The furnace ignitor not getting voltage is a common HVAC problem that prevents a furnace from lighting and delivering heat. This article explains likely causes, step-by-step diagnostics, expected multimeter readings, safety considerations, and practical fixes homeowners and technicians can use to restore proper operation. Accurate troubleshooting reduces repair time and helps avoid unnecessary part swaps.
| Symptom | Possible Cause | Quick Fix |
|---|---|---|
| Ignitor cold when furnace calls for heat | Blown fuse, open circuit, failed control board | Check fuse, measure voltage, replace faulty component |
| Intermittent ignition | Loose wiring, failing transformer, bad limit switch | Tighten connections, test transformer, replace switch |
| No 120/24V control power | Tripped breaker, disconnected power, blown transformer | Reset breaker, restore power, test/replace transformer |
How Furnace Ignition Systems Work
A furnace ignition system typically includes a thermostat, control board or gas valve, transformer, safety switches, and the ignitor (hot surface ignitor or spark ignitor). When the thermostat calls for heat, the control board sends voltage to the ignitor and opens the gas valve or energizes the spark module. If the ignitor is not receiving voltage, the problem lies upstream in the control circuit.
Types Of Ignitors And Their Voltage Requirements
Identifying the ignitor type clarifies expected voltages and testing methods. The two common types are hot surface ignitors (HSI) and spark ignitors (spark electrode/ignition module).
Hot Surface Ignitor (HSI)
HSIs are ceramic silicon nitride or silicon carbide elements that glow red when heated and require a 24VAC control signal or line-level voltage through a dedicated circuit depending on furnace design. They are resistive and do not spark.
Spark Ignitor
Spark ignitors use a high-voltage transformer or ignition module to generate sparks across electrodes. The control board or module provides a 24VAC trigger while the module creates the high-voltage pulses for sparking.
Common Causes When The Ignitor Is Not Getting Voltage
Diagnosing why a furnace ignitor not getting voltage requires checking multiple components in the control circuit. Common causes include: failed control board, blown fuse, open transformer, safety switch interruption, thermostat wiring fault, and wiring/connectors issues.
- Control Board Failure: Board relays and outputs can fail and stop supplying voltage.
- Blown Low-Voltage Fuse: Many furnaces have a 3A or similar fuse protecting the 24VAC circuit.
- Transformer Failure: If the transformer fails, no 24VAC will reach the gas valve and ignitor control circuit.
- Open Safety Switches: Flame rollout switches, limit switches, door switches, or pressure switches can open and prevent ignition voltage.
- Thermostat Or Wiring Fault: Incorrect thermostat signals or broken wires can prevent the call for heat.
- Disconnected or Damaged Ignitor Wiring: Fragile leads to the ignitor may break or short out.
Essential Tools And Safety Precautions
Before testing, gather a multimeter (AC/DC, True RMS preferable), insulated hand tools, a non-contact voltage tester, and manufacturer wiring diagrams if available. Always shut off power to the furnace at the breaker before touching wiring. For live voltage checks, use caution and follow safe procedures. If uncomfortable with mains or control circuits, contact a licensed HVAC technician.
Step-By-Step Troubleshooting Guide
The following workflow helps isolate why the furnace ignitor not getting voltage. Perform tests in order and record readings.
- Verify Power And Thermostat Call: Ensure the furnace has 120VAC at the service switch and breaker. Confirm the thermostat is set to heat and set above room temperature to call for heat.
- Check 24VAC Supply: With the furnace powered, measure across R and C at the control board or transformer secondary. Expected reading is roughly 24VAC. If absent, test transformer primary and fuse.
- Inspect Low-Voltage Fuse: Locate the small control board fuse (often 3A or similar). If blown, replace it, but investigate root cause that may have blown the fuse.
- Trace The Call To The Board: Measure voltage between W and C when thermostat calls for heat. If 24VAC is present at W but the ignitor still gets no voltage, the control board or relays likely failed.
- Test Safety Switches And Pressure Switches: Bypass each safety switch one at a time (only briefly and for diagnostic use) to see if the ignitor then gets voltage. Replace any switch that prevents the circuit from closing.
- Measure At Ignitor Leads: Check voltage at the ignitor connector during the heat call. If voltage reaches the connector but the ignitor remains cold, the ignitor is likely failed and needs replacement.
