Baxi [E109] fault codewhat it means and how to fix it

E109 is "Pre-circulation fault" in the 600, 800, Compact, Duo-tec 2 and Assure tables and "Precirculation alarm" on Baxi's professional page. Before ignition the pump runs for a few seconds and the board watches the temperature sensors for the small change that proves water is moving through the heat exchanger.

If it does not see it, it will not light, and it shows E109 instead of going on to produce E125 later. The 600 and 800 manuals list 109 among the codes that "it is possible to relight" by "pressing the Reset button for 1 to 3 seconds", and their fault-finding flow treats 109 and 125 together: "System fault — correct. Ensure that the boiler and system are fully vented. Check flow temperature sensor connections and position", with the sensor's cold resistance of about 10 kΩ at 25 °C as the test. So the causes are the circulation causes — a pump that is not turning, air in the pump or heat exchanger, a system with every valve closed — with one addition specific to the moment of detection: a flow sensor that is loose in its pocket or wired badly will report no change because it is not in contact with the water, and the board cannot tell that from a pump that has stopped.

On the selector-switch Duo-tec HE and Platinum the same check produces E193, which has its own page because the reset instruction there is different. Pressure is usually normal when E109 appears; it is a movement test, not a quantity test.

1. 1

   Check the pressure gauge reads at least 1 bar cold and bleed any radiator that is cold at the top. Air is Baxi's first suspect.

2. 2

   Make sure at least a few radiator valves are open and any thermostatic heads are turned up, so the pump has somewhere to push.

3. 3

   Press Reset for one to three seconds, once. Baxi list 109 among the codes this relights. If it returns, the pump or the sensor needs an engineer.