Viessmann [F.352] fault codewhat it means and how to fix it

F.352 — "Burner in a fault state — Combustion CO limit exceeded" — is the numeric table's most direct safety row. The electronic combustion controller on these boilers infers the air ratio from the ionisation current, and when the combustion it infers would produce carbon monoxide beyond the limit Viessmann have set, it shuts the burner. The measures point at one thing: "Check entire flue gas path for the following: Leaks. Flue gas back pressure caused by water pocket (if flue system fall is insufficient). Constrictions. Blockages. Repair flue system if necessary. Reset the appliance." A flue that leaks exhaust back into the air intake, a horizontal run without enough fall so that condensate pools and chokes it, a terminal nesting birds have found, or a crushed or kinked section all give the burner air it cannot burn cleanly, and the controller notices before a CO alarm in the room would.

This is a page with no householder step on purpose. The operating instructions' general permission to reset a locked-out burner once exists, but a code that says carbon monoxide should be read as an instruction not to: leave the boiler off, open a window if the boiler is in a living space, and if anyone has a headache, nausea or drowsiness that improves outdoors, treat it as a CO incident. The two-character 100-W B1HC has a related code, 0A, for its CO limiter; the Vitotronic 200-W reports combustion trouble through its Lambda Pro codes. F.347 and F.463 are the recirculation and contaminated-air rows that often precede F.352; F.416 is the flue gas sensor wrongly positioned, another route to the same inspection. When the engineer has repaired the flue and reset the boiler, a combustion analysis confirming CO under Viessmann's permitted 1000 ppm figure is the evidence that it is fixed.