Worcester 20/25 blowing internal fuse

[David G]

I replaced the fuse, tried unplugging the blue burner cable, and it powered up OK. I connected the burner cable up again and it blew again. I went inside the wiring box, behind the control unit and disconnected the transformer, fan and oil pump wires: the fuse did not blow. I reconnected the oil pump: the fuse did not blow. I reconnected the fan: the fuse blew. I disconnected the fan again and reconnected the transformer: the fuse blew.
Does this mean both the fan and transformer need replacing? It seems unlikely that they would both have broken at the same time, unless one's fault damaged the other. I had the transformer disconnected from the electrode leads for these tests, is that OK?
Or does it mean the control unit is faulty?

I was obviously hoping to narrow it down to one component to replace, but now I am confused.

Any help would be gratefully received,
David

 

[Burnerman]

I'll hazard a guess, but both components need to be tested individually. Look at the transformer (igniter unit) - are there any signs of burning or swelling anywhere or any smell of Bakelite burning?
It's easy enough to test the burner motor on the bench, by connecting 250v across it. Carefully does it!
John

 

[David G]

Thanks for the reply. The igniter doesn't look damaged, or smell. Do I need to remove the burner motor to test it, could I just wire it straight to a plug with a 13 amp fuse and see what happened?

I was just wondering what the normal sequence of operations is for the burner. Would the igniter unit be fired straight away? If not, and given that the fuse blows immediately, maybe this might excuse the igniter unit. But, if that is the case, why does it still blow the fuse when the fan motor is not connected? Does this point to the control unit?

 

[Burnerman]

The usual sequence for the burner operation is this:
When the burner starts, the motor with fan spins, naturally enough, and the ignition is also taking place.
This continues for a few seconds to purge fresh air through the flue - after these few seconds the oil supply is electrically switched on and combustion should result. The photocell then sees the established flame and if it's there, the oil supply continues.
I'd certainly test the motor to see if it will start up on its own and reach it's working speed - it's a brushless motor and should spin up smartly.
If you can test for a spark eventually, disconnecting the coil on the top of the oil pump will prevent the oil flowing.
John

 

[David G]

OK, thanks for that. I will try the motor tomorrow - I have been up since 4am, for work at 5am, so I am flagging a bit now. Can I test the motor in-situ or should I be removing the whole burner unit from the boiler?
How would I test for a spark, do I just need to supply a constant 240v to the igniter unit? Do I just listen for the spark or is there some way of seeing it?
Sorry for all the questions, as you can tell I am a beginner at this, just a DIYer willing to have a go.
David

 

[Burnerman]

I like to remove the burner from the boiler - it just makes access so much easier!
I actually have a remote supply of kerosene in my workshop, and a variety of electrical connections - so I can fire the burner up on the bench.
I'm no installer or heating engineer though - like you, I like to understand how things work!
Be lucky
John

 

[Agile]

I'm no installer or heating engineer though - like you, I like to understand how things work!

Be lucky
John

Take that with a pinch of salt!

John is very competent indeed with oil boilers as well as diesel cars!

 

[Burnerman]

Tony, you have made my day
Oh for some qualifications in either!
John

 

[David G]

I have been too busy with work today, so I am making a start tomorrow morning.
First I will try the motor from the mains, second try the ignition unit from the mains. If either blow the plug fuse, I will assume they need replacing.

David

 

[David G]

I have plugged the motor in, it seems to work fine and didn't blow the 3 amp fuse in the plug. I tried the same with the ignition unit, and it did seem to blow the first 3 amp fuse I tried. I realised I hadn't actually tried that fuse beforehand in anything else (it was from an open packet), so I checked another 3 amp fuse, and plugged it in again. This time it didn't blow the plug fuse. I plugged the HT leads in and tried again. There was no discernible sound of sparking, but the body of the ignition unit got warm. Should I hear sparking in this situation, and am I safe in assuming this means the ignition unit is at fault?

I have a horrible feeling that while doing my initial tests from the first post, I may not always have replugged the blue wire bundle connector the correct way round! I know it is only supposed to fit one way, but I noticed at one point that I had pushed it on at 90 degrees to the correct position, connecting 4 of the 6 pins, but in the wrong places Maybe I should reconnect the fan motor and oil pump, leave the ignition unit disconnected, and risk another internal fuse.

David

 

[Burnerman]

David, it does sound like the motor is fine......usually they fail without going open circuit anyway. Usual symptoms are not starting to spin (but speeding up when you twist the spindle) and not reaching full torque or RPM.
So long as you gave the electrodes of the igniter unit a chance of arcing against each other ( 3mm gap or whatever) and there's still no spark it would indicate that it is at fault and I'd go for a replacement.
The original fuse is a 3.15A slow blow type which is able to cope with a minimal surge in current, which could disguise things.
The plug connector can't be fitted the wrong way round (unless the locating lugs have broken off) but if you have managed that then expect a fault on the PCB, basic though they are. (Don't ask how I discovered that one )
Hope this helps!
John

 

[David G]

I think the locating lugs have broken off, but I still think it is worth a try reconnecting the motor and seeing if the boiler is happy without the igniter unit. As I said, I reconnected the igniter to the spark leads on the boiler, so I assuming the spark should happen and that I would hear it - I can't. If the fuse doesn't blow, I will order a new ignition unit and see how it goes.
By PCB, do you mean the burner control box, or the one at the top of the boiler, behind the timer and controls?

David

 

[Burnerman]

Its the one behind the boiler top panel, David....when I did this on my Heatslave boiler, it burnt all of the copper tracks on the PCB away. I was able to repair it with solder and thin single core cable though, and the incident was more than obvious when I looked at the thing!
John

 

[David G]

I reconnected the motor and left the igniter off, and it still blows a fuse. I unplugged the blue cable bundle, fuse OK. So, it is reconnecting the motor that blows the fuse, even though the motor appears to work fine when powered directly from the mains. I am wondering if wiring in the motor, is connecting some other circuitry up that causes the fuse to blow, but I just don't know.

I eventually spotted that the motor is actually wired directly between the yellow and black cables in the blue cable bundle, it doesn't go via the burner control box. So I left the burner control box off and made sure there was nothing else connected to the motor wires, and it blew a fuse. That eliminates the burner control box, and just leaves me with ...... THE PCB That was my last fuse, and I will have to go on a 10 mile round trip to get some more. It sounds like I should take the PCB out and have a good look at it.

David

 

[oilhead]

Don't quite understand where you say the motor is not controlled by the control box. There is a feed from the motor terminal to a post purge circuit depending on the exact model of heatslave. If you have had a faulty transformer, it is possible that this has also burnt out the control box (TF830)
Another way of testing each component is to link the feeds independently, one at a time, from the live feed terminal.