Vaillant 824/2 E F25 issue - system has pressure

[Grangey]

Hi all,

Have an issue with my Vaillant 824/2E, the boiler no longer ignites so I don't have heating or hot water at the moment. When hot water or heating is demanded, the boiler goes through its ignition cycle, sparts away, and just when you'd expect the gas to be released, it cuts out showing F.25

The system has pressure, and it holds, and on a whim I've replaced the pump in case it was this that had failed. The new pump is working and the start cycle confirms it, but gets as far as "S3" in the start up process before failing. When I drained the system form the lowest rad, the water seemed good so no obvious signs of gunk or sediment in the system (bar the boiler, all rads and pipework are less than 2 years old).

I've googled away, and cannot figure out the possible cause. Depending on what website you land on the F.25 can be;

"Water Switch Water Switch - plug disconnected System pressure too low Waterswitch activated" (from an online manual)

From another Vaillant website - "Flue Gas Temperature Too High (Not Applicable To UK Models)" - mines a UK model to my knowledge

And from another Vaillant publication I found somewhere "F.25. Water Switch, System Pressure too low. Pump Obstructed, Pump Running at low capacity, air in the applicant, system pressure too low, flow and backflow NTC reversed".

With the pump replaced and confirmed working, I'm not sure what else to do or try to get to the bottom of this? I'd like to try to sort this myself (I'm very handy) before I absolutely have to call someone in for financial reasons.

Update: NTC sensors now also replaced, not solved.

Thanks for your help in advance!

 

[ianmcd]

Faulty NTC replace both

 

[Grangey]

Thanks ianmcd, happy to give that a shot! Am I right in thinking these are the two I’ve pictured here (one of the heat exchanger, the other off the what I guess is the hot water bit?)

 

[ianmcd]

yes that is them

 

[Grangey]

Thanks Ian,

Replaced both NTC sensors, and for a second or two (literally) the boiler fully fired up and started to heat the hot water, raising it 2-3oc, before then failing again producing F.25 as it did before. Resetting just repeats the original fault, ie I can no longer get the boiler to fire up. The old sensors were covered in white sediment so clearly needed changing and made a small impact, but now at a loss as to what to try next.

Any other suggestions?

 

[Grangey]

Could it be the pressure sensor??

 

[Grangey]

Good news, seems to be working, looks like it may have been pressure sensor related but mainly the cabling.. but this has lead me to find what I believe to be a much bigger issue.

Looks like the main burner section has a leak on the side near the pressure switch. Some of the bolts were not even finger tight! I've tightened them now as much as I can but don't trust it at all, think it would be best to get a new gasket/flange at the least. Obviously not something I'd do myself so I've booked someone in to take care of this on Tuesday.

Been a great experience learning about all this though!