Potterton Suprima 40 problem - boiler won't stay running

[nmg196]

Hi,

I had the fairly common problem that my Potterton Suprima would "lock out" and flash it's red LED once per second. Going on the advice given by several in this forum, I replaced the PCB and wiring with the "upgrade kit" 5111603 PCB which is supposedly more reliable.

However immediately, I now have a different, but similar problem. The boiler will light up fine and start heating up the radiators as normal, after the water is hot, it will stop firing (flashing green) a few times while the house starts to heat up. However, after an hour or so, it will change and will flash the RED LED about once every 3-4 seconds or so.

According to the fault-finding guide supplied with the new PCB, a RED LED flashing every 5 (I make it more like 3) seconds means "check thermostat". The thermostat seems to be OK. It has a resistance of about 500kohms when warm and drops if I hold it between my fingers, so I refitted it with thermal paste. Same results at PCB end so wiring OK.

Does anybody have any ideas what might be causing this problem? Is it likely that my brand new PCB is faulty?! Or perhaps could the thermistor/thermostat still be faulty at high temp.

I don't get what causes the boiler to flash the LED at this rate (technically speaking). Is it that the resistor goes outside the normal range, or could something else be causing it?

I'm reluctant to call in Potterton who will charge me £225 and may not even fix the problem.

Nick...

 

[gazthepottertonengineer]

sounds like your boiler is overheating to me 3 questions for you ?

1.is the pump wired to the boiler

2.have you got 3 port valve or 2 x 2 port ?

3.have you got bypass on system

 

[nmg196]

There's a single live wire that goes to the pump. For some reason, there is no neutral wire connected. But the pump does switch on when needed (although it has always made a slightly funny noise.

I think I've got a single 3 port valve.

What's a bypass??

I checked the overheat thermostat seemed to be pretty much 0 ohms when I tested it while the boiler wasn't working, so to me, I would say that it's not overheating. Unless that is, the overheat thermostat is normally closed? Actually it can't be, because if I disconnect the overheat thermostat and start up the boiler, it never tries to light and immediately flashes the LED once per second which is different to what it's going now...

 

[nmg196]

I have noticed that the even if you switch the boiler off for quite a while, when you turn it on again, it still won't light and immediately starts flashing the red LED every 3 seconds or so. But at that point, the thermistor and overheat 'stat seem to be OK resistance wise, so what on earth else could cause it to not want to fire up if it has a call for heat?

 

[ollski]

can I just try to strip out the technical stuff?, are you saying that you're on a y plan valve and that the heating comes on with the hot water but won't work when the hot water is hot or independantly?

Pump neutral will be wired from the pump to the junction box I expect.

 

[nmg196]

The boiler won't come on full stop once the light has started flashing. It's not a hot water problem specifically.

 

[ChrisR]

I don't have one very handy to check but 500k sounds too high for a temp sensing thermistor. I believe the boiler decides to stop if the value is out of range.
Check the MI for a value, or wait until I or someone else can be ar5ed to go out to the van...

 

[nmg196]

Well it drops if I warm it up a bit, but it's definitely ~300k or more. I've been trying to find a replacement as they're only a few quid, but i can't seem to find anybody that sells them! (not if I search by potterton part number). Any ideas where I could get a replacement thermistor from?

Is there anything else that might be causing this problem such as a faulty pump or valve?

 

[nmg196]

Update: I noticed that while the boiler is "locked out" and the red LED is flashing once every few seconds, if I remove the thermistor from the pipe *without* disconnecting the wires to it and blow on it, the boiler *immediately* fires back up. So I'm now fairly certain that the thermistor must be faulty (perhaps at high temp only). If i leave it to it's own devices, the pipes have to drop down to pretty much room temperature before the boiler will come back on again (by which time the house is getting really cold).

So it looks like the thermistor is faulty and perhaps my PCB upgrade wasn't even necessary?! Oh well.

So the last remaining question is, where can I get a thermistor from ASAP? Would any local heating supplies companies happen to carry them, or will I have to order it directly from potterton (expensive?).

Thanks!

 

[damorad]

My experience?
I suffered daily resets (flashing red light), no hot water or heating. The odd thing is that sometimes it worked and sometimes it would not.
I removed the motherboard and re-soldered all the main joints, especially the sockets that the off board sensors connect to (I could see hairline cracks around the base of the pins). I'm convinced now that it was thermal expansion on these joints that caused the failures.

That was weeks ago, its been running sweet ever since....

£15 for that soldering iron was a sound investment

 

[nmg196]

Well I took the advice of the people in this and other threads and bought a new PCB off eBay for £95 (new type). Spent an hour or two fitting it, and it didn't fix the problem at all. In fact it was worse because the reset button would no longer fix the problem. Instead the led was flashing red every 5 seconds.

After quite a lot of faffing, I realised that if I removed the thermistor and blew on it, the heating would immediately come back on. So I've ordered a replacement thermistor.

However, before this even arrived, something else happened. At 6:30am, the mains RCD tripped and took out all the power. At 8, I flicked it back on did my little trick of blowing on the thermistor to start the heating for a bit. However, after about 2 minutes the pump made a very horrible grinding noise and stopped. It seems a ridiciously bad coincidence but it appears that the pump has now gone. I've checked that it has 240V and it's still not spinning (even when prompted to by using the screw mechanism designed to unstick it). I've also just noticed there is a small amount of water dripping from the 3 way valve!

What a nightmare! By the time I've finished I'll have replaced the whole heating system! GRRR. So at the moment, I'm stuck with no heating, and I can't find any plumbers in the Southampton area that can come out anytime soon.

I'm also not sure whether to pay British Gas £132 (or more if more than one part needs changing, which I think it does) or get a local plumber to do it (if I can find one).

I don't suppose anybody knows of any good plumbers in Southampton?

 

[ollski]

I don't have one very handy to check but 500k sounds too high for a temp sensing thermistor. I believe the boiler decides to stop if the value is out of range.
Check the MI for a value, or wait until I or someone else can be ar5ed to go out to the van...

Its quite a range though chris 28kohms to 1Mohm between 25°C and 82°C

 

[sccduck]

Chris R

Before splashing out on a new PCB could you just run through what I need to test with my multimeter and the expected values?

I'm sure this would be useful for a lot of tight fisted people, like me, on this forum.

Cheers

Sitting Duck