baxi barcelona, thermistor or control pcb?

[richiereynolds]

Hi, looking for some advice on my barcelona. Christmas before last I had to replace the flow thermistor and ignition pcb, one to get it to stay lit and the other to get it to spark.

Last Christmas I again had to replace the thermistor to get it to stay on until the water was beyond lukewarm.

About 2 weeks ago it just stopped coming on, no red lights or lockouts, just a little click when it was supposed to come on but no fan or attempt at lighting.

I put my multi meter across the thermistor set at 20K ohms and in one direction it read open circuit, in the other in read -4.95. Anyone know what that means? It's supposed to be between 0.5 and 20 K Ohms.

After a while though, I discovered that putting the multimeter across the thermistor caused the boiler to start! Even just connecting the wires that attach to the thermistor directly across the multimeter cause it to read -4.95 and the boiler to start.

Anyway, I replaced the thermistor yet again and it worked fine for 2 weeks but now it's back to the same problem, is the control pcb destroying the thermistors? Is the thermistor the real problem or a red herring?

Any help greatly appreciated.

 

[ollski]

Are you reading it with the wires connected?....if you are you will be testing the resistance of the whole circuit. If the thermistor is open circuit it is knackered, but when you put your meter across it you make a closed circuit and the boiler will fire.

 

[richiereynolds]

Thanks for the advice, you're right, I had the wires connected. Without the wires on it's reading open circuit on a cold pipe, I take it the point of this flow thermistor is to measure the temp on the flow?

Thermistor 1 lasted 4 months after I moved into the house, might have been there for up to 3 years before that.
Thermistor 2 lasted a year.
Thermistor 3 lasted 2 weeks!

Any idea what could be causing this?

Cheers, Richard.