Worcester Combi not firing in morning

[WHPES]

Got a problem with a Worcester combi not firing in the morning from cold.

Boiler approx 14 years old.

Pressing the reset button once only gets the boiler to fire OK.

The fault is intermittent and happens about once per week.

The fault is not connected with frosty weather.

The tank had been contaminated with water and there was dross in the oil line. Flushed line and emptied water out.

Changed pump and solenoid coil, made little difference.

Any suggestions?

 

[Rob Sale]

Have you tested the resistance on the Limit stats? if they are greater than 1 ohms they have a short circuit which could do this.
check aps aswell and thermistors.
clean venturi

Does it have a programmable room stat? i'd check the connections on that.

but hey i am not an oily engineer.

 

[dontknowitall]

Capacitor? Losing charge/whatever it is after a cold spell?

 

[Plumb Ugly]

Is it a gravity or suction oil line?

 

[SimonG]

I'd try a new capacitor.

But I did have one a couple of years ago, intermittent lockout on a morning and it was the transformer.

 

[Roger Welton]

Also had a dodgy satronic box cause bizarre intermittent lock outs.

 

[Bunker]

Is it a balanced flue? leaking gasses?

 

[Billy Bob Bob]

aps????? on a oil boiler

 

[Plumb Ugly]

Could be a micro leak on the oil line allowing air to build up in the line whilst the boiler is off over night then clearing after the first fire until the next extended off period

 

[Best]

First I would check the motor starts on the split second every time, (give it loads of starts to test, with your ear near the burner) - any slight hesitancy & I would think capacitor.
Normally Control box does that, once in a while lockout, but photocells can do weird things.
As has been suggested, check the transformer is making a strong & constant spark.