How do heating elements work?

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What is a heating element?

Photo of the heating element in a ceramic cooktop.

Photo: The heating element concealed inside a ceramic cooktop. This is one continuous element, beginning at the blue dot and curving around in a maze shape until it reaches the red dot. There’s no point in this element being any other shape or size: it has to concentrate heat precisely underneath a cooking pan—and this is the most effective way to achieve that.

A typical heating element is usually a coil, ribbon (straight or corrugated), or
strip of wire that gives off heat much like a lamp filament. When an electric
current flows through it, it glows red hot and converts the
electrical energy passing through it into heat, which it radiates out
in all directions.

Materials

Heating elements are typically either nickel-based or iron-based.
The nickel-based ones are usually nichrome,
an alloy (a mixture of metals and sometimes other chemical elements)
that consists of about 80 percent nickel and 20 percent chromium (other compositions of nichrome are available, but the 80–20 mix is the most common). There are various good reasons why nichrome is the most popular material for heating elements: it has a high melting point (about 1400°C or 2550°F), doesn’t oxidize
(even at high temperatures), doesn’t expand too much when it
heats up, and has a reasonable (not too low, not too high, and
reasonably constant) resistance (it increases only by about 10
percent between room temperature and its maximum operating
temperature).

In water heaters, the nichrome element is covered with an outer sheath made of stainless
steel, tin-coated copper, or INCOLOY® (an iron-nickel-chromium “superalloy,” which is rustproof, long-lasting,
and works well in hard-water areas). The sheath is insulated from the heating element by magnesium oxide,
an unusual material that’s a good heat conductor but a poor electrical conductor, so
it allows heat to flow from the nichrome but not electricity.

Types of heating elements

There are lots of different kinds of heating elements. Sometimes the nichrome
is used bare, as it is; other times it’s embedded in a ceramic
material to make it more robust and durable (ceramics are great at
coping with high temperatures and don’t mind lots of heating and
cooling). The size and shape of a heating element is largely governed by the
dimensions of the appliance it has to fit inside and the area
over which it needs to produce heat. Hair curling tongs have short, coiled
elements because they need to produce heat over a thin tube
around which hair can be wrapped. Electric radiators have long bar
elements because they need to throw heat out across the wide area of a
room. Electric stoves have coiled heating elements just the right
size to heat cooking pots and pans (often stove elements are
covered by metal, glass, or ceramic plates so they’re easier to
clean).

Photo of red hot nichrome ribbon heating elements inside a toaster.
Photo of electric kettle heating element covered in limescale.

Photo: Two kinds of heating elements. 1) The glowing nichrome ribbons inside a
toaster. 2) You can clearly see the coiled electrical element at the bottom of this kettle. It never glows red hot in the same way as the toaster wires because it doesn’t normally get hot enough. However, if you’re foolish
enough to switch your kettle on without any water inside (as I did once by accident), you’ll discover that
it is perfectly possible for a kettle element to glow red hot. That dangerous and disastrous episode permanently
damaged my kettle and could have set fire to my kitchen.

In some appliances, the heating elements are very visible: in an electric
toaster, it’s easy to spot the ribbons of nichrome built into the
toaster walls because they glow red hot. Electric radiators
(like the one in our top photo) make heat with glowing red bars
(essentially just coiled, wire heating elements that throw out heat
by radiation), while electric convector heaters generally have
concentric, circular heating elements positioned in front of or behind electric fans (so they transport heat
more quickly by convection). Some appliances have visible elements
that work at lower temperatures and don’t glow; electric kettles,
which never need to operate above the boiling point of water (100°C
or 212°F), are a good example. Other appliances have their heating
elements completely concealed, usually for safety reasons. Electric
showers and hair curling tongs have concealed elements so there’s (hopefully) no risk of electrocution.
Electronic cigarettes (e-cigarettes) also have tiny, battery-powered heating elements inside them that
generate the aerosols (vapors) their users inhale.

Heating element concealed inside a fan convector heater.

Photo: The looped wire element inside a fan convector heater. Left: You can see the black, three-bladed fan inside the heater case looking from the front. Right: Looking from behind, you can see the thin coiled wire of the heating element. The fan draws cold air through the holes in the back of the heater, past the element, and blows hot air out of the front. The element is very large and covers the same area as the fan.

Designing heating elements

All this makes heating elements sound very simple and straightforward, but there are,
in fact, many different factors that electrical engineers have to consider when they
design them. The first step in designing a heating element is understanding exactly how it will be used. This heated rear window in an old VW camper can is essentially a ribbon-type heating element bonded to toughened glass. The design considerations include making sure the element doesn’t block the driver’s view, sticks permanently to the glass, doesn’t damage the glass when it heats up, is powerful enough to melt frost and snow relatively quickly, and can be powered from the vehicle’s battery (or electricity supply).

A silver heating element in the back window of a Volkswagen camper van.

Photo: A heated rear window is essentially just a ribbon-shaped heating element bonded to the
glass. You can see the two contacts (on the right) where the element begins and ends.

In his excellent book on the subject (see references below), Thor Hegbom lists roughly
20–30 different factors that affect the performance of a typical heating element, including
obvious things like the voltage and current, the length and diameter of the element,
the type of material, and the operating temperature. There are also specific factors
you need to consider for each different type of element. For example, with a coiled element made of round wire,
the diameter of the wire and the form of the coils (diameter, length, pitch, stretch, and so on) are
among the things that critically affect the performance. With a ribbon element, the ribbon thickness and width, surface area, and weight all have to be factored in.

