RAVENHEAT CONDENSING COMBI H/W PROBLEM

[cee em dee]

My radiators are heating but water sometimes heats sometimes doesnt. There also seems too long a time gap when the boiler fires up when heating the radiators....HELP !

 

[Slugbabydotcom]

Eliminate the CH thermistor before you do anything else. They only cost about a tenner and if it isn't the problem now it will be sooner or later.
These thermistors go all the time and are more or less considered a service item by many. Problem occurs more when a system isn't flushed properly.
Then assuming you have a decent flow rate of water its time to start checking out your diverter valve if that doesnt help.
How old is the boiler? If its less than a year you could get ravenheat to sort it under the guarantee.

 

[cee em dee]

Thanks for your reply. The warranty has just expired so i plan to change the Thermistor myself. Will i just have to shut off the stop tap then drain the lowest hot water tap and drain the water on the DHW inlet ?

 

[Slugbabydotcom]

Will i just have to shut off the stop tap then drain the lowest hot water tap and drain the water on the DHW inlet ?

No....The CH thermistor is on the CH side not the HW side. I know that your problem is with the HW so I see how you had that idea, but that is just a symptom of this rather common fault.
Isolate the flow and return under the boiler.There is a drain valve near the pressure relief valve to let the excess water out. Use the PRV itself to release water at your own risk. [They sometimes dont shut off properly afterwards] Make sure the boiler is cold and electrically isolated. The thermistor is the green sensor on the right. Remove the electrical connector. A mains tester screwdriver can be used to lever back the plastic catch [ which always breaks off anyway!] I just happen to have a 13mm box spanner which is perfect for undoing and replacing the sensor.
It's all in the manual which you can get online from the Ravenheat site

 

[cee em dee]

The boiler does fire up when i turn on the hot water but then cuts out again when the water is starting to heat up then obviously goes cold again, then fires up and repeats the forementioned. Could the problem not be anything else, say, the potentiometer ? Thanks for all your info so far Slugbaby.

 

[Slugbabydotcom]

Could the problem not be anything else, say, the potentiometer ?

Thats a possibility. I have heard of them going but havent as yet had the pleasure of changing one. I did come across one with a blocked diverter valve and another where the DHW thermistor had gone [very rare]
I know the CH thermistor seems totally illogical when you read the fault finding. When you said

There also seems too long a time gap when the boiler fires up when heating the radiators

That was the clue that implied that the problem is on the CH side.
I just change it on sight. They cost about a tenner and it takes more than a tenners worth of my time to mess about although I sometimes swap the CH and DHW thermistor leads over for testing purposes only.
If you want to play about then I will tell you that the CH and DHW thermistors are both the same part as are the CH and DHW potentiometers so you could try swapping them about to prove or disprove what I strongly suspect is the cause.