Vaillant ecotex 831 - heating only working in service mode

Hotpipes · Dec 25, 2023

Wishing everyone on tgmhis forum a very happy Christmas!!

I am working on a combi boiler which has problem with heating only. Water is heating normally. Here are the symptoms

1. Signal from thermostat is not firing the boiler. It does not make any sound when heating is on from thermostat.
2. Heating Dial on the boiler does not trigger temperature change on the display screen when turned. Water dial does change the temperature.
3. Low temp/ high temp test in service mode works and radiators are heating.

Given above,
1. Could this be an issue with thermo stat? Does high heat test in service modw work if thermo stat is broken?
2. Could there be an issue with the heating dial on the boiler?

Please help.
Thanks

ShaunCorbs · Dec 25, 2023

Is there a rad symbol on the display when heating is on / calling ?

Hot water working ?

Desktop987456 · Dec 26, 2023

In service mode can you up or lower the temperature? If yes, there must be a setting somewhere (check Manual) to increase flame intensity, somehow it must have been set to 0 or turned off in your case. Try resetting the boiler as a first step.

I am not a pro but the same thing happened to me on viessmann, where boiler would only fire at 20% firing rate and will stay there though it worked fine in Serv mode. I contacted tecnical support and they asked me to change the gas control valve! I persisted and google helped me to get into setting to increase flame intensity.