Biasi Garda HE lukewarm water and Heating

[Steve412L]

Hi, I have a Biasi Garda HE M96.24 S/M Combi Boiler...about 10 years old. The burner fires up but only stays lit for between 5 and 15 seconds (ish). It the same whether it on just DHW, just CH, or with CH on and asking for DHW. The burner fires up...after a few seconds switches off...but no red light for about 30 seconds, then the red warning light (not the lockout lamp) comes on for about 5 seconds, and the burner fires up again. Warning light goes out, and the cycle repeats. The upshot is that it's off more than it's on, resulting in lukewarm rads and water that cycles from reasonably hot to cold....and a bathfull of lukewarm water!
Some people say it's the Plate HE, but I don't see how that would affect the heating? If it was the diverter valve surely one or the other would get hot? If it's the DHW temp sensor/thermistor...again why is the heating affected?
Any ideas would be much appreciated
Thanks in advance.

 

[CBW]

Have you checked the condensate pipe?

 

[Steve412L]

not yet but I will thanks....for blockage or frozen?...or either?

 

[CBW]

Either, probably more blockage, especially when connected to another source, eg a waste pipe.

 

[Steve412L]

Yep....checked the condensate pipe....not blocked or frozen