Potterton Suprima 50L - Thermistor resistance values needed

[HootingOwl]

Hi All - Recently replaced the PCB on a Suprima 50L (to cure the lockout problem) and all was well for a week. Called back after a week as the boiler was not coming up to temperature. Boiler is firing up, getting warm and then stopping, green light flashing.

Boiler thinks that flow temp is satisfied when the boiler is only at 40 degC. So I replaced the thermistor and got the same result. Thermistor is reading about 1.1Meg Ohm at roomtemp which drops to about 500K at 40(ish) degC.

Muppet at Potterton tech support said thermistor should read 1.3 meg at 20 degC, 760K at 80 degC and 500K at 40 degC. He got very obnoxious when I asked him to check the values he had quoted me - there is no way that the NTC can be 760K at 80 deg AND be 500K at 40 deg if it starts at 1.3Meg when cold. Boiler fires fine when i take the thermistor out of the pocket but burner cuts out as soon as the themistor gets warm.

Unlikely that the new thermistor is duff. Can anyone out there tell me what the thermistor values should be at 40 deg and at 80 deg? Potterton are no help here. Perhaps the PCB is duff (it was the latest type) - hope not. Client is having cold showers.
Many thanks
pete
(corgi reg. and waiting to enter the combustion chamber...)

 

[nickso]

the new pcb's do have a fault where they think the temperature is greater than it actually is.

its rare though. out of the several hundred suprima pcb's ive changed ive only seen this fault twice.

 

[mmzz]

I have got the exact same problem. Now on my second thermistor but problem still remains. I suspect the new pcb may be reading the wrong temperature values. Did you resolve the problem?

 

[HootingOwl]

yes - problem WAS the NEW PCB. There have been a couple of revisions to the new PCB. Beware PCB kits sold on EBay as some of these are the older type new ones that are duff.

I moaned to Baxi/Pot and they kindly sent me a new PCB (the plastic box part of the kit). Swapped it over and it ran perfectly well.

So as nickso said, the new PCBs have a problem too. Well, wouldn't be Baxi if it didn't, would it?

Worth a moan to Baxi as they must know about it.

 

[johnsmith0774]

hi guys, my potterton 50l is doing exactly the same thing as you are talking about here. turning on for 1 minute and switching off for 3 or 4 before firing again for a minute.

tried taking the thermistor out of pipe but keeping it connected and this didnt't work.

i bought a pcb 511603 on ebay, secong hand, and fitted it this morning. but the problem is still there.

any ideas? im not really keen on paying £300 for new pcb but i will have to if thats the case.

if i do have to can you tell me the latest part number? thanks

 

[Agile]

Muppet at Potterton tech support said thermistor should read 1.3 meg at 20 degC, 760K at 80 degC and 500K at 40 degC. He got very obnoxious when I asked him to check the values he had quoted me - there is no way that the NTC can be 760K at 80 deg AND be 500K at 40 deg if it starts at 1.3Meg when cold.

I saw an advert in the local paper for technical advice clerks, "no experience needed" !

Tony

 

[Boilerman2]

£300 should buy you TWO BOARDS!!

 

[HootingOwl]

the board kits sell on fleabay for about seventy quid. at that price they must be red hot, fresh from the employer's van.

but if you have replaced the pcb already i would be looking elsewhere for the problem. blocked heat exchanger, not enough flow, pump?

what is the actual flow temp from the boiler?

part no for the kit is 511-1603

 

[HootingOwl]

oh as for the resistance values, the 500K at 40 degrees is probably right as that is what i measured and baxi muppet said the same. i also think that 1.3 meg at 20 degrees is also feasible.
i think the value at 80 degrees must be 760 OHMS and NOT 'K' Ohms.

 

[tonybhoy]

hi guys, my potterton 50l is doing exactly the same thing as you are talking about here. turning on for 1 minute and switching off for 3 or 4 before firing again for a minute.

Could well be POCs recirculating in the boiler combustion chamber and causing vitiation, poss cause could be down to a faulty or badly fitted flue.

Had this recently on a FF boiler that recently had it's fan replaced but the air duct wasn't re-fitted properly causing the boiler to do as you describe above.

 

[nickso]

hi guys, my potterton 50l is doing exactly the same thing as you are talking about here. turning on for 1 minute and switching off for 3 or 4 before firing again for a minute.

Could well be POCs recirculating in the boiler combustion chamber and causing vitiation, poss cause could be down to a faulty or badly fitted flue.

Had this recently on a FF boiler that recently had it's fan replaced but the air duct wasn't re-fitted properly causing the boiler to do as you describe above.

was it a suprima? in my experience the only things that stop a suprima working are early PCB's and it being switched off.

 

[tonybhoy]

was it a suprima? .

No, an Ideal classic 240 FF, I think.

 

[Agile]

oh as for the resistance values, the 500K at 40 degrees is probably right as that is what i measured and baxi muppet said the same. i also think that 1.3 meg at 20 degrees is also feasible.
i think the value at 80 degrees must be 760 OHMS and NOT 'K' Ohms.

No it would not be that low, I think that 76k sounds most likely as they roughly halve their resistance every 20°C.

Taking the 1.3 M and the 500k for a 20° rise gives a factor of 2.6 times reduction. Applying that twice again for 60° and 80° gives 74k !

Tony

 

[johnsmith0774]

fixed it! it was the pump, obviously not sending the water round quickly enough and the boiler thought it had reached the right temperature.

it cycles at about every 5 minutes now on hot water. i think this may be acceptable given the short route of the pipework from boiler to tank and back. am i right or is 5 minutes too short????

on central hearing mode it it operates for....well i got bored waiting after 10 minutes so at least 10 minutes!

no more 60 second on and 5 minutes off!