Radiant RSF 24 Pilota

[BobCroft1]

I would appreciate some help on this problem.
In central heating mode the boiler works very well and with the temperature dial set at Max. the display lights go up to 80.
In hot water mode the boiler comes on and runs for a minute or two until the display lights get 60 and then the burner cuts off. At this point the '50' light flashes and the water from the hot tap runs cooler. Looking at the MI the flashing light indicates a heating sensor failure.
I checked the sensor resistance and it read 3.7Kohm @ 60°C. The MI states 10Kohms @ 25°C. (I also checked the DHW sensor which I believe is the same and got a reading similar to the MI value)
I tried turning the power on with the sensors disconnected and the '50' light flashed with both the CH demand and DHW demand off.

I would conclude the heating sensor is faulty but I would greatly value any input from others who may have experienced this problem

 

[Agile]

It sounds as if one of your sensors is going open circuit when it gets hot.

You can test for that by replacing one of then with a 6.7 K fixed resistor and seeing if the boiler then works normally. measur the resistance of the disconnected sensor as it gets hot.

They are cheap so its best to replace both while you have the cover off.

Tony

 

[BobCroft1]

Tony,
I appreciate the prompt response and I will try what you suggest. As a matter of interest how do you get the value of 6.7K is there some data somewhere on the value of the thermistor versus the temperature? I am familiar with how thermistors work but the Mi I have only gives one spot value. Good tip about changing both.

 

[Agile]

Its a more complicated formula for the resistance.

Simply its about 13k at 20° and about 2k at about 80°.

I chose 6.7k because its the preferred value so easily obtainable.

It will run the boiler on the basis of a flow temp of about 50° so it will be modulating back.

These are all just typical values and the exact values are not important for a simple test in your case.

Tony