Silicon carbide (SiC) heating elements are the workhorse of industrial high-temperature furnaces up to roughly…
MoSi2 Element Resistance & Why Old and New Can Be Mixed
The surprise: MoSi2 has a positive temperature coefficient
Unlike a stable metal resistor, a MoSi2 heating element’s resistance rises sharply with temperature. A cold element may read only a fraction of its operating resistance — sometimes 1/10 or less. That is why MoSi2 draws a huge inrush current at switch-on and why a reduced-voltage (often 1/3 supply) start is not optional.
What changes as the element ages
Over thousands of hours the hot zone grows a glassy SiO₂ layer from normal oxidation. This layer is an insulator, not a conductor, so it does not lower resistance — but it does make the element more brittle. The conducting MoSi2 core stays electrically stable, which leads to the practical rule below.
Why old and new elements can share a circuit
A common worry: “My new element reads 0.6 Ω cold and the old one reads 1.2 Ω — can I run them together?” The answer is yes, in parallel, and here is the engineering reason:
- At operating temperature both elements settle to a similar hot resistance because the resistance is dominated by the MoSi2 hot zone, whose cross-section and composition stay stable with age.
- The cold-resistance mismatch is large only at room temperature, exactly when you are using the reduced-voltage start that limits inrush anyway.
- Because they are wired in parallel, each element draws according to its own resistance; the slight difference balances out once hot.
This is confirmed against the full MoSi2 specification, and it is one reason MoSi2 is easier to maintain than elements that must be matched by serial number.
When you should NOT mix
- Different grades. Do not parallel an EQ1700 and an EQ1800 element — their rated temperatures differ and one will be overloaded. Keep grades separate as noted in the temperature-grade guide.
- Severely uneven wear. If an old element’s hot zone has thinned or cracked, replace it rather than pair it with a new one.
- Series strings. In series wiring the lower-resistance (newer) element takes less voltage and may not reach temperature; parallel is the safe topology.
Atmosphere still matters more than age
Resistance behaviour is stable across air life, but a wrong atmosphere can wreck an element fast regardless of age. Review how atmosphere affects MoSi2 life before moving elements between furnace types.
Replacing a mixed set of elements? Tell us your grade and we will match the right replacement.
Why Specify Molybdenum Disilicide (MoSi2) heating elements with HeatingDriver
HeatingDriver is a manufacturer and exporter of Molybdenum Disilicide (MoSi2) heating elements with nearly 20 years of experience supplying furnace builders, labs, and integrators worldwide. We stock standard grades and build custom sizes from your drawings, and we review your element drawings free of charge before production. Browse our Molybdenum Disilicide (MoSi2) heating elements catalog or contact our engineers for a quote.
Frequently Asked Questions
Q: Can a new MoSi2 element share a circuit with older ones?
Yes, in parallel. At operating temperature both settle to a similar hot resistance because resistance is dominated by the MoSi2 hot zone, whose cross-section and composition stay stable with age. Wired in parallel, each element draws according to its own resistance and the slight difference balances out once hot.
Q: Why does a new element read a much lower cold resistance than an old one?
MoSi2 has a positive temperature coefficient, so a cold element may read only a fraction of its operating resistance, sometimes a tenth or less. A mismatch such as 0.6 Ω new against 1.2 Ω old is large only at room temperature — exactly when the reduced-voltage start is limiting inrush anyway.
Q: When should elements not be mixed?
Do not parallel different grades such as EQ1700 with EQ1800, because their rated temperatures differ and one will be overloaded. Replace, rather than pair, an old element whose hot zone has thinned or cracked. Avoid series strings too: in series the lower-resistance newer element takes less voltage and may not reach temperature, so parallel is the safe topology.
Q: Does ageing lower the resistance of a MoSi2 element?
No. Over thousands of hours the hot zone grows a glassy SiO2 layer through normal oxidation, but that layer is an insulator rather than a conductor, so it does not lower resistance. It does make the element more brittle, while the conducting MoSi2 core stays electrically stable.
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