No hot tap water from combi-boiler but central heating is fine

My #Vaillant combi-boiler was working fine for both central heating and tap water, then suddenly there is no hot tap water. These are the clues that seem to imply the secondary heat exchanger would NOT be the problem:

• radiators heat up fine, which I think must prove the radiator side of the secondary heat exchanger is clear (OR does the secondary heat exchanger get bypassed when central heat runs)?
• the “hot” (cold) tap water flows with good pressure, which seems to suggest the tap water side of the secondary heat exchanger is probably clear as well.
• new secondary heat exchanger was installed in February 2019, which would be unlikely to clog this early.
• when I last showered the hot water was good and continuous. In 2019 when an exchanger clogged shower water would alternate between hot and cold. But in the case at hand I had solid hot tap water one day and not the next.

When I turn on a hot water tap, the boiler LCD should normally show an icon of a faucet to indicate that it has switched to tap water. That is not happening. Even if I turn off the central heating by turning the dial to the lowest setting, the boiler never switches to tap.

The guy in this video says it’s normally the central heating side of the secondary heat exchanger that clogs. Does anyone concur with that? Seems unlikely because the radiator circuit runs the same water through which has inhibitor. The tap water side would have new quite hard water constantly passing through the tap side of it.

There are no fault codes displayed when I press the “i” button for information. What a likely issue?

I guess this question might be important:

• Does the secondary heat exchanger get bypassed when central heat runs?

After some thought, I suspect bypassing the heat exchanger would be important, otherwise it would heat the tap water sitting still in the exchanger and cause unwanted pressure, correct? If that’s the case, it makes me think the radiator side of the exchanger could be clogged. But then I guess that does not explain the tap water icon not appearing on the lcd. When I turn on the tap, I hear no solenoid switching sound like I used to.

UPDATE

This morning the hot tap kind of works with the thermostat off and after letting water run quite a long time. The tap water icon appears somewhat chaotically, flashes every couple seconds (not sure if that’s normal). For the first couple minutes the status rotates between these undocumented status codes: 11, 14, 17. What do those mean?

The burner ignites for a second then quits (status 14). Like a car starting to rev but giving up.

Over the summer I drained the whole radiator system, flushed about ½ the radiators, and refilled with water and added the inhibitor. Just yesterday I added water to increase the radiator pressure. There is no filter on the radiator circuit. I wonder if my maintenance would have caused sludge to end up in the heat exchanger.

Then I turned on the heating and the tap water was shut out again. The status sequenced like this:

1. (status 4) heating mode: burner on
2. (status 5 undocumented)
3. (status 7) heating mode: pump overrun
4. (status 8) Anti cycling mode (after heating operation) ← what is that?