HeatingCodesWeil-McLain Ultra boiler b18 and E18: high limit and thermal overrun
What the codes mean
The Ultra control has several levels of overheating protection. In the supplement's words, the first level shows a soft lockout code when supply or return water reaches 203 degrees F: "b" followed by 18 on the supply side, or 19 on the return. The burner does not run again until the water temperature has dropped. The second level is a hard lockout if the temperature reaches 210 degrees F, shown as "E" followed by 18 (supply) or 19 (return).
A hard lockout requires pressing the RESET button on the display panel. Weil-McLain adds that a service technician must troubleshoot the cause and correct it before the boiler goes back into operation.
The related codes
| Code | Meaning (per Weil-McLain) |
|---|---|
| b18 | High limit operation: burner off until outlet water temperature drops below 203 degrees F minus the on differential. |
| b19 | Return water temperature higher than 203 degrees F: burner off until it drops. |
| b24 | Return temperature higher than supply temperature: burner off waiting for supply temperature to rise above return. |
| b30 | Temperature rise across the boiler more than 45 degrees F: burner off for a 150-second wait, plus one additional minute for each failed attempt, up to 20 times, then hard lockout. |
| E18 / E19 | Thermal overrun condition on the outlet or return. Hard lockout. |
| E25 | Temperature rise greater than 58 degrees F. Hard lockout. |
What the guide says to check for b18
The first note is context: "This should only happen when the heat demand is lower than the lowest firing rate of the boiler, such as operation of a single, small zone." That is a common winter situation when only one small zone is calling for heat. The checks are:
- Verify the boiler and system are full of water.
- Make sure the system does not have trapped air preventing proper flow.
- Inspect and verify system piping and components, including pipe size and circulator capacity, and that piping agrees with the boiler manual recommendations.
- Verify proper circulator operation.
- A possible loose connector on the boiler outlet sensor, or a possible bad outlet sensor.
Pressing RESET should restart the boiler immediately once the cause is corrected. The guide also reminds readers that there is an electrical shock hazard and that power should be off when working on wiring or components.
Safe homeowner checks for Weil-McLain Ultra boiler b18
- Note the exact displayWrite down whether the code is b18 (soft) or E18 (hard) and which zones are calling for heat.
- Check whether zones are openMake sure radiator valves, baseboard dampers and zone valves that you control are open and not blocked by furniture.
- Press RESET only if the display allows itA soft lockout clears itself. For a hard lockout, press RESET once, and call a technician if it returns.
- Call a technician for repeat lockoutsWater level, trapped air, circulators and sensors need tools to diagnose.
Frequently asked questions
Does b18 mean the boiler is broken?
Not necessarily. Weil-McLain says it should only happen when heat demand is lower than the lowest firing rate, such as one small zone. It also lists causes like trapped air, a circulator problem and a loose outlet sensor.
What is the difference between b18 and E18?
b18 is a soft lockout at 203 degrees F that clears itself. E18 is a hard lockout at 210 degrees F that needs the RESET button and a technician to find the cause.
What does 19 mean?
The same type of overrun on the return side instead of the supply side: b19 or E19.
Sources
- Part 550-100-029/0810: Ultra Gas-Fired Water Boiler Series 1 & 2, Control supplement and Service guide (Weil-McLain) Covers: Weil-McLain Ultra Gas Series 1 and 2 condensing boilers (Ultra-80, 105, 155, 230 and 310). The guide is dated 2010; newer firmware can differ.
Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.