oil burner start up

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Looking for technical help with my oil fired boiler.

It's a Worcester Danesmoor 20/25, more than 10 years old, which is getting increasingly hard to fire up. It runs fine once it's going, but trying to start it up either from cold or after it's switched off thermostatically results in lockout. If, after resetting lockout, I flick the temperature control on and off quickly it eventually fires, this used to take 5-10 'flicks' but now it's more like 25.

Bleeding the pump doesn't seem to have any bearing on the problem, and I have also changed the control box on the burner but that didn't help either. Fuel supply seems to be ok, I've changed the tank during all this so new heating oil and new filter. One heating engineer who I called out said it needed a new electric eye, but how can that be when it runs ok after firing?

Any advice gratefully received!
 
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What state is the cabling in for the photocell ? For what one costs, it can't hurt to replace it, as they can fail. Could also be a faulty pump, if this is also ten years old.
 
Thanks for that, I'll check out the photo cell tomorrow.

I forgot to say before but the heating enineer checked pump pressure and found it ok.
 
One heating engineer.....

He wasn't, otherwise he'd have fixed the problem.

Don't spend money on a photocell until you know that is the fault. You already have an unnecessary control box. Have you measured the value of the motor capacitor? That is the most likely cause.
 
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Thanks for your reply, Oilman, but I don't really understand your answer!

Why does the motor have a capacitor, or is it part of the HV ignition system? Also how do you test or measure capacitance?

I'm ok on mechanics but ,as you can see, I struggle with electrics.
 
There are two windings on the motor.
One is direct from the supply, the other is through a capacitor.
The capacitor shifts the phase angle of the current in one winding so there is some torque to start the motor.

As the capacitor gets old, the value decreases, and so the torque decreases.

Measuring it needs a capacitance meter. It would be cheaper to change it than buy a meter. But if you bought a meter, you would have it for other jobs.
 
Er... not really meant to do this, but the pump capacitor is required not just for starting ( as on some switch start induction motors ) but for running too.

However, since the motor runs the pump, surely if the capacitor was failing ( losing capacitance ) then the motor would be running slow and the pump pressure would be low.

I forgot to say before but the heating enineer checked pump pressure and found it ok.

That being the case then I would say the motor cap is OK.

Whilst I have not looked inside your boiler during ignition, the problem would seem to me to be a lack of actual ignition. This might be a weak or misalligned spark or the oil is not correctly mixed with air at the spark electrodes.

I would also discount the photocell because, if that was faulty, then the oil would ignite but go out a few seconds later as the flame had not been detected.

Tony
 
Er... not really meant to do this, but the pump capacitor is required not just for starting ( as on some switch start induction motors ) but for running too.

The capacitor has dropped in value, not just vanished.

However, since the motor runs the pump, surely if the capacitor was failing ( losing capacitance ) then the motor would be running slow and the pump pressure would be low.

I forgot to say before but the heating enineer checked pump pressure and found it ok.

That being the case then I would say the motor cap is OK.

The motor speed is determined by the mains frequency. Even if the capacitor is very low and will not start the motor, the motor will run if it is given a flick.

The pump will produce the correct oil pressure even if the motor does run slow. This is one reason for sooting, i.e. pump works fine, air supply low.


Note: correct spelling of misaligned
 
I had taken the view that the OP would have told us if the motor was not starting up at all. Also that if that was the case the engineer who attended would have been likely to have realised what the likely problem was.

So I was only considering a failure to light with the motor running.

To the OP, when it "fails to light" is the motor always running or is the problem that the motor does not start?

The oil pump will still give the set pressure if the motor is a bit slow because the pressure is regulated by a bypass bleed.

Tony
 

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