For my sins, I have a 24ci which was installed in the cellar below the level of all the rads so it gets all the debris 
The engineer (friend of a friend) came because the expansion vessel had gone, which he reckons was due to overfilling after the pressure sensor got clogged. Anyway, he fitted an external expansion vessel for me and cleaned out the sensor. There was a fair bit of black grit. It fired up but as soon as you draw any amount of hot water, it throws an F19 (NTC not connected). If CH is off, the hot water works for a while then goes to warm (discovered this after he left). It seems if you turn of the water then back on it starts to warm up again. If the CH is on, you get an F19 within a few secs with the HW tap full on. He said that points to some kind of thermistor fault or the pcb.
He didn't have the right thermistor on the van so tried one from, I think, a Vaillant. I have no idea if this is still 10kOhm like the original part; anyway, same problem. To save the cost of another call out, he suggested I fit a new thermistor 1st as it's cheaper and if that doesn't work the next thing to try is the more expensive pcb. If both thermistors give the same fault, can you think of any reason a new one would be any better? Is there anything else to try? Given that there are new pcbs on eBay for around £65, is it worth going straight for that or even buy another 24ci for spares (a bit of a lottery, I appreciate)?
He advises fitting a Magnaclean to mop up the crud which is causing havoc with the system. Makes sense to me.
I'd be grateful for qualified opinion.
The engineer (friend of a friend) came because the expansion vessel had gone, which he reckons was due to overfilling after the pressure sensor got clogged. Anyway, he fitted an external expansion vessel for me and cleaned out the sensor. There was a fair bit of black grit. It fired up but as soon as you draw any amount of hot water, it throws an F19 (NTC not connected). If CH is off, the hot water works for a while then goes to warm (discovered this after he left). It seems if you turn of the water then back on it starts to warm up again. If the CH is on, you get an F19 within a few secs with the HW tap full on. He said that points to some kind of thermistor fault or the pcb. He didn't have the right thermistor on the van so tried one from, I think, a Vaillant. I have no idea if this is still 10kOhm like the original part; anyway, same problem. To save the cost of another call out, he suggested I fit a new thermistor 1st as it's cheaper and if that doesn't work the next thing to try is the more expensive pcb. If both thermistors give the same fault, can you think of any reason a new one would be any better? Is there anything else to try? Given that there are new pcbs on eBay for around £65, is it worth going straight for that or even buy another 24ci for spares (a bit of a lottery, I appreciate)?
He advises fitting a Magnaclean to mop up the crud which is causing havoc with the system. Makes sense to me.
I'd be grateful for qualified opinion.
