Ravenheat CSI 85T hot water problem

[ed102]

Hi guys

I have a Ravenheat combi boiler (CSI 85T) thats about 3 years old, the central heating works fine but the hot water in intermittent. There is a lot of info on here suggesting it could be the diverter valve, but if someone could confirm or suggest other likely causes after I have explained the problem I would be grateful.

The boiler will fire up and give hot water for about 2 mins regardless of whether CH is on or not. The boiler will then go out resulting in cold water, it will then sometimes relight again but then go out again. When the hot water stops the plunger that activates the pressure switch on the diverter valve stays out, if it was a faulty diverter valve would the plunger not go in switching the boiler of? This suggests to me that the problem lies elsewhere.

Thanks

Ed

 

[gas4you]

You sound correct in your thoughts about the DV.

I would replace the ch thermistor first. Cheap and easy to do.

 

[Agile]

As Dave says it may well be a thermistor, but whilst he says change the CH one, I would expect it to be the DHW thermistor at fault as you say your CH runs fine.

You can check this guess by temporarily replacing the thermistor with a 15k fixed resistor and then measuring the resistance of the boiler thermistor during hot water delivery and seeing what the hot water does.

The likelyhood is that the thermistor is going open circuit. These are very simple components with the thermistor wires welded onto the pins and sometimes this weld comes apart with the thermal cycling and the connection becomes intermittent.

Tony

 

[ed102]

Thank for the quick responses, much appreciated. One other thing I forgot to mention is that occasionally the central heating will come on all on its own even though it is off on the timer and thermostat, the only way to turn it off is by the main switch on the boiler. Does this re-enforce the thermistor problem or just confuse matters! I will change the thermistor tomorrow anyway and see what happens.

Ed

 

[Agile]

If the thermistor becomes rather high resistance the boiler assumes its getting cold and thrns on the boiler for a few minutes to nwarm it up.

One expects this is done with the CH thermistor but it could be done with either or both.

Tony

 

[ed102]

Thanks for all the advice, I have replaced the HW thermistor and it seems to have cured the problem.

Ed