Baxi Combi Duo tec Problem I just can't solve-Any ideas?

[Derek Ball]

My boiler is displaying no faults., I have no problems with the hot water. It is the central heating circuit which is driving me mad. The boiler fires up very quickly for about 30 seconds to about 85degrees and then cuts out. The pump continues to run and the temp slowly falls to about 30 degrees and then it fires up again for another 30 seconds and I watch the temperature rise very quickly to about 85 and then it cuts out again-the pump never stops running. We have had every radiator and every pipe drained and flushed, And presently have a litre of 'descaler' circulating within the system.
We have had a new 'Diverter Valve' fitted. Just what can be causing the rapid 'on/off' cycling of the boiler? Any ideas please?

 

[Wayners]

I don't know the answer, so can't say if you have fault. Just chipping in.
The room thermostat is controlling boiler.
Our work boiler ( massive) shows 75c but the radiators can take several hours to heat up, so the boiler temperature is not the same as the radiator temperature.
It's also possible that the boiler keeps circulating water but the wall thermostat is happy the house is warm, so the boiler won't fire up. Eventually the house starts to cool and the boiler fires up but the water has cooled to 30c.

Turn the thermostat all the way up and see what happens

 

[CountryFan]

Not sure why it’s posted in your projects, but my guess would be an isolation valve, return filter or stuck diverter.