Intermittent fault, Eurocal with Sterling ST108

[myles]

Got called out to a Eurocal boiler with a Bentone Sterling ST108 burner last week that I serviced last month (since found that the issue predates service, no ignition issue during service), Usual thing where it has an intermittent fault. So I arrive on Friday afternoon and the boiler wont light at all. I changed the electrodes as the ceramic had a big crack right across from one electrode to the other (prob from an over-tightened bolt) also changed the HT leads I had advised homeowner about this during service. Did usual elec tests, pump pressure, fan etc and everything seemed in order except it still wouldn't ignite. Apparently the boiler wouldn't light for the previous two days. I spent a bit more time double checking electrics for loose connections but no change. Looking at flow chart for the burner and it basically came down to a faulty electrode transformer. Had to go as it was getting on and homeowner said its fine as they don't really use boiler much! Anyway I ordered a new transformer and went back today, Called for heating and boiler lit up straight away without even touching it! Customer said boiler had been lighting fine over the weekend but didn't this morning. Then the next problem came where I tried replicating the fault of it not firing up so turned boiler off and on etc boiler ignites every time lol. Took the opportunity while it was running to test the thermostat which turned boiler on and off when turning dial up and down from 0 so far so good. Stuck fire-valve tester @60c and put thermostat phial in which shut the boiler off as expected (thermostat set at 50c). Turned thermostat up to 80c and burner went through sequence but wouldn't ignite and ran to lockout again so now thinking maybe the thermostat is faulty. Overheat stat worked so ruled that out, Photocell is fine with no obstructions and fga comes back spot on when its running (no change from when I serviced it) I then changed the fuse for a new one and boiler ignited straight away so again turned on and off by various methods and each time burner ignited without fail! Don't really want to be swapping out parts if its something simple that one of you guys has come across before! If any other oilies have any idea I would appreciate any advice via PM or whatever (don't want to many clues on here for DIYers) :dizzy2:

 

[SimonG]

Thermostat and lockout are not linked. Whats the combustion like when it is running? Check position of electrodes. Any damp in the control box?

 

[SimonG]

Went to a firebird tonight. Rdb burner, they have had problems since firebird came out last year at end of warranty. Flame dropping out and then firing back up. Photocell was about 1/2" out of its holder. Pushed it home and ran like a train. Easy money

 

[myles]

Combustion is spot on when its running, no damp in box that I could see, electrodes are within specified limits checked they were right with my calipers when I put them in, I was leaning more towards the control box but cant be 100% as its so intermittent!!!

 

[myles]

Went to a firebird tonight. Rdb burner, they have had problems since firebird came out last year at end of warranty. Flame dropping out and then firing back up. Photocell was about 1/2" out of its holder. Pushed it home and ran like a train. Easy money

I wish this one was that easy lol best I had was back boiler pilot not lit [emoji106]

 

[SimonG]

Try dropping the oil pressure and air a touch. Ive seen some were the fan blows the spark away from the nozzle, if tbat makes sense.

 

[myles]

Yeah makes sense, guess its worth a shot I'll see how its running now over next few days (going to service their shop boiler next week so will find out then) cheers!

 

[Best]

You could take a look at the spark at electrodes by disconnecting the solenoid & taking a look in the front of blast tube. Try it loads of times to see if soark ever suddenly gets weak or fails.
Control box can also be very intermittant - failing one day and working all the next day.

 

[Bunker]

Cause and effect.

You have to ask yourself why did the transformer fail? Its usually because the control box is chattering or keeping the ignition on for too long. Its also often caused by the photocell being partly obstructed or faulty - it sees the flame, doesnt, sees it, doesnt - every time ignition is initiated, the transformer runs for 12 seconds and will burn itself out. As you have changed the electrodes, check the view of the photocell or try running the burner with the pcell out, covered, then show it a torch after the coil clicks.

 

[Pipes Plumbing]

Have you checked that the photocell isnt dirty or damaged in any way?

Had an RDB3 doing similar recently, where sometimes it would start, flame out, start, flame out etc but only once in a while. Most of the time it would click into life as normal.
When I pulled the cell it turned out that it had a film of crap on it from where the initial butchers that had installed it had left the settings so far out. The end of the photoresistor itself was slightly offset in the rubber too, as if it had been rammed in against the side of the housing previously.
Cleaned and straightened it out and away it went. No issues since (plus the combustion setting are now right!)