Ignition Modules — HSI and Spark

The ignition module is a sequencer. Given a call for heat and permission from the safety string, it runs a pre-programmed sequence — purge the combustion chamber, energize the igniter, open the gas valve, check for flame, and if flame isn’t proven within its timing window, shut the gas valve and either retry or lock out. That’s the whole job. Everything else — temperature control, blower management, comfort features — lives on the integrated furnace board or is handled by separate controls.

In a residential context, you’ll meet three ignition strategies: Direct Spark Ignition (DSI), Hot Surface Ignition (HSI), and Intermittent Pilot (IP). Each has its own module types, failure modes, and diagnostic habits.

The three strategies

Direct Spark Ignition (DSI). A high-voltage transformer produces a continuous spark at an ignition electrode positioned near the burner. When the module commands ignition, spark fires and the gas valve opens; flame establishes directly at the main burner. Flame is proven by a separate flame sensor rod using rectification, or occasionally by the same spark electrode (on smaller units). Common on oil-fired boilers, some power burners, and gas fireplaces. Fast startup, fewer moving parts than IP, but the high-voltage spark circuit is a failure point.

Hot Surface Ignition (HSI). A silicon-carbide or silicon-nitride igniter is powered with 120 VAC for 15–45 seconds until it glows red-hot, then the gas valve opens and combustion air flowing past the igniter ignites the gas-air mixture. Flame is proven by a separate sensor rod. Dominant in modern residential gas furnaces because it’s reliable, quiet, and doesn’t require a high-voltage spark circuit. The igniter itself is the wear item; HSIs fail from thermal cycling, contamination, and mechanical shock.

Intermittent Pilot (IP). When a call for heat starts, the module energizes a spark to light a small pilot flame, proves that pilot flame with a rectification rod, then opens the main gas valve. Between calls, no pilot burns. More complex than DSI or HSI (has both pilot and main valve operations) but safer than a standing pilot in several ways. Still common on older equipment and on some modulating boilers.

Inputs and outputs

A typical integrated gas furnace module (or the ignition section of an integrated board) has these connections:

Understanding these connections is the map for diagnostic work. When something fails, you’re asking: which input is missing, which output is missing, and is the failure at the module or at the device the module is trying to drive?

The ignition sequence in timing

A typical gas furnace ignition trial, timed:

t=0     Call for heat. Safety string closes. Module receives 24V input.
t=0     Module energizes inducer.
t=~5s   Pressure switch closes (draft proven).
t=5s    Module starts HSI warmup. 120V on igniter.
t=5-50s HSI warms to operating temperature (35-45 sec for silicon-carbide).
t=~50s  Module energizes main gas valve. Gas flows through igniter.
t=50s   Flame establishes at burners. Module starts flame-prove window.
t=~52s  Flame signal reaches minimum µA. Module transitions to steady state.
t=~52s  Module de-energizes HSI (it's done its job).
t=~80s  Heat-on delay expires. Module energizes blower.

Silicon-nitride igniters have much shorter warmup (5–17 seconds typical) because they reach ignition temperature faster. Check which type you’re dealing with — ohmmeter reading at ambient usually differentiates: 11–20 Ω for silicon-nitride, 40–90 Ω for silicon-carbide.

If flame doesn’t prove within the trial window (typically 4–7 seconds), the module shuts the gas valve, purges, and either retries or locks out depending on configuration.

Retry and lockout logic

Modules vary, but typical residential logic is:

• 3 trials, soft lockout. Module attempts 3 ignition trials. If all three fail, the module enters a 1-hour soft lockout and will try again after the timeout.
• 3 trials, hard lockout. Same as above but no auto-retry. Must be reset by power-cycling the thermostat or the breaker.
• 1 trial, immediate lockout. On some older modules and on commercial-grade controls. One failure and the module locks out.

Universal replacement modules

Honeywell’s S8610U and S8660 series are commonly used as replacements for a wide range of original modules across many brands. These are configurable via jumpers or DIP switches for trial timing, retry count, and flame threshold.

When using a universal replacement:

1. Read the original module’s part number and its install sheet. Many originals have specific timing the replacement needs to match.
2. Set the DIP switches according to the replacement’s instruction sheet for that original. Wrong settings can produce lockouts that weren’t happening before or run the igniter too long and burn it out.
3. Verify flame signal after install. Universal modules sometimes have different flame thresholds than the original; a system that was running near the threshold may now lock out.

Diagnosing module vs. component

This is the most common diagnostic question: is the module itself bad, or is the module healthy but the thing it’s trying to drive is bad?

The rule: a module is a sequencer that waits for inputs and commands outputs. Check the inputs and outputs, and the module’s role becomes clear.

• Inducer doesn’t run on call for heat. Check 24V at IND output. Present → inducer itself is bad. Absent → module not commanding. But don’t condemn the module yet; the module might be waiting for an input that isn’t arriving (safety string open, pressure switch already closed, internal lockout state). Check status LED.
• HSI doesn’t glow but should. Check 120V at IGN output. Present → HSI bad. Absent → module not commanding (or output relay failed on the module).
• Gas valve doesn’t open but HSI glowed. Check 24V at MV output during trial. Present → valve bad. Absent → module not commanding.
• Flame lights, drops out, module locks out. Flame signal is too low. Module is doing exactly what it’s supposed to do. Problem is in the flame-sensing chain (rod, ground, polarity).

From the field

A callback on a furnace that kept locking out after ignition. The tech before had replaced the module, the gas valve, and the flame sensor. All new parts. Still locking out. The µA reading was 0.4 — just below the threshold — with a perfectly clean new sensor rod.

Checked the outlet with a plug-in tester: reversed polarity. Someone had rewired a circuit in the garage years earlier and swapped hot and neutral. The flame rectification circuit couldn’t reference neutral correctly, and µA readings ran artificially low. Fixed the outlet, µA came up to 3.5, problem gone. None of the replaced parts were actually bad — the module had been accurately reporting a low-signal condition the whole time, and the previous tech had been treating the symptom instead of the cause.

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Check your understanding

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01An HSI ignition module runs through its trial: inducer on, pressure switch closed, HSI glows, gas valve opens, but flame never establishes. The module locks out. What's the most likely problem?

AThe ignition module has failedBSomething in the gas-path or combustion-geometry side — no gas, low gas pressure, orifice blockage, burner fouling, or HSI positioned wrongCThe flame sensor has failedDThe pressure switch is miscalibrated

02A silicon-carbide HSI reads 75 Ω at ambient. After install, the module locks out on every trial with a 'no flame' code. What should you check?

AThe HSI is likely bad — replace itBInstall orientation and handling — silicon-carbide is damaged by skin oils and mechanical shock; a visually intact but contaminated igniter will glow weakly and won't ignite gas reliablyCThe module needs its DIP switches resetDThe flame sensor is miswired

03When diagnosing whether an ignition module is at fault or whether the module is correctly reporting an external fault, what's the most useful single action?

AReplace the module and see if the problem goes awayBMeasure the module's output voltages during the trial — 24V on MV during ignition, 120V on IGN during warmup, etc. — and verify each output matches where the sequence should be at that momentCCheck the module's resistance at its power terminalsDJumper past the pressure switch

An ignition module is a disciplined little state machine. When you understand the states and transitions, and you can measure at each step, diagnosing “the module is bad” vs. “the module is fine and something else is bad” becomes a matter of 10 minutes with a meter, not a hour and a parts cannon.