Alpha 240x intermittent not firing

[ianshearer]

Symptoms:-
Pilot lights ok when gas valve depressed, fan goes to high speed, pilot lights.

On CH demand, pump runs, primary flow switch operates, but fan does not go to high speed- except that sometimes it does. It will then cycle as expected for an hour or 2 , then fail to start and sit there with the pump running.

The primary flow microswitch measures OK on a meter, I even ran it for a while with a jumper across its terminals, and the heating still stopped

Prior to the above:-
I replaced the fan, as it was noisy and wasn't always generating enough suck to operate the APS
I then couldn't relight the pilot as the fan didn't go to high speed when the gas valve was depressed, so I replaced the IGN PCB.

I have checked as many other things as I can think of, any comments appreciated

 

[gas4you]

Try testing the ch thermistor resistance cold, although it could be breaking down intermittently from the sound of it.

Does it fail to ramp up to full speed at any particualr time, eg when the boiler is hot?

 

[ianshearer]

Dave, I guess it fails when the boiler is hot, as it is likely to start up ok when switched off and on when, found to be cold and not working

The sensor measured 14k at a (chilly) room temperature, warming it up in situ with a cigarette lighter, it goes to 13k and then rises slowly to 14 as it cools. Manual says it is nominally 11k at room temp.
can they break down intermitently ?

 

[gas4you]

13K sounds a bit high to me, especially as the book says 11K.

When you heat it up the resistance should drop quite significantly.

Yes they can break down intermittently. They are only around £10 each, so if I am ever in doubt I personally replace it/them.

Unfortunately I'm not one of the 'electronics' wizzos to know ntc's precisely as some lead you to believe they are

 

[Gas2Air]

Maybe it's time you had someone qualified and experienced to deal with the fault?

 

[ianshearer]

Time to get someone qualified and experienced?
Gas2Air, Yes it is, but they are hard to find, and unfortunately the qualified and experienced person I thankfully came across and normally get is, quite rightly, on holiday, and my lodger is freezing, so I feel I should be doing something.

I am qualified and experienced for electronics, and I know enough to know not to mess with the gas side (ok, the fan is to do with combustion, thin ground there )
I'm slow and carefull, and so far the boiler is running beautifully quietly with the new fan, the failure to light the pilot light fault is corrected with the new ignition board, and I am going with Dave's qualified suggestion that the ch thermistor is at fault.

The boiler will be getting an annual check shortly by the corgi registered engineer I know
Thanks to all the people who have helped me with this and previous enquires

 

[ianshearer]

As Dave said, the ch thermistor was a bit high resistance compaired to the new one I got and fitted today, time will tell if it stays fixed, fired up ok, but sometimes it would.
Cheers for that Dave

 

[ianshearer]

After an experienced professional had a look at this and did not find an obvious fault, I paid for an Alpha fixed price repair, very fast response, he replaced the main PCB, which was pretty much the only thing not either replaced or well checked out. ~It was expensive, but no more than some places charge for the PCB, so a result.