Ravenheat rsf100et hot water cycling on / off

[wayneb7729]

Good people out there some advice please.

Ravenheat rsf100et been installed for approx 4 yrs - no problems. Suddenly dhw cycling on / off. [ch working just fine]
Dhw temp set midway.
Get manual out and follow instructions - switch to hot water only.
Take cover off and watch boiler flame whilst hot water tap running- it appears to fire up and the burner flame increases if the temp control is increased.
However it sounds like water is almost boiling and pipe very hot before boiler flame switches off - there is no overheat light illuminated.
Clearly water then runs cold for some considerable time before boiler once again fires up.
Read as many threads as possible and decided it was the dhw control thermistor.
Replace wet for wet - still have problem.
Now here's the thing. When the hot water is running if the ch thermistor is touched or unplugged the boiler switches off! Therefore does this also need replacing?
No wires are crossed etc
Some advice here would be very much appreciated
Thank You

 

[gas4you]

Do you live in a hard water area? could be the heat ex scaled up if you didn't have a scale inhibitor fitted at installation.

 

[wayneb7729]

Thanks for the prompt reply

Yes it is a hard water area - fernox [or similar] was put in originally.

 

[clf-gas]

both thermistors work together, from your post touching the CH thermistor seems you've fault found yourself, takes some engineers years to learn that, check the wires fof loose connection on the spades, if you've got access to a multi meter then check continuity, if wires ok then cange the thermistor if reading less than 15ohms when cold.

ps. the DHW thermistor should read same when cold, between 15 and 20 ohms.

 

[wayneb7729]

Marvelous - gives me something else to try tomorrow - thank you.

incidentally to the first replier is there something that can be put into the boiler re hard water as it is a particular problem where i live?

 

[Slugbabydotcom]

the DHW thermistor should read same when cold, between 15 and 20 ohms.

Should be between 500 ohms and 20k Ohms

As I recall if you take the thermistor/s out and stick them under your armpit for 10 mins [36.9ºC] you get a reading of about 7-8k

CH thermistor has got to be the most common fault on most Ravenheats. I check them first whenever there is a water temp. fluctuation problem.

 

[gas4you]

You would have to fit an external water scale inhibitor to the cold inlet supply on the boiler. This would only help prevent the DHW heat ex from scaling up. The best ones are the cartridge type that you replace annually.