Pilot light keeps going out

[chrishutt]

The ex vessel is holding pressure so I switched on the CH. True to form after 25 mins the pressure gauge was showing 2.5 bar

It would help if you could report what the expansion vessel pressure is when the system pressure increases towards 2.5 bar (to measure this you will need to leave a pressure gauge connected to the schrader valve).

In theory the expansion vessel pressure should exactly equal the system pressure - if not (i.e. ex. vessel pressure remains low) you have a blockage on the connection between system and ex. vessel, as was suggested earlier by corgigazza.

 

[corgigazza]

Its stops you from lighting because it is overheated, remove vent screw in pump check shaft for revs, if it is going round the impellor still may be faulty drain down remove 4 screws from pump body remove pump head, when filling back up listen to aav for expelling air other wise this may also be faulty.

ex vessel hose may also be blocked

boiler stat may also be faulty

 

[engineer01]

I have a Swiftflow 80, the pilot goes out due to a faulty thermistor I have found out. It never happens when I am watching it, but at unexpexted times, so I had to diagnose the boiler properly, taking measurements.

They are expensive, £32 online, almost £50 from a local supplier, so make sure 1st before buying.

I substituted a 2.5 K Ohm resistor for the heating thermistor (the one on the left with two thin black wires coming out), and the heating was on, but the burner then extinguished, this eliminated a faulty PCB. So it was the thermistor, as this measured only 4.7 K Ohms, and out of spec when hot, it should be 2.5 K Ohms at 82 Deg C maximum. When cold it is over 10 K Ohms.

Assuming the pump is OK and spinning, not noisily but, with the impeller turning, not just the shaft, and the flue fan is OK, if it is cutting out when on heating only, it is overheating due to a duff PCB or Thermistor.

Do not alter the pots on the PCB unless your thermistors are OK, and you know how to calibrate it, as it can be adjusted for burner pressure, and temp for heating, and hot water, all seperately, Caution!

To eliminate the PCB, when the heating is on, pull out the thermistor terminals, and push in a 2.5 K resistor, if the burner goes out, the PCB is OK, if not, it has a fault.

To be sure check the thermistor when the boiler is very hot; it should read 2.5K, if it does, it is OK.

Another clue is that the hot water may be very hot when running the heating, but OK when not running it.

The hot water has a thermistor too, but this tested OK, and allowed the boiler to modulate when running low ouput hot water flow. 2.5K when hot, and 10K plus when cold.

Due to sharing the heat exchanger, if the heating water is too hot, then so will the hot water for a while, utill it has colled it down by heat transfer of the excessive heat produced by the fault on the heating.

As a fail safe, if the temperature goes over 90 deg C, the high limit stat cuts off the thermocouple supply, and this shuts the pilot valve, closing off the gas supply.

Hope this helps, I wish I had a VCW-Sine 18 T3 W, not as troublesome, and less moving parts to fail, better made, etc.

They don't make them like they used to!