Applicable Systems
Lochinvar Knight Boiler (SMART SYSTEM Control)
Symptoms
The Lochinvar Knight boiler SMART SYSTEM control displays fault message E03, indicating a high limit temperature fault. The boiler supply water temperature has exceeded the high limit setpoint, causing an immediate safety shutdown to prevent overheating and potential steam formation.
Possible Causes
- Pump not running or flow restricted, preventing heat removal from the heat exchanger
- High limit setpoint set too low for the application, causing nuisance trips
- Air in the system blocking water flow through the heat exchanger
- System valves closed, preventing water circulation
- High limit switch itself faulty, tripping at a lower temperature than set
Troubleshooting Steps
- Step 1: Check the system circulator pump operation by feeling the pipes for temperature differential
Supply pipe is hot and return pipe is cooler, confirming flow - Step 2: Verify all system isolation valves are open
All valves are in the open position - Step 3: Check the boiler water pressure gauge to confirm the system is full
Pressure is at the normal cold-fill value (typically 12-15 psi for residential) - Step 4: Bleed air from the system at all air vents and the boiler air vent
No air comes out of the vents; water flows steadily - Step 5: Verify the high limit setpoint matches the system design (typically 180-200F)
High limit setpoint is at or above the design operating temperature
How to Reset
To reset: 1) Fix the pump, valve, or air issue causing low flow. 2) Allow the boiler to cool. 3) Press the RESET button on the SMART SYSTEM control. 4) If the high limit trips again immediately, the switch or sensor may be faulty. Caution: Do not increase the high limit setpoint to eliminate nuisance trips. The high limit protects against potentially dangerous overheating.
Common Replacement Parts
- 100225: Lochinvar high limit switch (Set to the temperature specified in the system design documents)
Frequently Asked Questions
- Q: What is the high limit and why is it important?
The high limit is a safety device that shuts down the burner if the boiler water temperature exceeds a safe maximum (typically 180-210F for residential boilers). Without it, a pump failure or flow blockage could cause the water to boil, creating steam pressure that can damage the boiler and piping.
- Q: Why does the high limit trip when the pump is running?
Even with the pump running, the high limit can trip if there is air in the system (blocking flow through the heat exchanger), if system valves are closed, if the pump is degraded and not moving enough water, or if the high limit switch itself has drifted and is tripping at a lower temperature.
Source: Lochinvar Knight Boiler Service Manual KB-SER-08, Troubleshooting Table
Category: Boiler