Temperature sensor

[joolz44]

Hi everyone,

Does anyone know how long a temperature sensor should last in an Ariston A/23 MFFI?

I installed a new a year ago because the heating kept turning itself on, now I'm having the same problem.

Many thanks

Frank

 

[plumbplumb]

If it's a wet pocket sensor it would depend on the quality of the water in the system

 

[joolz44]

I'm not sure what a wet pocket sensor is. it was installed by an expert. I have read here that there might be sludge in the water. Is it worth replacing water or adding a cleaner to it?

 

[Gasguru]

Wet pocket sensors are screwed into the pipework and come into direct contact with the water. ie. you need to drop the boiler pressure etc to replace them.

Dry pocket sensors can be replaced without having to worry about the water side.

Water quality rarely causes a thermistor to fail. Sludged water may create an insulating film on the sensor, but that would just effect the
response time and possibly cause other issues. The majority of thermistor failures result in the sensor going open circuit either permanently or intermittently. It's very common to find the sensor reads ok at cooler temperatures but once a particular temperature is reached goes open circuit or the resistance fluctuates.

On the EuroCombi boilers I've come across, if the flow thermistor goes open circuit it will kick into frost mode and the heating will come on.

I once had a customers boiler that would occasionally run the heating after the washing up had been done. As the kitchen tap was turned down to reduce the flowrate to rinse the the dishes, the boiler temperature will soar. If they then turned off the tap every so often the flow temperature would be sufficiently high to cause the flow thermistor to fail going open circuit. The boiler would then assume it's very cold and run the heating. It's a dumb frost mode on these and the radiator temperature will get quite high before the boiler drops out of frost mode.

 

[joolz44]

Thanks very much for that. I'm not sure if it's wet of dry pocket sensor - hopefully it's dry and I don't remember the gas man turning the water off to replace the sensor before. In which case, I think I will replace the sensor and see what happens. I was just a little disappointed that the sensor that was replaced last year has not lasted longer. And hopefully it is that and not a bigger job.

many thanks

 

[Gasguru]

It's wet, bleed all the radiators and drop the pressure with the boiler drain-off on the right.

 

[joolz44]

I have bled the rads. No excess air from them. Iz the boiler drain off the round tap on the down pipe that alters the bar pressure? If so, I turned it so that the needle got out of the blue section. I did all this several days ago, but the problem still exists. Have i done it right?

 

[Gasguru]

Bleed the radiators, drop the pressure using the boiler drain-off valve on the right hand side manifold (inside the boiler). Check the vessel..see FAQs. Then replace the flow thermistor.

 

[joolz44]

Thank you for your advice. That actually sounds like it's beyond my experience. I think I'm going to have to call in an expert. Thank you.