I have a Ferroli Optima 701 and it seems to have a problem with intermittent DHW. The British Gas engineer has been out and has changed a couple of sensors, (CH temp and 88oC limit sensor) but the fault still persists. He has also cleaned out the venturi to no avail. Initially the loss of DHW was only apparent when the CH was on. BG suggested a build up of scale in the heat exchanger which was causing it to retain heat and trip the limit stat @ 88oC. However, recently, the DHW is now running hot then cold in cycles even though the heating is off and the temp gauge reads a low reading.
When I switch the hot tap on, I can see the boiler firing up then after approx 60 seconds, the boiler flame extinguishes and the water runs cold. After approx 20seconds, the boiler re-ignites and the water runs hot. This continues in cycles and is more apparent when the boiler is warm (well warmer than cold).
Does anyone have any ideas? Is it worth getting the heat exchanger de-scaled (not covered by BG) or could the fault be with something else?
The boiler is 17 years old and has served me well, I'll be sorry to see the old girl go if a replacement is in order.