Weil-McLain A30 Error Code — What It Means
The A30 code on Weil-McLain Ultra-series boilers indicates a high-temperature lockout. The boiler has shut down because the sensed water temperature is rising too quickly or has exceeded the control’s limit logic. This is a protective shutdown to prevent overheating damage. The control detects that heat is building up faster than it should, which typically means the boiler is firing but water is not circulating through the system properly.
The lockout is not an ignition fault. Instead, it signals that the boiler cannot move heat away from the combustion chamber fast enough. The most common real-world cause is a failed or weak circulator pump, but air trapped in the system, restricted flow from sludge or a clogged strainer, improper piping or bypass valve settings, and a faulty supply temperature sensor can all trigger the same code. Until the underlying flow or sensing problem is corrected, the boiler will continue to lock out on A30.
Before You Replace Anything
Homeowners often replace the temperature sensor first, but the real culprit is usually poor system flow. Before ordering a sensor, verify that the circulator pump is running and that the system has been purged of air.
Common Causes
- Failed or weak circulator pump (~40%) The primary system circulator is not running, has a worn bearing, or is cavitating so that water is not moving through the boiler fast enough to carry heat away.
- Air trapped in the system (~25%) Air pockets in the boiler or distribution piping reduce flow and prevent proper heat transfer, causing rapid temperature rise.
- Restricted flow from sludge or sediment (~15%) Clogged strainers, closely spaced tees, or sludge buildup in low-flow zones block water movement and trigger overheating.
- Improper piping or bypass valve position (~10%) Incorrect primary-secondary piping, a bypass valve set too open, or reversed flow direction allows the boiler to overheat.
- Faulty supply temperature sensor (~10%) A drifting or shorted thermistor reports an impossibly rapid temperature rise to the control, even when actual water temperature is normal.
Quick Diagnosis
Answer these to narrow it down fast.
Is the circulator pump running when the boiler calls for heat?
No: The pump has failed, lost power, or is not receiving the signal from the boiler control. Check electrical connections, fuses, and pump rotation by hand (power off). Replace the circulator if it will not start.
Does the system pressure gauge read in the normal range (typically 12-25 psi cold)?
No: Low pressure indicates a leak or inadequate fill. Top up the system to the correct pressure and bleed all air before restarting the boiler.
After purging air and confirming pump operation, does the boiler still lock out on A30 within a few minutes of firing?
No: The problem was air or the circulator. Monitor the boiler over the next few cycles to confirm stable operation.
Step-by-Step Fix
- Confirm the exact boiler model and download the correct Weil-McLain manual from the manufacturer’s website, because diagnostics and menu navigation vary by control family.
- Check the fault history in the contractor or diagnostics menu on the control display to confirm that A30 is a high-temperature lockout and to see whether it is recurring.
- Verify circulator operation by listening for the pump motor when the boiler calls for heat, feeling for vibration at the pump body, and checking that the pump shaft rotates freely when power is off.
- Measure circulator electrical supply if the pump is suspect: confirm line voltage at the pump terminals and check for current draw while the pump should be running (consult your model’s table for expected amperage).
- Purge air from the entire system using manual air vents on the boiler and all zone loops, and confirm that automatic air eliminators are functioning and not clogged.
- Inspect piping, bypass valves, and flow direction to make sure the boiler is piped per the installation manual and that any internal or external bypass is set to the correct position for your system type.
- Check the supply temperature sensor if flow is good but the control still reports rapid overheating: measure thermistor resistance and compare to the sensor chart in the manual, and look for loose or corroded wiring.
- Inspect and clean flow restrictions including the boiler inlet strainer, closely spaced tees, and any inline filters, flushing or replacing components as needed to restore full flow.
- Reset the lockout after correcting the underlying issue by following the manual’s reset procedure (typically pressing and holding a button on the control), then monitor temperature rise and circulation during the next firing cycle.
Parts Often Needed
| Part | Notes |
|---|---|
| Weil-McLain boiler circulator pump | Amazon | Verify pump model and flange size from your boiler’s installation manual or the nameplate on the existing pump before ordering. |
| Weil-McLain supply temperature sensor / thermistor | Amazon | Match the sensor part number to your exact Ultra control model; resistance values and connectors vary by series. |
When to Call a Pro
Call a licensed heating technician for A30 diagnostics and repair. Although checking that the circulator is running and purging air are tasks a confident homeowner can perform, diagnosing electrical faults in the pump circuit, measuring sensor resistance against manufacturer charts, inspecting complex primary-secondary piping, and safely working around a live gas-fired boiler all require professional training and test equipment. If the circulator has failed, a pro will also verify that the replacement pump is sized correctly for your system’s head and flow requirements. Finally, if the boiler locks out repeatedly even after the obvious checks, a technician can use the control’s advanced diagnostics to isolate intermittent sensor faults, control-board issues, or hidden restrictions that are not apparent during a simple visual inspection.
Rough cost: A pro service call runs about $200-500.