Why Do Our Heaters Have An Upper Temperature Limit?

One of our readers recently wrote in to ask why heaters have an upper temperature limit. Great question. While there are many factors that determine a heaters upper temperature limit, understanding temperature limitations in a heater is essential for maximizing heater performance and ensuring safety.

Electric heaters are limited in temperature by the properties of the materials they’re built from. Materials’ properties change with temperature, and they can only take so much abuse (heat) before their properties are not able to behave in the manner intended. There are two types of changes in temperature that happen to materials in the way of degradation: excessive oxidation or dielectric breakdown. Very quickly put, the metallic parts of a heater fail due to oxidation. That’s when materials either break, as in snap or separate, or allow a path for electricity to get from the heating circuit to a component that should be electrically insulated, like the heater sheath. Dielectric breakdown occurs when an insulating material does not insulate as it once did, resulting in a short from the circuit to the sheath or between live components.

A lot of the design of a heater has to do with preventing electricity from flowing where it shouldn’t, like from the circuit to ground. That’s one definition of heater failure. It’s one thing to burn yourself from touching an energized heater (silly person), but it’s another altogether if you were to get electrocuted. That’s why there are safeties in place in our homes and businesses to protect us, mostly as ground-fault circuit interrupters (GFCI) or Residual Current Device (RCD).

The dielectric strength of an insulating material, which is its ability to prevent electricity from flowing through it, decreases with temperature. That’s just the nature of the beast and there’s nothing we can do about it. We’re talking about ceramic materials, magnesium oxide powder which is used as an electrical insulator in cartridge heaters, and mica or aluminum oxide used in band heaters. Insulating materials don’t become full-on conductors when they get too hot but they can allow too much current to flow through them, resulting in a short, usually to ground. That leakage current can initially be as small as a few milliamps, but it is still dangerous. That decrease in dielectric strength at higher temperatures is something that limits the upper temperature of a heater. The change in the insulating properties of a material depends on the material itself, often expressed in volts/mil, and its temperature.

Oxidation occurs when oxygen in the atmosphere combines with metal components altering the material. A shiny new nail that’s been exposed to the elements over many years becomes an old rusty nail. That rust is the oxidation the nail has been subjected to where the iron in the nail turned into iron oxide, giving it that characteristic dark orange rust color. Iron oxide is not iron and it doesn’t have the same properties as iron. If you were to hammer an iron nail and a stainless steel nail into a fencepost and leave them alone for several years you would return to find that the iron nail had rusted and was darker in color and that the stainless steel nail was as shiny and new looking as the day it was placed there. That’s great. Stainless steel doesn’t rust. Or does it? At room temperature stainless steel doesn’t rust. Get it hot enough though, and it will indeed oxidize and degrade. Oxidation occurs faster at higher temperatures, even with materials that are designed to resist oxidation, making things challenging for electric heating elements. There are other, even higher oxidation-resistant materials like Incoloy and Inconel that oxidize too, but it takes a few more hundred degrees (°F) to get them to oxidize. That shady straw color that appears on a heater even if it’s only reached 500 or 600°F is a thin oxide layer. At that level, things aren’t too bad. It’s when you get to the 1100°F or 1200°F range (and above) that things start to get a bit dicey. Oxidation on resistance wire can cause it to break. Oxidation on resistance wire and the inside of a heater sheath can create a conductive path between the two. The first failure mode occurs when the oxidation degrades the integrity of the resistance wire, allowing it to break. The second failure mode occurs when the oxide layer gets so thick, both growing inward from the sheath and outward from the resistance wire, that the two surfaces get close enough to each other to allow electricity to flow. That’s a failure to ground.

Oxidation is not always bad. It does provide an airtight seal over the material that was originally oxidizing, thereby preventing it from oxidizing any further. The oxide actually becomes an airtight protective layer. If a heater is left on and held at a fairly constant temperature it’s a lot better than cycling it between hot and cold many times. With cycling, that oxide layer can crack, exposing the inside unoxidized material to air, meaning it will oxidize further when it next gets to a high temperature.

There are many varieties of resistance wire alloys, and many of them are developed for resistance to the formation of oxides. Chromium and aluminum are common alloying agents that create tough oxide layers that are less prone to cracking from thermal cycling.

The best thing to do in order to lessen the occurrence of these types of failures, be it from dielectric breakdown or oxidation, is to not operate a heater as hot as it can. A heater in a large heatsink in a tight fitting hole or clamped tightly around a large mass (and retightened once at temperature to take up the slack from thermal expansion) will help pull heat away from the heater quickly, getting it to where it needs to go away from the heater and putting less stress on it. As counterintuitive as this may sound, one of the worst things for a heater is heat. Operating a heater as coolly as possible will prevent early heater failures as discussed here and prolong heater life.