Oil CH boiler ignition duration = how long should it keep sparking?

The oil CH boiler's a Worcester Danesmoor HPJU25OOO. It's nearly 20 years old, and has been 'professionally' serviced annually - and it has been fairly trouble free.
About 6 months ago, it started locking out - but the local service guy got it sorted out. Unfortunately, it has started doing it again (although I've turned its internal thermostat up a bit, and it seems to be OK at the moment) - and I'm going to call him again tomorrow to have another look at it.
Anyway, my question really relates to the ignition (which I want to ask the service guy about, when he comes).
For the past couple of years, I have been aware of the following:
1. While the ignition is sparking, there has been a considerable increase in interference to the radio. My previous boiler was very bad, and needed the insertion of a suppressor in the ignition lead. This boiler used to quiet - but it now seems to have got pretty bad. [I'm pretty sure that neither the ignition lead nor the igniter have been changed.]
2. The time the ignition runs for seems excessive long. The ignition sequence is roughly - The motor starts - and at the same time, the igniter starts sparking (as heard on a nearby radio). - After about 3 seconds, there is a slight click (presumably the oil valve opening or pump starting). - After about 5 more seconds, there is a 'whump', and the boiler ignites. At the same time, the intensity of the interference drops slightly (presumably the flame is partially shorting the spark-gap). - The boiler continues to burn as it should, but the ignition spark (the interference) also continues for typically 30 to 40 seconds.
So has anyone got any advice as to whether this sort of behaviour is normal? I don't think there is any connection with the lock-out problem, but it would nice to know if the long ignition duration is typical - and if there's likely to be any service adjustment to shorten it to something more sensible.
--
Ian

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On Sun, 5 Feb 2017 19:57:23 +0000, Ian Jackson wrote:

That shouln't happen, pretty sure ours produces no interference at all. <wanders off to bolier with MF radio tuned to inter station space> maybe a brief "tick".

Odd. Are you sure the interference is the igniter and not the motor?

AFAIK all burners have the pump driven by the fan motor. Click I'd say if the valve opening but 3 seconds pre-run seems short.

We get <click><whump> after about 10 seconds of pre-run.
Ignition should be very quick, might be a weak spark (due to knackered supression component(s)) and/or bad adjustment and/or erroded electrodes in relation to the nozzle.

That seems wrong as well. The flame detection should turn the ignition off once it detects a flame and turn it back on if the flame fails. Failure to ignite within a few tens of seconds will lead to a lock-out. Maybe the photocell is sooted up?

If several things become marginal, weak ignition > long time to ignite, poor flame detection > longer time to detection, worn nozzle bad spray pattern/big droplets hard to ignite, can all add up to a lockout.
The Riello burner in our bolier has the following sequence:
Power on Motor/pump runs for 12 seconds Valve opens Flame detected? No Igniter on     5 seconds elapsed? No Goto Flame Detected? Yes LOCKOUT, Valve closed, Igniter off, motor off, lockoout indicator on Yes. Igniter off, Valve open, Motor on Goto Flame Detected?
--
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Dave.
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As I said, the previous one (a Potterton, probably installed in the early 70s) needed an additional suppressor. I simply asked the annual service guy if anything could be done about the interference, and he immediately produced a suppressor from his van. However, I would have expected a more-modern boiler to have resistive ignition lead.

I hear it on the FM band, where it's pretty vicious. But it's essentially wideband.

I think it's unlikely that the interference is from the motor, as it (the interference) stops after about 40 seconds.

Well, it might actually be 3 or 4 - but it's certainly not very long. Apart from being long enough to ensure that the oil has got up to the correct pressure, why would it need to be longer?

I doubt if spark is weak, because boiler fires up readily, without any obvious hesitation.

I'm sure that the neither the igniter nor the ignition lead have ever been changed during one of the annual services - or when the guy came 6 months ago to fix the lockout. He reckoned that the nozzle (the one thing that HAS been changed annually), although the correct size, might not be the right one (variations of the profile of the hole?) - and he changed it. He also replaced the solenoid coil, which has a reputation for going o/c when hot (a look at posts in online forums confirms this). His other work was re-setting the oil pressure (which was too low).

