Baxi [E28] fault codewhat it means and how to fix it

E28 is the flue thermistor fault in the Baxi code set, and it is among the most frequently seen Baxi codes in service. The flue sensor watches the temperature of gases leaving the appliance, which on a condensing boiler should be comparatively low — capturing that heat into the water is the entire point of the design. Two situations produce this code and they need different work. The sensor may have failed, in which case the flue temperature is fine and only the measurement is wrong. Or the reading may be accurate, in which case heat is not passing into the water where it belongs. Baxi's guidance on the related E131 points at a possible blocked internal heat exchanger waterway, and that is the important possibility here. A deposit layer of scale or magnetite inside the exchanger insulates the metal from the water well enough to raise flue temperatures measurably, and the condition develops over years rather than suddenly.