Oil burner will only fire when cool - please advise

14 Mar 2014
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United Kingdom
Bit confused on this one, burner will fire perfectly from cold until the boiler stat stops calling for heat, then it cools until the stat calls for heat again, then it purges and attempts ignition but goes to lockout. It won't fire again if the lockout button is pressed until about 45mins later when it has cooled then starts again perfectly.

It's a warmflow 120he condensing boiler with a riello rdb 2.2 oil burner.

240v to the control box.

New pump was fitted recently.

Oil supply is fine.

Solenoid valve itself seems fine, not seized and all of the oil ports are clear.

Electrodes are gapped correctly.

Nozzle isn't coked up.

Photocell is good, getting 300ohms with light & infinity when dark.

Replaced the solenoid coil and its wiring.

I'm guessing the control box. Again the burner fires up perfectly when pressing the lockout button but only when it is cool, but will not fire at all when warm.

Any advise or guidance is very much appreciated.
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Let it run up to temperature and try to start. If it locks out disconnect the p/cell and try again. If it starts, then replace the photocell. If not then it must be control box.
Thanks for the reply.

When it was cold I disconnected the photocell and sure enough it fired up & then shut down within 5 secs just as it should.

When its warm it just will not fire at all no matter what I try.

If I'm beside the boiler and I start it from cold (it starts perfectly) and let it get up to temp, just as the boiler stat tells it to stop when it gets hot enough I then increase the stat temp immediately and it still goes to lockout even though it was running perfectly 2 or 3 secs previously.

That's what confuses me if it is the control box, it will run perfectly at any temp on the boiler stat but just will not start again unless it cools down.

It only locks out when it attempts to start again when it's warm, it's not tripping when it reaches any temp.

The control box itself appears fine, no cracks or warping, no water ingress.
Theoretically, the boiler 'control stat' is normally wired to the control loop of the burner control box and on each re-fire via either timer or control stat demand the burner control would repeat its normal sequence automatically - however if the boiler 'limit stat' should ever activate, for safety reasons it should trip the lockout safety circuit requiring a manual reset of the lockout button for a burner restart.

Without being clairvoyant, in your current situation the problem seems as if the limit stat is being used as the control stat via the lockout wiring loop, a cool down period would allow closing of limit stat contacts for a restart but only via lockout button reset.

Whilst i hope this is not the case, i would have the two wiring circuits (boiler stat + boiler limit stat) checked out anyway.

Any boiler overheat or limit stats must trip-out the lockout circuit so as to cause responsible person intervention and inspection.

...and the bell ringing clues are ->

a. "i then increase the stat temp immediately and it still goes to lockout even though it was running perfectly 2 or 3 seconds previously"

b. "It won't fire again if the lockout button is pressed until about 45mins later when it has cooled then starts again perfectly.

good luck
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Did you try firing it up from warm with the photocell disconnected? It is not clear from your reply.
Also, when you try to fire up from hot, have you checked for voltage to the solenoid leads with the photocell connected and disconnected?
Thanks for the replies everyone, they're all very much appreciated.

Mr Therm - that's exactly what it seems like to me. The complete boiler stat unit was replaced 6 mths ago as excessive oxidisation on the capillaries had caused it to fail, it's a dual safe type with the manual overheat reset, so I'm assuming its good. The overheat is not activating and it will bring the boiler to any temp that it is set at, be it 65 or 75 or 85 fairly accurately as I've tested the ambient temp of the shroud with an ir thermometer. Everything your saying makes sense but I don't understand why the boiler stat itself isn't tripping its manual reset before the control box acts oddly. Only thing I can presume is that maybe the circuitry within the control box is expanding with the slight increase in ambient temp causing it to act up.

Jeff the gasman & slapper - I'm thinking that also, maybe with the slight increase in surrounding temp the solenoid valve itself is expanding & constricting its operational movement. I removed the valve itself and it wasn't gunked up and all oil ports were clear. There was maybe 1mm of lateral travel on the valve itself, though I don't know if that value is standard.

Oil head - sorry, yes I did try firing it up when warm with the photocell disconnected, again same problem, just goes straight back to lockout. As for the solenoid coil itself I'm getting 100ohms resistance both hot and cold, and again when both hot and cold the coil is magnetising no problem, with me removing it and inserting a screwdriver within the coil, so I can only assume the voltages to the coil are good and correct when both hot and cold.

The whole thing is very confusing, I'm a qualified diesel fitter and mechanic so I'm familiar with working with high voltages and fuel systems etc & I know the attitude of any boiler tech would just be to replace the entire unit as it would be more economically viable than hours of testing. Again it is very very confusing.
I'd go for a replacement control box at this point, but I'd also like to check for an ignition spark when it won't fire up from hot.
John :)
Thanks burnerman.

Is there a satisfactory way to test both?

I imagine the control box is not something many people would bother testing, with it just being replaced complete if it is suspect. Other than the tests I've performed as above, I don't see much else that can be done.

Problem is that if I buy a control box complete & solenoid valve & regular service parts just for good measure, the total cost involved and the effort required for fitting, it could end being close to the price for an entire new burner unit itself.

As for testing for a spark, is there any other test that can be done other than me removing the burner when warm and attempting ignition when exposed to the atmosphere and visually looking for a weak spark or no spark etc. I imagine there could be the possibility of fine hairline cracks on the electrodes themselves that could expand when warm causing open line and therefore no spark and ignition failure. The wires themselves from the control box to the electrodes look good and I could test them for voltage, but, they are not accessible when the burner is in use as they sit inside the air chamber just before the blast tube with the control box itself forming an air seal.

It doesn't help as well that I cannot find an electrical schematic to read anywhere, for any of the electrical components, let alone something as relatively complex as the internal circuitry of a control box.

Thanks again everyone for reading & for your replies, but I'm still massively confused about the whole situation.
Remove the burner, Remove the solenoid from the valve, insert a keep through the coil and look at the spark. you cannot just discnnect the coil as the control box checks the circuit on this burner as with other Riellos.
Have you checked with a pressure gauge to see if you have fuel when warm?

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