Safety and steady heat are the hallmarks of these electric heaters

Oil filled space heaters are a good choice for heating individual rooms of your home if you don’t have central heating, or if you want to save money on heating by turning the central heating down and heating selected rooms.

But there are a couple of myths we should clear up first. I did a quick survey of other websites that cover these heaters, and found the same two kinds of misleading information on many of them. Let’s cover each of these myths in detail.

1. Efficiency of electric heat in general

People trying to sell you an oil filled space heater will try to convince you that oil radiator heaters are more efficient than other types of heaters. There are a couple of problems with the efficiency argument. In the first place, electric heat is one of the least efficient forms of heat in terms of its ability to convert a fuel source into heat. There are two conversions you need to think about. The first conversion is that you have to convert the energy source to electricity. If the energy source is a fuel such as coal, natural gas, oil, or nuclear, then it is used to create steam which in turn drives a turbine to produce electricity. This conversion process is typically 30-40% energy efficient, because 60-70% of the energy from the fuel (or nuclear fission) is waste heat at the power plant. So from the outset, electric heaters are a maximum of 40% efficient when the electricity is generated from a thermal source – compared to natural gas furnaces which can reach efficiencies of up to 96%.

The second conversion occurs in the electric heater itself. Your electric heater converts electricity to heat at 100% efficiency – because of the second law of thermodynamics, there is really nothing else for the electricity to turn into other than heat. Even electric heaters with a fan, in which some of the energy goes to driving the fan, are still 100% efficient, because eventually the kinetic energy used to drive the fan produces friction in the moving air molecules, and in the moving parts of the fan, and that kinetic energy turns to heat because of the law of entropy. So when you see marketeers trying to convince you that their electric heater is 100% energy efficient, you will know that they are trying to dupe you into thinking their heaters are more efficient than someone else’s. And when they tell you their heaters are almost 100% efficient, they’re trying to dupe you with modesty. The fact is:

All electric heaters are 100% efficient at converting electricity to heat, but electricity is still the least efficient way to produce heat from a fuel source.

2. Efficiency of oil filled radiators vs. other electric heat sources

The other myth you’ll see repeated endlessly is that oil filled space heaters are more efficient than other forms of electric space heaters. As the above explanation makes clear, there is simply no way for this to be true. What is true is that oil filled space heaters are more effective in certain situations, namely where you want a very steady flow of heat. Why is that? Because oil filled space heaters have thermal mass: instead of heating an air space directly as a convection heater does, or heating solid objects from a distance as a radiant heater does, an oil filled space heater heats the oil inside the heater, and that oil in turn heats your living space. The oil acts as a buffer, slowly letting out heat, so that you won’t burn yourself or set fire to your house with a red hot heating element exposed to the air. So to clear up myth #2, the fact is:

Oil filled electric heaters are no more efficient than any other form of electric heat. They are just more effective in situations where you want diffuse, evenly distrubuted heat over a large air space.

Efficiency of the heater vs. efficiency of the heating regimen

Many people claim to save energy by heating with electric heaters such as electric oil rad heaters. The truth is that the savings come not from the type of heater, but from the flexibility that electric heaters offer. When you have central heating, it can be pretty hard to set different temperatures in different rooms at different times of day. And your whole house may wind up being either too warm or too cold. But you can place electric heaters strategically around your house – or even wheel them room to room if the rooms are on the same floor – and only heat the rooms you need to use at the moment. This is the main place that cost savings come into play with electric heaters of any type: the fact that they allow you to heat a smaller space than a central heatng system typically does.

Oil filled vs water filled

There are a couple of water filled space heaters on the market, such as the Westpointe water filled convection heater. What are the differences between water filled and oil filled space heaters?

The point of a liquid-filled space heater is to absorb heat from an electric resistance element, and then release it slowly. Both water and oils provide thermal mass that quickly soaks up the heat and slowly distributes it into the surrounding air. But oil has greater thermal inertia than water, so while oil filled space heaters take longer to warm up, they also continue releasing heat for longer after their power source is shut off. If you need heat quickly – for example, you come home late at night to a freezing cold bedroom and want to warm the room up quickly – an oil filled space heater is probably the least effective way to warm the room up: by the time the oil in the heater has warmed enough to provide substantial heat to the room, you’ll either have fallen asleep under a mountain of blankets, or have died of hypothermia. A water filled space heater will warm more quickly, and therefore warm the room more quickly, but for this type of quick heat the best solution is a convection space heater, which will warm the air directly without an intervening liquid.

