The Low Water Cutoff (LWCO) Explained
A low water cutoff stops the boiler firing if water level or flow drops too low — preventing dangerous dry-firing. Here is how it protects you.
Firing a boiler with too little water inside is one of the most destructive things that can happen to it — and potentially dangerous. The low water cutoff is the safety device that prevents it, and on many BC installations it is required. Here is what it does.
What it does
The LWCO senses whether there is enough water in the boiler. If the level or flow drops below a safe threshold, it cuts power to the burner so the boiler cannot fire dry. Once water is restored and the cause is fixed, it allows normal operation again.
Why dry-firing is so damaging
Without water to carry heat away, the heat exchanger temperature skyrockets in seconds. That warps and cracks metal, destroys exchangers and can create a genuine hazard. The LWCO exists specifically to prevent that scenario.
When it trips
- A leak somewhere in the system has dropped the water level.
- A fill or feed problem means the system never reached pressure.
- Air binding is preventing proper flow through the boiler.
Key takeaways
- The LWCO stops the burner firing when water is too low — preventing dry-firing damage.
- A tripped LWCO almost always means a leak or fill problem to investigate, not a faulty device.
- Many BC installations require an LWCO; never bypass it.
Frequently asked questions
My boiler won’t fire and I think the low water cutoff tripped — what now?
Treat it as a signal that water level or flow dropped for a reason. Topping up and resetting may restart it, but if the level falls again you have a leak or fill fault that needs diagnosis. Repeatedly overriding an LWCO is unsafe.
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