On Monday, 4 March 2019 22:01:32 UTC, Bob Minchin wrote:
On 04/03/2019 21:27, tabbypurr wrote:
On Monday, 4 March 2019 21:21:02 UTC, Bob Minchin 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?

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