In case you are interested in the specifics: water has a thermal conductivity of 0.6 watts per meter kelvin, while mineral oil has a thermal conductivity of about 0.14 W/(mK). So it will take roughly four times longer for oil filled space heaters to warm up as water filled space heaters, but they will keep on releasing heat four times longer as well.

This brings up another myth of oil filled space heaters: Some manufacturers claim their heaters are filled with a ‘special heat conserving oil’. But the thermal conductivity of oils ranges from 0.1 to 0.21, which relative to water would mean that any oil is a special heat conserving oil. It’s just marketing.

Drawbacks of oil filled space heaters

There are three main drawbacks worth considering when you are thinking of buying an oil filled space heater:

  • Oily or chemical smell on first use
  • Bulkiness and difficulty of moving around
  • Risk of oil leaks

Oily or chemical smell on first use: Oil filled radiator heaters are sealed units, and if properly manufactured there should be no oil appearing on the surface. However many consumers report unpleasant odors from these heaters, especially the first few times they are used. There are two reasons for this: the first is that tiny amounts of the oil used to fill the space heater may be present on the surface. The second is that the paint coating on the space heater may give off a chemical smell. However, in most cases the smell goes away within a few days. Many manufacturers recommend turning on their oil filled space heaters in a well ventilated place for a few hours before they are first used in a more confined space.

Bulkiness and difficulty of moving around: Naturally an oil filled space heater is heavier than a ceramic space heater or a radiant heater. Most of these units come with casters so you can roll them from room to room. But there are a couple of problems – often the casters don’t roll well on carpets, so if you have a lot of carpets or rugs, be prepared for the occasional snag. And some models are poorly designed in terms of their mobility, in that they don’t provide proper handles to hold while moving. While oil filled space heaters don’t generally get hot enough to cause burns, you’ll be hard pressed to hold onto the radiator itself for more than a few seconds without feeling discomfort, which makes it hard to move a heater that doesn’t have proper handles.

Risk of oil leaks: This is the biggest drawback of oil filled radiator heaters. If they are poorly made, you could be in for an unpleasant surprise down the road. A slow leak may manifest itself simply as an unpleasant smell like that of heated motor oil – just enough oil is seeping out to get onto the surface of the heater, and the heating cycle causes volatile organic compounds to evaporate from the surface, creating the odor. If your heater starts producing this smell after burn-in, check it carefully for any sign of leaks. And if it produces the smell for more than the first day or two of use – that is, if burn-in doesn’t address the initial smell problem – send it back to the seller.

But even worse than a slow oil leak with its accompanying unpleasant odors is the sudden leak of large quantities of oil. While this is a rare occurrence, it can be a costly one. A carpet soaked in mineral oil is not going to be easy to clean, and the smell will be with you long after you have tossed the heater in the trash bin.

Best uses for an oil filled space heater

The main reason I would recommend oil filled space heaters is for people who pay different electricity rates for different times of the day. Because oil has good thermal inertia, you can arrange for your heater to fire up when electricity is cheap, and still get some benefit from the heat when electricity is more expensive and the radiator is switched off. For example you can put an oil filled space heater on a heavy duty electric timer (be careful to buy one that supports the high amperage of space heaters – not just any timer will do), and have the timer switch on an hour before you wake up, since electricity in winter tends to be cheaper before 7am, before people are out and about and manufacturers and offices start using large amounts of electricty. Most people can get buy with temperatures in the high 50’s and low 60’s in their bedroom if they are in pajamas and buried under blankets, but it’s hard to get dressed in a room that cold, so setting an oil filled space heater to heat up an hour before you wake up is a pleasant and efficient way to wake up and dress in comfort.

Another good reason to buy oil filled space heaters instead of convection fan heaters is that oil filled space heaters are virtually silent. If you have trouble sleeping to the sound of white noise, the fan of a convection fan heater will keep you up. The only noise you will hear from an oil filled space heater is the occasional creaking caused by thermal expansion or contraction of the radiator metal, as the unit warms or cools, but this is infrequent and almost inaudible because these heaters tend to keep a fairly steady temperature. Even baseboard heaters – or convection heaters without fans – tend to be noisier, as their thermostats cause the heat to turn on and off, which makes the metal in those heaters expand and contract more noisily.

