Vokera Linea 735 Intermittent Central Heating

[Alex Beach]

I have a Vokera Linea 735 combi boiler powering my central heating and hot water. When starting from cold (hard reset, power off) the heating fires up nicely following a demand for heat from the time clock (no additional controls). Following a number of hot water requests the heating stops working, the pump is running but the burner is never ignited for the heating. The only burner ignition that occurs is for hot water or pre-heat. The pump will continue to be run until the demand for heat is removed (time clock off), resulting in very cold radiators (boiler reporting temperatures such as 22 Dec C on the heating loop). If a soft reset is performed (switch to the off position) when the switch is returned to the "heating and hot water" position the heating continues to be inhibited.

The only way to get the heating back is to remove the power from the boiler. Once this hard reset is applied the heating works as normal again until it gets itself stuck in the heating inhibit mode after a few hot water requests.

Does anyone have any ideas?

 

[aggis]

When firing on dhw & temp rises, turn water off, is then the blr temp low? [ 20 40c] if so probably primary ntc.
If high temp check operation of divertor motor