- Inspect Ignitor Resistance: For HSIs, measure resistance with power off. Typical HSI resistance ranges vary by model (30–120 ohms often for silicon nitride; consult data). An open or extremely high resistance indicates a broken element.
- Check Transformer And Relay Outputs: Confirm the transformer secondary supplies 24VAC under load. Inspect relays on the control board that switch power to the ignitor; replace the board if relays are dead.
Expected Voltage Readings And What They Mean
| Test Point | Expected Reading | Interpretation |
|---|---|---|
| R to C (control transformer) | ~24VAC | Healthy transformer and control power |
| W to C (thermostat call) | ~24VAC When Calling | Thermostat is signaling heat |
| Ignitor Connector (during call) | 24VAC Or Board-Specified Voltage | Ignitor should receive voltage; if not, upstream fault |
| HSI Resistance (power off) | Manufacturer Range (e.g., 30–120Ω) | Out-of-range or infinite = failed ignitor |
Diagnosing Specific Faults
Blown Control Fuse Or No 24VAC
A blown 3A fuse or lack of 24VAC indicates transformer or short. Replace fuse and test. If it blows again, isolate shorted wiring, terminal block, or faulty component drawing excessive current.
Thermostat Wiring Or Configuration
If W shows no voltage when calling, inspect thermostat wiring and configuration. Smart thermostats may need power stealing or a C-wire; missing common can prevent proper signaling.
Open Safety Or Limit Switches
Limit, rollout, or door switches often open the ignitor circuit. Locate these switches and test continuity. Replace any switch that won’t close under expected conditions.
Control Board Or Relay Failure
If the board receives W but does not energize the ignitor output, board relays, triacs, or driver circuits may be faulty. Control board replacement may be required after confirming upstream signals are correct.
Transformer Under Load
Some transformers show 24VAC with no load but drop significantly under load. Test voltage while the furnace is calling for heat. A large voltage drop indicates a weak transformer that should be replaced.
Fragile Ignitor Leads And Connectors
HSI leads are fragile and can break close to the ignitor or connector. Inspect wiring harnesses for chafing or breaks. Replace connector assemblies or the ignitor if leads are damaged.
When To Replace The Ignitor Versus Repair Controls
Replace the ignitor if it is open or has visible cracks, or if it fails resistance checks yet no voltage is present at the connector. Replace control board, transformer, or safety switches when upstream voltage is absent or commands are not passed despite correct thermostat signals.
Typical Costs And Parts
Parts and labor costs vary by region and furnace model. Typical replacement costs: ignitor part $50–$150, transformer $50–$150, control board $200–$600, safety switch $20–$100. Professional labor can add $150–$400 depending on complexity.
Preventive Maintenance To Avoid Ignitor Voltage Issues
- Schedule annual furnace tune-ups to check controls, transformers, and safety switches.
- Replace air filters regularly to prevent overheating and tripped limit switches.
- Inspect wiring and harnesses for wear and secure loose connections.
- Keep the furnace compartment clean to avoid debris affecting switches or sensors.
When To Call A Professional
Call a licensed HVAC technician when the issue involves live mains wiring, complex board diagnostics, multiple failing components, or if safety devices triggered (smell of gas, pilot failures). Professionals ensure safe testing of high-voltage and gas-related parts and proper board replacements.
Troubleshooting Checklist For Technicians
- Confirm thermostat call and 120VAC supply.
- Measure transformer secondary (R-C) under load.
- Inspect and test control board fuse and relay outputs.
- Test continuity of safety and pressure switches during a call.
- Measure voltage at ignitor connector during the ignition sequence.
- Swap in a known-good transformer or control board if available for isolation testing.
- Document readings and replace the failed component with OEM or equivalent parts.
Key Takeaways
When a furnace ignitor not getting voltage, the problem is usually upstream: blown fuses, transformer failure, open safety switches, or control board faults. Systematic voltage and continuity checks isolate the fault. Replacing parts without proper testing risks unnecessary expenses; starting with basic checks saves time and money.
If unfamiliar with electrical diagnostics or gas-fired appliance safety, contacting a licensed HVAC technician is the recommended course of action to ensure safe, code-compliant repairs.