On the other hand, if white noise helps you fall asleep or stay asleep, maybe a convection fan heater is the way to go.

Safety is another consideration: it is much harder to burn yourself (or for a small child to burn themselves) with an oil filled radiator heater than with a convection or radiant heater, because the heating element heats the air only indirectly through the oil, not directly. And it is much harder to tip over an oil filled radiator because of its bulk and weight.

Finally, other than the caster wheels that some oil filled space heaters provide so you can easily move the units around, these electric heaters have no moving parts to wear out, unlike convection fan heaters which have a continually moving fan.

Best brand of oil filled space heater

I would recommend the Delonghi brand of oil filled space heater over most others. For one thing, they have been building these units for many years. They have a solid reputation for dependency. For another, many Delonghi heaters are made in Italy where standards of manufacturing quality are reasonably high. Delonghi does seem to have moved some of its manufacturing to China, which may explain why some of the more recent models get the occasional bad review, but on average 8 out of 10 Delonghi customers seem to be at least satisfied with their purchase (with nearly half being extremely satisfied) and Delonghi offers a better selection than just about anyone else.

A digital model is a worthwhile additional investment. While digital models are more expensive, they offer finer-grained control over temperature: you can actually set your heater to a particular temperature and have that temperature maintained. Most oil filled space heaters simply have a number dial, and you choose a setting between 0 and 9 or 1 and 10 without really knowing at what temperature the thermostat will shut the heater off. With a digital thermostat model, the heater will shut off when the desired temperature is reached, so if you know that 70F is your comfort temperature, you can set the heater to that temperature.

Honeywell makes one digital oil filled space heater that comes highly recommended: the HZ-709. This heater is a little more pricy than some but is very solidly built, includes a 12 hour timer so you can set it to switch off, for example 9 hours after you start it, if you want to be sure it doesn’t keep running after you’ve risen and gone off to work. Its quality is backed up by a 3 year warranty, and consumers report it is very quiet and odor-free. You can set it as high as 85F and you can also choose a heating speed, from low through medium to high, so whether you want a slow build-up of heat or rapid heating you’ll have your pick.

One final thing to bear in mind about oil filled space heaters: if you have any sensitivity to dust, you’ll find these heaters preferable over fan-based convection heaters – or even forced air furnaces – because there is no steady blow of air sending dust swirling about the room. For anyone with allergies an oil filled space heater can provide plenty of comfort in an enclosed space, at a fraction of the cost of keeping an entire home warm.

13 replies
  1. Kate
    Kate says:

    We have my gran’s old cottage in wales and want to leave some heating on over the winter whilst nobody is living there so that it doesn’t get damp – it is heated by a Rayburn which we can’t leave on…so I was thinking of getting some oil filled electric heaters – just 2 maybe – and leaving them on low. Is this safe though?

    • Robin
      Robin says:

      If you want to keep your gran’s cottage from getting damp in the winter, rather than a heater I suggest a low temperature dehumidifier as described in the “Extra Cold Basement?” section of my article Energy Efficient Dehumidifiers. A low temperature dehumidifier will take moisture out of the air, will produce heat, and can operate at temperatures as low as 5C. For example, the Frigidaire 50 pint dehumidifier is a low temperature dehumidifier. It can be hooked up, with a drainage hose, to a nearby drain, so that it can draw moisture out continuously.

      I don’t know how cold it gets in the part of Wales where your cottage is. If it does get below 5C, then you could combine an electric heater and a dehumidifier. Put the heater on a plug in thermostat and set the thermostat to only turn on below 7 degrees. That way the heater will only heat the cottage when it is cold enough that the dehumidifier will shut off. The end result of this combination will be to minimize moisture and minimize heating costs. The alternative is to use just an electric heater and a plug in thermostat that shuts the heater off above a certain temperature (electric heaters have thermostats too but most of them cannot be set to a low temperature) but I am pretty sure you will use more electricity with just a heater, or get less moisture out of the air, than with a dehumidifier or a dehumidifier and heater combination.

      As to whether oil filled heaters are safe, yes, they are. But they do not give any advantage for your situation over any other type of electric heater. The advantage of an oil filled heater is that whether they are drawing current or not, they provide roughly the same amount of heat over long periods because of their heat reservoir (the heated oil). This advantage is useful if you are sleeping close by the heater, but there’s no real advantage in an empty cottage and you might as well go with a cheaper convection heater. Just don’t get a radiant electric heater.

