Lochinvar Knight Flow Switch/LWCO — Flow Switch or Low Water Cutoff is not Making
Flow Switch/LWCO
Flow switch or low water cutoff is not making. Either the optional flow switch or the low water cutoff (LWCO) is not making, so check boiler pump operation on a call for heat, closed valves or obstructions in the boiler piping, system water level and purging, the flow switch jumper, and replace fuse F3 on the control board or the LWCO as needed. The table classifies it as Lockout (manual reset).
Applies to
This low-water or pressure protection code is displayed by the SMART SYSTEM control on Knight fire-tube boilers (KHB floor 055–285 and WHB wall 055–399), as documented in Knight Service Manual, doc 100305279 Rev E — Table 3-4 fault codes (KHB floor; the WHB service manual Rev G prints the same table) (100305279 Rev E). Codes can differ between controller generations, so always check the manual that matches your unit.
- Lochinvar Knight Floor Mount (KHB)
- Lochinvar Knight Wall Mount (WHB)
- Lochinvar Knight KHB055
- Lochinvar Knight KHB085
- Lochinvar Knight KHB110
- Lochinvar Knight KHB155
- Lochinvar Knight KHB199
- Lochinvar Knight KHB285
- Lochinvar Knight WHB055
- Lochinvar Knight WHB085
- Lochinvar Knight WHB110
- Lochinvar Knight WHB155
- Lochinvar Knight WHB199
- Lochinvar Knight WHB285
- Lochinvar Knight WHB399
Controller: SMART SYSTEM control. Source manual: 100305279 Rev E (2024-04).
Why a code is not a diagnosis
A water-pressure or low-water code can mean a genuine leak or empty system, a failed expansion tank, a faulty fill valve, or a sensor problem. The appliance does not distinguish these.
Safe things to observe
- Read the pressure gauge and compare it with the normal cold-system pressure your installer set.
- Look for drips or wet spots near the appliance, pipes and the relief-valve discharge.
Do not
- Do not keep re-pressurizing a system that keeps losing water.
- Do not cap a relief-valve discharge pipe.
How professional service helps
- A technician finds and repairs the leak, checks the expansion tank and fill assembly, and verifies the sensor circuit.
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Parts and service
Parts commonly involved
These parts appear in the documented text for this alert. A code reports a condition, not a failed part — a technician confirms the cause by testing before anything is replaced.
Control board / PCB
The control board — also called the PCB, or printed circuit board — is the boiler's main computer.
What a technician checks →Low-water cutoff
A low-water cutoff is the safety device that stops a boiler from firing when there is not enough water inside it.
What a technician checks →Flow switch
A flow switch proves that water is actually moving through the boiler before it fires and while it runs.
What a technician checks →Circulator pump
The circulator is the pump that moves heated water from a boiler out to the radiators, baseboards or in-floor loops and back again.
What a technician checks →
Service for this alert
- Gas Boiler Repair
Licensed gas technicians from ROMA Heating & Cooling diagnose and repair residential and light-commercial gas boilers across…
- Emergency Boiler Repair
Priority gas boiler repair for homes and buildings in Greater Vancouver with no heat, no hot water, a locked-out boiler or a leak.
Every boiler part explained — what it does, how it fails and what a technician checks.
Documented for Lochinvar Knight Floor Mount (KHB) and Lochinvar Knight Wall Mount (WHB).
Frequently asked questions
Can I clear this code myself?
Clearing a code without fixing its cause usually just brings it back. Note the code, check the safe items on this page, then call a licensed technician. If your appliance manual describes a user reset for this exact code and it stays away, no call may be needed.
Boiler locked out? Call with the alert text ready.
The brand, model and this alert are passed along when you request service online, so the technician arrives prepared.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
- Knight Service Manual, doc 100305279 Rev E — Table 3-4 fault codes (KHB floor; the WHB service manual Rev G prints the same table) · 100305279 Rev E · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.