Yes - I would have thought so. I wonder if the 40 seconds of sparks is a default maximum?

Or even faulty. There's certainly something keeping the spark going for far longer than necessary.
The previous boiler flame detector was a spiral bi-metal probe protruding into the flue, with a spindle that drove a microswitch. Otherwise, the whole sequence of events was driven by a small electric motor, with a long spindle turning a row of around 6 cams - each with a microswitch. If there was a lockout, it took a full 7 minutes for the sequence to get back to the start, while in the meantime, the motor and fan ran on to purge the unburnt oil fumes.

As you can gather, I don't know what my boiler is supposed to do - but it's probably similar,
I've rung the local service guy (who, last time, definitely seemed to know what he was doing). He's going to call back to arrange a visit, and as it's nearly time for its annual service, I'll get him to give the boiler the full works.
Anyway, thanks for all the info. While I don't want to appear to be a 'know-all' customer, it's handy to know which sensible questions to ask, and what background evidence might help him to diagnose the problems.
--
Ian

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On Mon, 6 Feb 2017 11:23:34 +0000, Ian Jackson wrote:

Don't think ours has a lead, the igniter has a couple of insulated sockets that the electrodes slide into with a locking screw to enable adjustment.

Yee gads, that's bad...

Inclined to agree, just thinkimg of the age, mind you ours is of a similar age or older with no hint of motor trouble. (SSSSHHHHHH...)

short.

To purge the boiler/flue of any oil vapour if the valve is weeping a bit. The right oil vapour/air mix will go BANG big time not just whumpf...

Apart from the time between click and whumpf when the igniter has been running since the motor started.

The words Heath and Robinson spring to mind. B-)

Have a look at the burner fitted to your boiler and google any maker/model information you can find. You'll probably be able to download a manual for it that ought to contain the start sequence and timings. There aren't many burner makers out there.
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Dave.
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On Sun, 5 Feb 2017 19:57:23 +0000, Ian Jackson

Hay, I was waxing lyrical about oil boiler QRM in the Smart-meters thread in URAM.
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People still use analogue radio?
And a pilot light is better than lighting the bloody thing each time.
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On Mon, 06 Feb 2017 01:52:50 -0000, James Wilkinson Sword wrote:

There are no digital radio signals here...

It's a pressure jet boiler not an oil fired Aga.
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People still use radio?

I have a gas boiler with a pilot. It gives off no interference.
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It's obviously much less practical to have a pilot light in an oil boiler.

Would you expect it to?
However, my 'previous previous' boiler (in my 'previous' house) used a hot coil of wire to ignite the oil vapour (sort-of like a glow plug) - and (of course) it too didn't give off interference. I would have been surprised if it had.
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Why can there not be a tiny oil jet? Does the jet have to spray on to a hot sheet of metal?

No, I'm just saying it's better than having to ignite the gas every time.

I've lived in two houses with oil boilers, neither interfered. I don't know how they functioned.
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On Mon, 06 Feb 2017 16:05:25 -0000, James Wilkinson Sword wrote:

Well I guess the waste heat stops the boiler freezing and the £100/year it costs isn't worth worrying about.

The pump would have to run all the time.

Which jet? Your ficticious pilot jet or the main burner jet? The main burner jet operates at the low 100's PSI and atomises the oil into a very fine spray with a defined shape to match the cavity into which it sprayed, ignited and burnt.
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My gas bill isn't that much in the months it's not used.

Oh.

Either.

I thought there was a hot plate. Maybe I'm thinking of a Primus Stove.
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On Mon, 06 Feb 2017 21:21:20 -0000, James Wilkinson Sword wrote:

A kerosene stove (or lantern) needs to be preheated and manually pressurised before trying to light it. It'll then need to be regularly pumped back up during use.
You may be thinking of an Aga type burner, they run 24/7. They can be turned up and dowm by controlling how fast the oil drips into the burner tray where it is distributed to wicks that vapourise and burn the oil, like a candle or wicked (not mantled) oil lamp. The burn should be an awful lot cleaner than either of those though!
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