Baxi [E.01.20] fault codewhat it means and how to fix it

E.01.20 is the flue over-temperature lockout on Baxi's two-part and dotted displays, the counterpart of E130 on the three-digit ranges. The 200/400 Combi manual defines it as "maximum flue gas temperature reached (> 140 Deg C)", the Combi 2 manual as "maximum flue gas temperature reached", and the two manuals give different-looking remedies that describe the same failure from two sides. The Combi 2 table is blunt: "EXCHANGER ON FLUE GAS SIDE BLOCKED — check the cleanliness of the exchanger".

The 200/400 table gives the circulation list — "vent the air in the heating system, check the water pressure, check the circulation (direction, pump, valves), check the cleanliness of the heat exchanger, check that the sensors are operating correctly" — because flue gas runs hot for two reasons: the water side is not taking the heat away, or the combustion side is so fouled that the flame heats the exhaust instead of the water. The 200/400 range prints two more flue-temperature codes around this one, E04 06 "critical flue gas temperature reached" with the same remedy and E04 04/05 for the flue sensor itself, and its user manual groups them under "flue temperature sensor fault", so an owner may see any of the three. This page is rated danger because the flue is where the boiler's combustion products go, and a heat exchanger blocked on the flue side is a combustion fault: a burner that cannot pass its exhaust through the exchanger is a burner whose combustion is being disturbed.

The reset — one second on a Combi 2, at least three seconds on a 200/400 — will clear the code, and a boiler that trips once on a very cold morning with a marginal system may run for weeks afterwards, but the manual's remedy is a cleaned exchanger and nothing a householder can do stands in for it.