Examples of how heating elements are used in a simple coil, in a stove plate, in a space heater, and in a hair dryer.

Artwork: Where and how is a heating element going to be used? That’s the first thing to consider when you think about what kind of element you need. Here are four more everyday examples, with the element colored red in each picture. Clockwise, from top left: 1A) A simple coiled element; 1B) Two coils in a ceramic stove plate (green); 1C) Two coiled elements in a basic space heater with reflectors (blue) to “beam” their heat into the room; 1D) Ribbon elements in a hair dryer with a fan (yellow) to blow their heat forward. Artwork from US Patent 5,641,421: Amorphous metallic alloy electrical heater systems by Vladimir Manov et al, courtesy of US Patent and Trademark Office.

And that’s only part of the story, because a heating element doesn’t work in isolation: you have to consider how it will fit into a bigger appliance and how it will behave during use (when it’s used, or abused, in different ways). How, for example, will your element be supported inside its appliance by insulators? How big and thick will they need to be and will that affect the size of the appliance you’re making? (For example, think about the different kinds of heating elements you’d need in a soldering iron, the size of a pen, and a large convector heater.) If you have an element “draped” between supporting insulators, what will happen to it as it gets hotter? Will it sag too much and will that cause problems? Do you need more insulators to stop that happening, or do you need to change
the material or the element’s dimensions? If you’re designing something like an electric fire with multiple heating elements close together, what will happen when they’re used individually and in combination? If you’re designing a heating element that has air blown past it (in something like a convector heater or a hair dryer), can you generate enough airflow to stop the element overheating and dramatically shortening its life? All these factors have to be balanced against one another to make a product that’s effective, economical, durable, and safe.

Does a heating element need a high or a low resistance?

You might think a heating element would need to have a really
high resistance—after all, it’s the resistance that allows the material to generate heat. But
that’s not actually the case. What generates heat is the current
flowing through the element, not the amount of resistance it feels.
Getting the maximum current flowing through a heating element is much
more important than forcing that current through a large resistance.
This might seem confusing and counter-intuitive, but it’s quite easy
to see why it is (and must be) true, both intuitively and
mathematically.

Intuitively…

Suppose you made the resistance of your heating element as big as you possibly
could—infinitely big, in fact. Then Ohm’s law (voltage = current
× resistance or V = IR) tells us the current flowing through your
element would have to be infinitely small (if I = V/R, I approaches
zero as R approaches infinity). You’d have a whopping great
resistance, no current, and therefore no heat produced. Right, so
what if we went to the opposite extreme and made the resistance
infinitely tiny. Then we’d have a different problem. Although the current I might be huge,
R would be virtually zero, so the current would zip through the element like
an express train without even stopping, producing no heat at all.

What we need in a heating element is therefore a balance between the
two extremes: enough resistance to produce heat, but not so it
reduces the current too much. Nichrome is a great choice.
The resistance of a nichrome wire is (roughly) 100 times
higher than that of a wire the same size made from copper (an excellent conductor), but only a quarter as much as a similar-sized graphite rod (a fairly good conductor) and maybe only a million trillionth that of a really good insulator such as glass. The numbers speak for themselves: nichrome is an average conductor
with only moderate resistance, and not remotely an insulator!

Photo of the soleplate of an electric iron, showing the connections to the heating element.

Photo: The soleplate of an steam electric iron, seen from above, with the water tank and everything else removed. The heating element is incorporated into the base, which
is a hollow metal tank that fills up with cold water and boils it to make steam. I’ve colored the two electrical connections to the heating element red and blue. Unlike the simple electric fire in our top photo, which operates at maximum power all the time, the element in an iron has to cycle between high- and low-power states so it can cope both with
ironing and being left idle on its stand. When you’re ironing cold and damp material,
you need the iron to be supplying high power so it doesn’t cool too much. When you
rest the iron on its stand temporarily, it needs to switch to much low power quite rapidly
so it doesn’t overheat and burn out the element. That’s achieved using a simple
thermostat.

Mathematically…

We can reach exactly the same conclusion with math. The power produced
or consumed by a flow of electricity is equal to the voltage times the
current (watts = volts × amps or P = VI). We also know from Ohm’s law
that V = IR. Eliminate V from these equations and we find the power
dissipated in our element is I2R. In other words, the heat is
proportional to the resistance, but also proportional to the square
of the current. So the current has much more effect on the heat
produced than the resistance. Double the resistance and you double the
power (great!), but double the current and you quadruple
the power (fantastic!). So the current is what really matters.

It’s easy to calculate that the resistance of the filament in
a typical incandescent lamp is a few hundred ohms. I’ll leave the calculation
to you!

Resistance heating?

We often refer to electrical heating—what heating elements do—as “Joule heating” or “resistance heating,” as though resistance is the only factor that matters. But, in fact, as I explained above, there are dozens of interrelated factors
to consider in the design of a heating element that works effectively in a particular appliance. The resistance isn’t always something you control and determine: it’s often determined for you by your choice of material, the dimensions of the heating element, and so on.

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