  2. Heanly Hyral
    Heanly Hyral says:

    Should I buy a 9 fin or a 15 fin? I know a lot of people say that it’s all about the wattage but I think that the 15 fin would be efficient since it would cool the room faster because of the larger surface mass and since it has more oil inside then the thermometer rest intervals would be longer – though this is just a personal opinion and I need second opinions. Any answer would be appreciated.

    • Robin
      Robin says:

      You are right that a larger surface area will distribute the heat to the room more quickly, and a larger thermal mass would likely mean the rest intervals are longer. However neither of these has any effect on overall electricity consumption. In terms of the total heat produced, total watt hours consumed is the only factor that counts. If anything, a larger thermal mass would mean (A) slower heating time when you turn the heater on, and (B) more heat wasted on an empty room, if you turn it off and leave the room for an extended period. If you are keeping the room at the same temperature all the time, then neither of those issues applies.

  3. John Phillips
    John Phillips says:

    Great article. Thanks. I have heard that you should place an oil filled radiator in the coldest part of the room. Any views on room placement?

    • Robin
      Robin says:

      The best place for an oil filled space heater is in the center of the room, so that the heat radiates from there to all parts of the room. The closer to an outside wall the heater is, the warmer that part of the room will get and therefore the more heat will escape through the wall. The closer to an inside door the heater is, the more heat will flow through the door itself, or through the doorway when the door is open, into other parts of the living quarters, and the less heat will remain in the room. If it’s a bedroom and your goal is to stay warm at night, having the heater near the bed makes sense.

      I haven’t heard that you should place it in the coldest part of the room. I would be interested to know the rationale, but it seems to me that is probably the worst place to put it from an energy efficiency standpoint. (It may be the best from an overall comfort standpoint though, come to think of it – since the thermostat will keep it running longer if it is the coldest spot, which will make that spot – and the rest of the room – warmer.)

    • Leslie
      Leslie says:

      I just got an oil filled radiator and the directions said to place the heater on the floor (on the floor? No duh … really?!) beneath the coldest window in the room, or any other location. Or any other location? Then why say beneath the coldest window. Just put the damn thing anywhere you want but make sure it’s on the floor! 😉

  4. Robyn
    Robyn says:

    Just took my oil heater out of storage…plugged in and it started to make a crackling sound and smell funny…Is that normal?

    • Robin
      Robin says:

      The crackling sound is normal, that’s just the oil being warmed up. That is, if the crackling sound is a slow one as of oil in a frying pan gently sizzling, or metal expanding.

      If the sound is sharper and faster, it could be a wiring issue. Try turning the heater on high, then unplugging. If the crackling sound stops immediately on unplugging, and the burning smell is coming from close to where the wire enters the heater or where the thermostat is, it may be a wiring issue. If the crackling sound continues for a few seconds after unplugging, it’s still just the oil.

      If the smell is oily, there may be a small leak in the heater. Run your hands around the heater to check for any leaks or oily spots. Another possibility is something got onto the outside of your heater while it was in storage, and the heating is cooking that something and drawing out its smell.

  5. Sherry
    Sherry says:

    I found a couple of oil spots under my heater. When I checked I found it was leaking from the heater. Should I get rid of the heater or is it easy to repair?

    • Robin
      Robin says:

      If the heater is still under warranty I would suggest contacting the seller or manufacturer for a warranty replacement.
      I don’t think you’ll find anyone willing to repair a leaking oil heater, the cost of the repair will probably exceed the cost of a new heater.
      This is not likely something you could fix yourself. You might be able to solder over the leak but I wouldn’t trust it, and it would be a shame to assume the leak was fixed only to turn the heater on, leave it running, and come back to find oil all over the floor.

  6. Betty
    Betty says:

    I have a small 5 foot by 6 foot cloak room that is well insulated with no heat. Can I put an oil heater in it for the winter time in northern Wisconsin?

    • Robin
      Robin says:

      The room you describe seems ideal for a personal space heater. You’ll probably only want something with 750-1000 watts capacity for a space that size, otherwise it will cycle on and off frequently. I’m not sure why an oil filled space heater would be a better choice than a plain old electric heater. Certainly a small ceramic heater would be much more compact, and they are quite cheap.


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