Boiler not giving out hot water, but heating is working fine.

[Trybius]

I have a Vaillant EcoTec 937 Plus boiler.
A few days ago, it had blown it's 3amp fuse overnight, which I replaced. The boiler powered up fine, and happily supplied the house with CH.
However, as soon as a hot tap was turned on, it cut off the gas after a short while.

I called Vailiant for an appt, but they didn't turn up, so I called one of their approved fitters.
He came to the house and in the end he called Vailiant, and they suggested changing the diverter valve and PCB.

He changed both, but it had no effect. After that he took out the secondary heat exchanger, and did some sort of acid clean on it.

He essentially gave up after that, so now I'm back to square one!

I can see the diverter valve engaging when the hot water is turned on. I can see the flow symbol, so it detects the call for hot water. The boiler engages and the temperature display starts to rise very quickly. It gets to about ~90 and then the gas shuts off (presumably a safety feature). No error codes are displayed.

If I leave the heating on, and don't touch a hot tap, it will happily run and warm the entire house.

Does anyone have any suggestions what it could be? Before I face another heating engineer, I would like to have some ideas to make sure what they are suggesting makes sense, before I get charged for slowly replacing every component in the bloody boiler one by one!

 

[Alec_t]

Dodgy internal thermistor/temperature-sensor?

 

[Trybius]

Would there be a way to test that?
If they are what supply the display with it's temp, it seems relatively correct - if the heating is on it sits around ~45 constantly, once the hot water is turned on it raises over 10-20 seconds to ~85, and then once the boiler turns off, it slowly climbs down a degree every 5-10 seconds.