Vaillant thermistor fault

Resistances don't sound that far out. Normal criteria are not having a resistance too low such that a few volts used in sensing does not produce much self heating and yet the resistance is not so high to need high impedance circuitry that can suffer from damp, dirt electrical noise etc

The end result tends to be a working resistance around 10k

and being almost the same value... but maybe not quite close enough for the boiler's liking. Or maybe an iffy connection on one. Oh, 12k was at about 30C primary temp.

NT

Well you could piss about with variable resistors to try and con it into firing to work out the range of resistance but .... Bosch thermistors are only a few quid but i dont know the cost of vaillant ones. looks like £19 on ebay?

An ideal job for the resistance decade box that I "liberated" during a laboratory clear-out some decades ago. I think I have only used it once since then, but I have always hung on to it just in case. Perhaps I will park it on top of my Vaillant (as insurance against never having a thermistor failure again).

I'm a bit puzzled. I have what purports to be the spares manual, some sections of it match the boiler some just don't. Eg the HX is round, but the manual shows it as a rectangle. Still, one picture that does match gves the same part no as one that doesn't, so I guess that's good enough.

Off to ebay to type that number in tomorrow. I just hope the error really is with the thermistor. Thanks folks.

NT

The question I'd be asking is: Were these two faults related to one another. IE can you think of a reason for both occurring at the same time? If not then maybe you have not found the root cause if it yet. Brian