Oil filled space heater - why the oil?

I see ads for portable electric space heaters that brag they are oil filled.
I don’t see how that would help heat anything, and it does make them heavy.
What’s the oil do for a space heater?

The oil is a good receptacle for heat, so once it is heated it should require less energy to keep the room warm. Of course it has to be heated, which takes energy. But there’s a ‘break even’ point where the energy expended in heating the oil has already been spent and keeping it warm (and radiating) takes less energy.

You could try this experiment: Put a pot on the stove and heat it. See how long it takes for it to drop to room temperature. Now fill it with water and boil the water. Remove it from the heat and see how long it takes to drop to room temperature.

Basically, the oil in the heater is a thremal mass.

Same thing it does in a frying pan - help transfer the heat from the source to the metal walls of the radiator. In this application, the oil is called a heat transfer fluid. It also helps to evenly diffuse the heat through out the radiator. If the raditaor was air filled, you’d probably end up with a single hot spot at the bottom, or wherever the heating element is.

More on heat transfer fluids

I just thought of a better way to put that - the oil allows the heating element to operate at a higher temperature without the danger of burning the nearby insides of the radiator. The oil acts as a heat sink to the heating element, and a heat source for the radiator.

All very helpful, especially the link.


I can’t see why this would be true. Whether the heater contains oil or not, the heat it delivers to the room has to come solely from the electricity it consumed. How does a reservoir of oil make this more efficient?

By drawing more heat from the actual heating element, the element operates at a somewhat lower temperature than if there were no oil present. The element is more efficient at turning electricity into heat when operating at a lower temperature (I think actually they are designed to be most efficient at a particular temperature range, but can’t find a cite).

At higher (and thus, presumably, less efficient) temperatures, where (besides into heat) does the energy go?

I have an oil filled space heater/radiator. It takes longer, IME, to warm the room (compared to a tradional space heater). But once it gets going it keeps it hot. Stays hot for a long time, too. So you can turn it off, and it keeps giving off heat. It also seems to be less of a fire hazard.

Is that true? I assumed that when the oil dropped below a certain temperature the heating element would come on at full-power to warm it back up above that temperature. So the heating element would not be operating at a lower temperature. But it would not be operating contiuously, as it would if there were no thermal mass.

Think of your refrigerator. Most of the time it is ‘not running’. When the temperature rises the compressor turns on and cold air is circulated until the temperature drops to the specified level. Then the compressor turns off again. If the cooling system were to operate continuously, then the temperature will continue to drop to the limit of the system instead of the desired limit. I believe the items in the refrigerator, including the air, serve as a thermal mass.

Or think of driving your car. On initial accelleration my Jeep gets about 4 mpg. At 60 mph I get about 23 to 25 mpg. Like the oil-filled heater, it takes a lot of energy to get to the desired level of operation (in the case of the Jeep, speed; and in the case of the heater, temperature) but once the level is reached it takes less energy to keep it there than it did to get there.

There is no way to make a heater of this type more efficient. If you draw a box around the room you have some Power coming in from the electricity and some heat leaving the room. Once that power is in there it doesn’t matter if it is running a space heater or a treadmill it all ends up as heat (or noise, or light but that ends up as heat too). The point of a space heater is to optimize how that heat is transferred into the room.

The reason oil might be used as a fluid is that it will operate at a lower temperature. The reason for this is that if you dump 1000 J of energy into air it will increase the temperature greatly becuase, of among other things, a smaller amount of mass per volume. That 1000 J of energy into oil will only raise the temperature slightly compared to air becuase it, among other things, has a greater mass. This is a good thing becuase you want your heater operating at a low temperature so you don’t severly burn yourself.

This is true if you only consider how much energy is put into the room. The goal of a heater is not simply to put heat energy into a room, but to keep the people in the room comfortable…this is something that can be done with more or less energy depending on the approach taken.

If you overheat part of the room, or all of the room some of the time in order to get the average (or minimum) temperature where you want it, then effiency is reduced. Even heating is slightly more efficient than “lumpy” (either in time or space) heating.

An oil filled heater offers greater surface area than a simple wire element, so it can operate at lower temperature, and thus heat the space more evenly. The oil also holds heat, so while the electrical heating element may cycle on and off, the radiating surface stays at near constant temperature, so you don’t get cold waiting for the thermostat to cycle the heating element on again.

Radiant heaters can be very efficient, as they attempt to heat the people directly, rather than all the air in the room. The problem is that they overheat the side near the heater, while leaving the other side cold…still this can be pleasent if you have a well insulated chair or somesuch to avoid heat loss from there.

That may be so, but (as treis notes) for there to be a difference in efficiency, some energy has to escape the system under one scheme but not under the other.

So in order to support the idea of a difference in efficiency, you need to show how some electrical energy that normally escapes is preserved within the system (i.e. the room being heated) when using an oil-filled heater. I don’t see how this can be.

As Kevbo notes, there are indeed complications associated with heating the people within a room. But when the issue is the efficiency of a heating element (as stated in CJJ*'s post), the answer is straightforward: essentially all electric heaters are equally efficient.