On 04/03/2019 22:01, Bob Minchin wrote:
On 04/03/2019 21:27, wrote:
On Monday, 4 March 2019 21:21:02 UTC, Bob MinchinÂ* wrote:
On 04/03/2019 21:13, tabbypurr wrote:

Vaillant ecotec plus 418 from about '07 or '10 or thereabouts. Pump
stopped running, boiler gave error codes
F22 dry fire
F72 NTC thermistor value out of range
Pump sorted, but boiler won't run reporting F72.
Thermistors measured at 12k & 12-13k - I can measure them more
accurately if needed.
Anything I should know or check before getting replacement thermistors?


NT

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 **** 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).