil.

In this case, it is not possible to repair the damage. Or rather: anything is possible, but a repair is not worth it since the fix costs more than getting a new heater. I guess to fix a leaking metal body, you would have to weld the leak. And then you would have to ensure that the weld is oil-proof. And you’d also have to ensure that there are no other leaks. A repair doesn’t make sense for most people.

In case your oil heater leaks, I would recommend you get a new heater. View my favorite recommended oil-filled radiator here (click the link to view it)

What to do when your oil heater loses oil

Even though we just found that oil heaters generally don’t lose oil because they are an enclosed system, you might still feel like your oil heater loses oil. Normally, this should not happen, but let’s troubleshoot the issue and find out together if your oil heater really loses oil.

First, you have to let the oil-filled radiator cool down. This is to let the oil settle and to be able to safely handle the heater. Also, letting the heater cool down will lower the pressure in the metal body.

Try to shake your heater. You should hear oil moving around. Do you think the amount of oil reduced compared to when the heater was new?

Let’s do an experiment. Put your heater on a scale to weigh it. Note the weight. Wait for 1-2 weeks. Then weigh it again. Did the weight change? If yes, this means that your heater leaks oil.

Check if there are any oil stains around the heater. And also check the heater surface. Can you find any leaks? If you find oil stains somewhere, this is a sure-fire sign your heater lost oil.

Otherwise, if your heater weight reduces, but you can find no oil stains, that might be a sign of oil evaporating. This is harder to detect.

In all cases, getting a new oil heater is the best decision. Not only do leaking oil heaters have lower heating efficiency. They are also a safety risk since less oil is present to distribute the heat.


As long as your oil heater is of decent quality and is running reliably, it should never ever run out of oil. The oil is enclosed in a metal body where it can not escape from. Also, the oil is not burned as a fuel but merely used as a transport medium for heat. This is why the oil is never used up and why oil heaters never need to be refilled.

If you think that your oil heater leaks, but you are not really sure about it, you can weigh it and check the weight difference within 1 or 2 weeks. If the weight of the heater changes, it likely loses oil.

Unfortunately, when your oil heater leaks oil, it is always the best decision to replace it with a new high-quality oil-filled radiator that will last you for years ahead. Trying to repair faulty heaters is not worth it.

credits: title image by quadZe

About the Author

Daniel Hirsch

Daniel is an electrical engineer, blogger, and author. He studied electrical engineering and information technology and decided to blog about heaters after working in the temperature sensing industry.