Valliant Boiler - Help me troubleshoot pls

2 Jun 2012
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United Kingdom

Have a Valliant EcoTec plus 637 system boiler that is showing no error codes and S.0 (no demand for heat), but it is not heating.

Have checked that it is receiving switched 240V heat demand into PCB, so believe it's a boiler issue.

Boiler was working fine up until needing to fix small leak in UFH manifold and swap over TRV rad valve. When boiler and heating system turned on again the boiler didn't respond to heat demand. Water pressure showing 1 bar on the LCD.

I noticed that water pressure was very low when I switched it off, to the point that I would have expected it to stop heating but it didn't.

Have tried resetting, power down and on. Have had a look at PCB and other than some spark discharge at the Ignition Transformer connection there are no obviously blown components.

It probably needs some pressure in the expansion vessel, but that wouldn't explain the S.0 code.

Is there a way to manually force it to heat, to try and isolate the problem?

My thinking is that if it is getting demand for heat but responding that no demand for heat then it may be PCB.

What else should I be thinking about?

Last edited:
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I don’t think it’s the pcb for a status code of S.0. Is the temperature up on the boiler? Are the flow and return valves open under the boiler? Any other valves shut?
Boiler LCD shows 0 degrees. Its room temp (but not zero). Hasn't fired up since yesterday morning.

The boiler wasn't touched so all valves etc remain open. Could it be a sensor issue thinking that water temp is hot enough that doesn't need to heat?
Could be an airlock from the repair, or check the motorised valve. Does the hot water work?
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Could be an airlock from the repair, or check the motorised valve. Does the hot water work?
No - nothing works. The boiler just sitting there silent.

Have checked for airlocks but found none but will keep looking. Would an airlock create this outcome?

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