Flexible heaters are devices that can be molded to the shape of the heated object. Most products can be bent around a small radius, and can be bent more than one without damaging the internal heating elements. Flexible heaters use two types of heating elements: etched foil and wire wound. Etched foil elements are etched in the heater's surface. They are low-profile and durable. Wire-wound heating elements are less common than etched foil or other low-profile elements. Length and width are important dimensions to consider when selecting flexible heaters. Specifications for power requirements and outputs include: maximum operating (sheath) temperature, AC voltage required, heating capacity, and watt density. Maximum operating (sheath) temperature is the highest temperature that the heater's sheath (or protective cover) may reach. This is not the maximum temperature a heated substance may reach. Heating capacity is the wattage which the heater can deliver. Watt density is a good measure of how quickly the heater can transfer heat to a heated surface. To calculate watt density, suppliers divide the available wattage by the heated surface area.

Most flexible heaters use a sheath or sleeve as a protective outer covering for the heater elements. There are usually several choices for sheath or sleeve material. Examples include aluminum, brass, copper, fluropolymer, high temperature (HT) foil, iron, nickel alloy, polyimide, rubber, steel, stainless steel, and synthetic rubber. Aluminum provides good electrical and thermal conductivity, high reflectivity, and resistance to oxidation. Brass is a copper-based alloy that offers good strength, excellent high temperature ductility, reasonable cold ductility, good electrical conductivity, excellent corrosion resistance, and low magnetic permeability. Copper is one of the best conductors of heat and electricity. Fluropolymers such as Teflon ^® (DuPont Dow Elastomers) are highly-resistant to temperature, chemical reaction, corrosion, and stress-cracking. HT foil is usually made of ceramic materials (ceramic foil). Stainless steel is a chemical and corrosion resistant alloy that can have relatively high-pressure ratings. Flexible heaters may also use synthetic rubber such as neoprene for the sheath or sleeve material.

Flexible heaters differ in terms of features and options. Some products are corrosion resistant, explosion-proof, finned, or portable. Others are suitable for cryogenic use or provide dynamic power regulation. Hazardous location heaters are designed for use in places where there is a risk of fire or explosion. Underwriters Laboratories (UL) and other organizations rate these devices. Indirect fired heaters use a secondary heat exchanger to separate the breathable air stream from the combustion air stream. Features for flexible heaters also include overheat protection, independent timing controls, and internal temperature detectors.