Applicable Systems
Lochinvar Knight Boiler (SMART SYSTEM Control)
Symptoms
The Lochinvar Knight boiler SMART SYSTEM control displays fault message E04, indicating a low water pressure fault. The system water pressure has dropped below the minimum safe operating pressure, causing the control to shut down the boiler to prevent damage from low water conditions.
Possible Causes
- System leak in piping, radiator, or boiler heat exchanger
- Expansion tank failed or waterlogged, unable to maintain system pressure
- Makeup water feed valve closed or pressure regulator failed
- System was not properly filled and pressurized after installation or service
- Automatic air vent leaking, slowly depressurizing the system
Troubleshooting Steps
- Step 1: Check the boiler pressure gauge to confirm the current system pressure
Pressure reads at least 12 psi (residential) or the design pressure for commercial - Step 2: Inspect all visible piping, radiators, and connections for leaks
No visible water leaks at any joint, valve, or connection - Step 3: Check the expansion tank by tapping it - top should be air, bottom should be water
Tank is not waterlogged; top half sounds different from bottom half when tapped - Step 4: Verify the makeup water feed valve is open and the pressure regulator is functioning
Makeup water valve is open; regulator maintains 12-15 psi output - Step 5: If system was drained for service, refill and pressurize to the correct cold-fill pressure
System pressure stabilizes at 12-15 psi cold fill with no rapid drop
How to Reset
To reset: 1) Fix any leaks in the system. 2) Refill and pressurize to the correct cold-fill pressure. 3) If the expansion tank is waterlogged, drain and recharge it (or replace if the diaphragm is ruptured). 4) Press the RESET button on the SMART SYSTEM control. Caution: Do not add cold water to a hot boiler. Allow the boiler to cool below 100F before refilling to prevent thermal shock to the heat exchanger.
Common Replacement Parts
- 100190: Lochinvar expansion tank (2 gallon) (Size the tank to the system water volume and design temperature rise)
- 100195: Lochinvar makeup water pressure regulator (Set to maintain 12-15 psi system fill pressure)
Frequently Asked Questions
- Q: What pressure should my boiler system be at?
For a typical residential boiler system, the cold-fill pressure should be 12-15 psi (equivalent to about 1 bar). The pressure will rise 4-6 psi when the system is hot. If the pressure drops below 10 psi when cold, the low pressure fault will trigger. Commercial systems may have different pressure requirements.
- Q: Why does the expansion tank matter?
The expansion tank absorbs the volume increase when water is heated. Without a functioning expansion tank, the heated water has nowhere to expand, causing the pressure to rise until the relief valve opens. When the system cools, the lost water volume causes the pressure to drop, potentially triggering E04. A waterlogged expansion tank is the most common cause of pressure problems.
Source: Lochinvar Knight Boiler Service Manual KB-SER-08, Troubleshooting Table
Category: Boiler