Intermittent fault; Systemate 2000 won't get out of bed in the morning

This is an intermittent fault. First thing in the morning there is no hot water (the one that delivers hot water instantly from the small reservoir before the heat exchanger kicks in to deliver the hot water it obtains from the boiler) and no matter how long you leave the hot tap running the boiler will still not get fired up. However, if you switch the Systemate to electric emergency backup and leave it there for a quarter of an hour or so you can then switch it back to use the boiler and everything will work normally for the rest of the day. This will happen several days in a row sometimes and then not happen again for weeks or months.

I'd be grateful for suggestions as to what could be wrong.

The unit is hot?
If so when asking for DHW.
It can / could be one of the sensors or the DHW pump.
You could have an ACB problem.
Have you ever tried switching the whole system off.
Waiting a few minutes and switching back on?
Mike

Is the unit hot? Neither of us can remember, but the next time it happens will take note and post back. It would defy logic, wouldn't it, since the reservoir which acts as the heat exchanger is so large and takes a day to cool down and warm up again and as I said it doesn't take that long to get the unit going again. But the tiny reservoir which is kept hot for the first few litres of hot water has definitely gone cold.

ACB = air circuit breaker?

And yes, since everything these days is essentially just a motherboard, I should have thought to switch off and on again. Hopefully it's not a motherboard fault since we had ours replaced 3 or 4 years ago while the whole unit was new in 2002 and has had most of the other bits replaced under maintenance. Intermittent faults are a bummer, no?

Thank you for your suggestions.

ACB
Appliance control board.
Ref changing it 3 to 4 years ago counts for nothing.
Power surges and storms can take them out.
Have you checked the ACB for fault codes?
Mike