Boiler clicking on & off

[Graham81]

Hi All

WB240RSF in which I have replaced the diverter & DHW heat exchange due to the heat slipping to the central heating circuit when demand for hot water. However during hot water demand the boiler will continually light then go out in a cycle. The water will still get hot but the boiler keeps cyclling in this pattern. Now I have watched the plunger that activates the micro switches & that is staying out all the time so it is not that causing it. Does anyone have any other ideas as to what would cause such a thing?

Thanks

 

[SortedPlumbing]

dhw sensor?

 

[Graham81]

Thanks for the swift reply. What sensor is the DHW one? Is it the one above the pump or the one at the rear of the boiler? Or maybe even another one haha

 

[SortedPlumbing]

not sure , never worked on one of these.......should tell you in instructions!

Seen similar fault on other makes
not alot of things it can be.

 

[Graham81]

I've just rotted out the manual and found the sensor. I metered it out & all appears to be fine. Small resistance when no demand and then then no resitance when hot water demanded. That is correct yes? I checked the central heating sensor and that behaves in the same manner.

 

[Agile]

Can you tell us the measured resistance when cold?

Tony

 

[Graham81]

Like an idiot I didn't note down the values but I will check again in the morning. On a side point could micro switches that got wet but were then dried out cause such a problem???

 

[Agile]

Any electrical component that gets wet needs to be throughly dried out. On a microswitch that usually means opening them and using forced hot air.

Then they need testing for both a low ON resistance ( < 0.3 ohms excluding test leads ) AS WELL AS a high insulation resistance, normally over 2 Meg.

Tony

 

[Graham81]

Sorry for the delayed response. I measured the resistance of the hot water sensor with no demand/heat & I get a reading of approx. 1.7kohms; is this correct?

 

[Graham81]

PS I guess the easiest way to see if it is the micro switches playing up would be to purely short the cables manually and see if the boiler still clicks on/off?

 

[clf-gas]

PS I guess the easiest way to see if it is the micro switches playing up would be to purely short the cables manually and see if the boiler still clicks on/off?

Dont go shorting nothing.

From the molex plug at X2, the resisstance on the red wires should read 1.5 / 25 kohms.
This is not always the case as some meter out sa good but infact are chuffed.

Firstly try replacing DHW sensor and if this does not cure problem then it is definately the driver board.

 

[Graham81]

Right all in all I have now replaced the following & yet the boiler still continues to cycle on & off with HW demand.

-Water to water exchange
-Diverter Value & diaphragm
-Diverter micro switches
-DHW sensor

The cycling is not occurring at set intervals but is between a couple of seconds & say ten seconds in no apparent pattern; but the boiler always immediately relights??

Anymore ideas other than maybe driver board.

Thanks

 

[Agile]

I would have checked the combustion aspects at a very early stage.

Unfortunately, I do not like giving advice about the air pressure switch to anyone who is not CORGI registered.

Tony

 

[Graham81]

Thats fair comment. Are there any electrical tests I can carry out that would let me know what kind of control the boiler is sensing that is making it feel necessary to cycle like it is?

Thanks

 

[Agile]

You could measure the voltage across the NO contacts on the APS.

Unlikely to be a problem on your model, but you could measure the NO and NC contact resistance when closed to ensure its less than 0.4 ohms.

Tony