As W0jtek92 wrote, it is probably a scale problem.
It would be advisable to descale the exchanger every few years to avoid such situations, and not to wait until there is a problem visible to the naked eye.
Added after 9 [minutes]: .
I see you have included a photo of the exchanger.
I'm not familiar with the Termet but in my Vaillant the tubes are only pressed in with an o-ring type seal and a special pin to prevent accidental fall out.
However, when I first dismantled the exchanger in my boiler after x years of operation, it took a lot of force to pull out the tubes and one element could be pulled out only after removing it together with the exchanger.
I am not trying to scare you, but I know that the exchanger disassembly may not be easy and pleasant, and you need a set of new, preferably original gaskets to be able to assemble the whole thing.
If something goes wrong and you have a leak, you will lose not only the DHW but also the central heating system, so it is better to do such experiments on your own outside the heating season, because there is no time pressure, which encourages mistakes, and in case of failure there is no dispute that everyone in the cottage is freezing.
It would be advisable to descale the exchanger every few years to avoid such situations, and not to wait until there is a problem visible to the naked eye.
Added after 9 [minutes]: .
I see you have included a photo of the exchanger.
I'm not familiar with the Termet but in my Vaillant the tubes are only pressed in with an o-ring type seal and a special pin to prevent accidental fall out.
However, when I first dismantled the exchanger in my boiler after x years of operation, it took a lot of force to pull out the tubes and one element could be pulled out only after removing it together with the exchanger.
I am not trying to scare you, but I know that the exchanger disassembly may not be easy and pleasant, and you need a set of new, preferably original gaskets to be able to assemble the whole thing.
If something goes wrong and you have a leak, you will lose not only the DHW but also the central heating system, so it is better to do such experiments on your own outside the heating season, because there is no time pressure, which encourages mistakes, and in case of failure there is no dispute that everyone in the cottage is freezing.
