A hot surface igniter — HSI — is the small silicon-carbide or silicon-nitride element that glows red-hot to light the gas burners on an electronic-ignition furnace. It replaced the old standing pilot: instead of a flame burning around the clock, the control board energizes the igniter for a few seconds at the start of each heat cycle.
That element is brittle, and repeated heating and cooling cracks it — which is why a failed igniter is the most common no-heat call on modern furnaces. Typical symptoms are a furnace that clicks and tries to start but never lights, no heat on a call, or a visibly broken igniter. Igniters are fragile enough that even touching one with bare fingers can shorten its life, and most last only three to seven years — far less than the furnace around them.
The good news for your wallet: the part is cheap, $15 to $100, and a professional replacement usually runs $100 to $425, often around $300, including the service call. Because a no-heat furnace feels like an emergency, the igniter is also a spot where overselling happens — its symptoms (clicks, no light-off) overlap with a bad flame sensor, a flame-rollout or pressure-switch lockout, or a control-board fault. A competent tech checks the inexpensive igniter and flame sensor before quoting a board or a new furnace.