Saunier Duval Xeon 40 40FF Boiler Fix Repair

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Faulty Boiler

Boiler on all day and it started doing this clicking and not firing up (no pilot light)

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  • PCB 900847 repair for Saunier Duval Xeon 40 aka Glowworm

Taking the PCB 900847 from bottom of boiler

Following the instruction book fault finding section, Neon 1 and 2 were lit.

The fault tree gets you to test some (fan?, I cant remember now) voltages , the first test had 240v, however the second didn't - rather it fluctuated between presumably 0 and 240 as the relay went on and off. My multimeter couldn't move fast enough.

Unfortunately this wasn't therefore a yes or a no. Assuming no, the third test was a third voltage test with same ambiguous result. Either way these both pointed to a fault with the air pressure sensor (top right of pic with clear tube.) So I started looking at this.

If you take the (red and clear) tubes off an attach a spare rubber tube to it, sucking or blowing a bit should make a microswitch click depending on which one [one sucks and clicks and other you blow and it clicks] If you put continuity tester on the microswitch this should make/break the connections. This appeared to be ok.

So at this point I dispensed with the manual and thought I would look at the PCB, suspecting dodgy electrolytic capacitors (anything with these that gets remotely warm wont last long, this boiler was over 10 years old). Anyhow after removing the board which is quite tricky [need patience and care not to break any clips], the/a fault was quite obvious.....dry joint on back of one of the relays. The caps all looked fine (not bulged).


Saunier duval xeon 40ff pcb top.jpg

To remove with minimal removal of wires (from memory)

  • Unplug everything
  • Remove earth from choc block 3th in from bottom left row (one that goes to the metal chassis)
  • Remove 3 mains in (+/-/Earth) from top right of choc block
  • Pop the 4 plastic PCB mounting clips in, the are odd and have a sort of plastic sprung bit you just push back flush with small screwdriver
  • Remove thin wire from top left, its very tight spade connection (I had to poke a flat bladed tip into this to widen it to get it back on it was so tight)
  • Wiggle board out gently

PCB Bottom

Saunier duval xeon 40ff pcb bottom.jpg

Closeup of Dry Joint on relay

I imagine this had taken a long time to arc-corrode away the solder joint, ending up in no connection.

Saunier duval xeon 40ff relay dry joint.jpg

Resoldered, closeup of Dry Joint on relay

Easy resolder job, used a small drop of some nice Carr's Green flux!

Saunier duval xeon 40ff relay joint resoldered.jpg

And now the boiler is all ok again!

Hope this saves someone a few hundred quid on a callout and new PCB !


Well it has carried on working for a couple more years, then had problem with pilot not lighting (got a CORGI person to fix that) but while waiting for him I looked at the PCB and some more joints were showing fracturing so I reflowed them too - so suggest doing all of them if you ever get the PCB out!

Saunier PCB Back Repair.JPG

Also possibly as a result of the pilot not lighting and it continually trying to ignite it - appears to have heated up R11 to point of it burning the colours off. IT still worked but replaced it anyway.

Saunier R11 Resistor.JPG

[I noticed people selling refurb PCS's on eBay had a higher power dissipation 1/2W resistor here instead of this 1/4W one.]

If your colour banding has burned off the original its a 27kOhm (Red, Violet, Orange - Gold).


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