Alpha [E02] fault codewhat it means and how to fix it

Alpha give this one as "overheat boiler lock out", with the mechanism as "overheat sensor has activated". The instinct on reading that is to assume the appliance is overfiring. It almost never is, and Alpha's own cause list says so: they name a pump or restricted flow problem, a blocked heat exchanger, air in the heat exchanger, and external valves or circuit to check. Every one of those is about water failing to move, not about gas.

What is happening is that heat is being produced at the normal rate and not carried away, so it accumulates in the exchanger until the sensor trips. That is the whole fault. It also tells you what a fix looks like: the pump, the air, the valve or the sludge, rather than anything to do with the burner. Repeatedly resetting an appliance that keeps tripping E02 is how a two-hundred-pound pump replacement becomes a heat exchanger, because each reset runs the burner into a circuit that has already demonstrated it cannot take the heat away.

1. 1

   Let the appliance cool completely before doing anything else. A hot overheat thermostat will not reset, and you will conclude the appliance is dead when it is simply still warm.

2. 2

   Check the system pressure is between 1.0 and 1.5 bar cold. Low pressure and poor circulation frequently arrive together.

3. 3

   Open every thermostatic radiator valve fully, and check any service valves on the pipework near the appliance are in line with the pipe rather than across it.

4. 4

   Feel your radiators once the system has run for twenty minutes. Cold at the top means air, which you can bleed. Cold at the bottom means sludge, which you cannot.

5. 5

   Reset once. If E02 returns, stop and book an engineer rather than resetting again.

   Repeated resets on an overheat fault run the heat exchanger without adequate flow. That is the single most expensive component in the appliance and this is exactly how it gets destroyed.