sime boiler problem

[frank777]

hello hope somebody can help, I have a sime friendly boiler, and have a problem with dhw, water pressure at all taps good, but boiler fires up on hot water demand, tap water only warm, then boiler shuts off and then fires up again after water has gone cold,, this cycle continues, this happens whether bolier is on dhw or ch mode, ch is okay. if I turn down the restrictor on the divertor valve to slow the rate down slightly,the boiler shuts off, the expansion vessel has been recharged to 1 bar
I am a retired ex gas service engineer but finished before the main influx of combis, however I have some knowledge of them, but not that great.
I fitted a new divertor valve 2 years a go, I am suspecting this again, but 2 years seems short time to go again, just wondered if any of the kind people on this site could give me their opinion
regards frank

 

[cross thread]

sounds like primary flow thermistor. or as one gas man to another bin the "friendly" boiler and fit a new modern condernser (vaillant) and get decent hot water performance