Combi boiler getting too hot (so then cycling)

19 Jan 2007
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United Kingdom
Hi all,

I'm posting a new thread about this as I've tried quite a few things recently, and the old one was getting a touch cluttered.

Anyway, I've got a Ravenheat RSF84ET combi boiler which is getting too hot and so cutting out (after 20 seconds or so, it cools, so re-fires, and the cycle starts again). This happens with the HW and CH. :(

The boiler uses a single heat exchanger for the CH and HW. In HW mode, the diverter valve short-circuits the CH loop, so a small amount of water is heated and pumped round and round the heat exchanger, adjacent to the HW pipes. In CH mode, the CH loop operates through the rads, as you would expect.

The short CH loop (inside the boiler) has a CH thermistor and overheat stat. If the loop gets above 85 degrees, the CH thermistor tells the PCB to stop. If it goes closer to 100 degrees, the overheat stat just kills the 240V.

From my measurements with a spare thermistor, the CH loop gets above 85 degrees so it shuts off the flame. But I don't know why the loop gets so hot! The manual points to the thermistors and potentiometers, which have all been replaced.

Possible thoughts are:

a) the pump is not working properly

b) the heat exchanger is scaled on the CH side (but I am in a nice soft water area, and draining the system produced lovely clear water, not black sludge!)

Both of these could theoretically stop the hot water getting away from the boiler fast enough (although all 7 rads get nice and hot!).

How can I prove either of these thoughts? Any others?

Thanks for reading this far! :)

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