Weil-McLain A100 Error Code — What It Means
A100 is not a verified standard Weil-McLain boiler fault code in manufacturer documentation. The most commonly documented code from Weil-McLain AquaBalance boilers is A01, which indicates no burner ignition or ignition failure leading to lockout after repeated attempts. The boiler tries to light, fails multiple times, and locks itself out for safety. If your display shows A100, verify the exact code against your specific model’s manual or wiring diagram, because code meanings vary across different Weil-McLain control platforms and series.
If you do have an A01 ignition lockout, the fault is typically related to the gas supply, ignition or detection electrode, gas valve, or condensate drainage. The control module records the fault and enters lockout mode. A simple reset may clear the code temporarily, but if the underlying problem remains, the lockout will return on the next call for heat.
Before You Replace Anything
Many homeowners replace the gas valve or control module without first checking whether the gas line was purged on a new install or whether the condensate trap is blocked or double-trapped. Verify gas supply pressure and inspect the ignition electrode before ordering expensive components.
Common Causes
- No gas supply or unpurged gas line (~25%) On new installations or after service, the gas line may not be purged or the manual gas valve may be closed, preventing ignition.
- Ignition or detection electrode fault (~25%) A dirty, misaligned, or failed electrode cannot create spark or sense flame, triggering lockout.
- Faulty gas valve (~20%) The gas valve may fail to open or deliver the correct pressure, blocking fuel flow to the burner.
- Insufficient gas supply pressure (~15%) Supply pressure outside the 3.5 to 11 in. w.c. range prevents reliable ignition and causes repeated lockouts.
- Blocked or double-trapped condensate drain (~10%) A clogged condensate trap or looped drain hose creates back pressure that can interfere with combustion and trigger safety lockout.
- Dirty heat exchanger or overdue maintenance (~5%) Accumulated soot or debris in the heat exchanger restricts airflow and can prevent proper ignition or flame sensing.
Quick Diagnosis
Answer these to narrow it down fast.
Does the boiler spark when you call for heat after resetting the control?
No: The electrode or control module may be faulty. Inspect the electrode for contamination or damage and check wiring connections.
Is the manual gas valve fully open and has the gas line been purged?
No: Open the valve and purge air from the line by bleeding gas at a downstream union or test port, then reset and retry.
Is the condensate drain hose looped, kinked, or is the trap full of debris?
No: Condensate drainage is not the issue. Focus on the ignition electrode, gas valve, and heat exchanger cleanliness.
Step-by-Step Fix
- Press and hold the reset button on the control module for 1.5 seconds to clear the lockout and allow the boiler to attempt ignition again.
- Verify gas supply by confirming the manual gas valve is fully open and, on new installations, purge air from the gas line at a downstream union or test port.
- Call for heat and observe whether the ignitor sparks at the burner. If you see or hear no spark, turn off power and inspect the ignition electrode for dirt, corrosion, or misalignment.
- Check gas supply pressure using a manometer at the inlet test port. Manufacturer guidance specifies 3.5 to 11 in. w.c. for proper operation.
- Inspect the condensate trap and drain hose for blockage, looping, or a double-trap condition. The drain must not loop and should run continuously downhill.
- Examine the heat exchanger for soot, debris, or signs of incomplete combustion. Clean or flush according to your model’s maintenance schedule if overdue.
- Test the gas valve for proper opening and closing by observing burner flame on a call for heat, or have a technician measure valve voltage and gas flow if the valve does not open.
Parts Often Needed
| Part | Notes |
|---|---|
| Ignition / Detection Electrode Assembly | Amazon | Match your exact Weil-McLain model and control platform; verify part number from the boiler nameplate or service manual. |
| Gas Valve | Amazon | Confirm voltage rating and gas-type compatibility (natural gas or propane) before ordering. |
| Control Module | Amazon | Only replace if other components test good and the module itself fails diagnostic checks or will not exit lockout after all other causes are ruled out. |
When to Call a Pro
Call a qualified HVAC or boiler technician for any work involving gas supply pressure measurement, gas valve replacement, electrode alignment or replacement, control module diagnostics, or heat exchanger cleaning. Gas appliances require precise combustion tuning and pressure verification to operate safely. If you have purged the gas line, confirmed the valve is open, and reset the control but the lockout returns immediately, a technician should measure gas pressure, inspect the electrode with a multimeter, and verify proper venting and condensate drainage. Do not attempt to bypass safety lockouts or modify gas piping without the proper training and tools.
Rough cost: A pro service call runs about $150-350.