Potterton Suprima only refires when cooled - not a lockout

[NotSoFast]

Hi,

I have a Potterson Suprima 60. It is 12 years old and has the original PCB. It was last serviced about 6 months ago and is attached to a Boilermate II. I had a couple of issues with the Boilermate (mixer valve failed, delay timer PCB fried) which I've fixed. The Boilermate is now working as designed which has highlighted a bit of a problem with the boiler.

The Boilermate makes a call for heat. The boiler responds until at some point it shuts down. On shutdown it will revert to displaying Ext. Call for Heat (Green LED flashing, Red LED off). After a time this changes to Boiler Temp Control Satisfied (Green LED flashing, Red LED on). Meanwhile the Boilermate continues to call for heat (checked with a voltmeter) as the Boilermate boiler pump circulates increasingly cooler water.

If left to its own devices the Boiler seems to re-fire when the water in the pipes drops to around 50c (yep, I've spent a lot of time hanging round the boiler waiting for things to happen ). If the Boiler is switched off/on at the mains it will fire immediately for some indeterminate period (often as little as 30 seconds, sometimes more than 5 minutes).

I've read lots of threads on here but can't find these exact symptoms. Its not a lockout (green LED off, red flashing manically) but is it more likely to be a faulty thermistor/thermostat or is the PCB on its way out?

 

[Agile]

I think that I know what the problem is but as its inside the combustion chamber then a gas engineer will be needed to access it.

Tony

 

[nickso]

agile thinks it's the thermistor.

 

[Agile]

Close, but the thermister does not need replacement.

 

[NotSoFast]

I think that I know what the problem is but as its inside the combustion chamber then a gas engineer will be needed to access it.

Tony

Care to enlighten me? I'm not planning on fixing it myself, I just want to be better informed when the engineer turns up in the morning esp. as he's the guy who serviced it (should he have spotted it during the service?), I've only just moved there and I've no idea how good he is.

 

[matt1e]

agile thinks it's the thermistor.

judgeing by the op's description of the problem

I think it's more than likely a problem related to the senser (thermistor) too! whether it's a faulty sensor or bad termal contact,wiring etc
followed by the temperature control or associated wiring

Close, but the thermister does not need replacement.

I wouldn't bet on that statement tony, read the ops discription again ie
it fires, heats then then shuts down and sits with a temp satisfied indication with
no attempt at re-ignition
no blocking
no lock out
then

If left to its own devices the Boiler seems to re-fire when the water in the pipes drops to around 50c

I know what you think it could be
but I'd be thinking, (it "thinks" its up to temp) firstly

Matt

 

[NotSoFast]

I think that I know what the problem is

I know what you think it could be

What a couple of teases...

Shall we play the Hotter / Colder guessing game (well it is a central heating forum).

Is it... um... oh... dunno... the Air Pressure switch?

 

[matt1e]

I think it's more than likely a problem related to the senser (thermistor) too! whether it's a faulty sensor or bad termal contact,wiring etc
followed by the temperature control or associated wiring

where was I teasing there?
(well maybe teasing agile )

Matt

 

[NotSoFast]

where was I teasing there?

By not telling me what you think agile thinks it is (I think )

Anyhoo, the engineer has been and gone. I am none the wiser and fifty quid poorer

He stuck his gauges on the pipes. The boiler fired and re-fired on demand at the correct(ish) pipe temperatures. Whatever I did, the darn thing wouldn't reproduce the problem for him (why does this always happen ).

He didn't think it was the thermistor - he didn't check it but he said, in his experience, they hardly ever fail and the pipe temps suggested it was working OK. He said it might be the PCB but as they have no diagnostics he can't check it and couldn't guarantee replacing it would solve the problem (fair enough, at least he wasn't simply trying to take my money). He said, he'd come back if it started happening 'more frequently' (it seems to have happened every time I want a bath - which is at least twice a week ) but there was no more he could do right now.

So, should he have done more / different checks? Should I have told him to replace the thermistor (I'm very reluctant to do this as I wouldn't want a rank amateur telling me how to do my job), should I find a different engineer (if so, how do I get a good one) or do I simply have to wait until I can reproduce the problem on demand (and buy more deodorant in the meantime)?

 

[Ray84]

thermistor or pcb not much else left on them to go wrong. id suspect the pcb but maybe the engineers looking at it from the view he doesnt want to look stupid charging so much changing a board for a recall next week but more than likely that i usually change the thermistor first and it makes no odds

Ray

 

[NotSoFast]

Unsurprisingly the problem returned pretty soon after the engineer left.

I've taken a look at the PCB and there's scorch marks around some of what I presume are resistors in the middle of the board. Guess I'm going to have to get it replaced. Would a direct replacement refurb be OK or do I need to get an engineer [1] in to fit the later model?


[1] not sure I want the bloke who didn't look / failed to spot this back.