Pellet Installation - Do I need a chimney renovation or not?

Soon we will have our pellet heating system installed. There is a two-flue chimney present. The stove builder now says that no chimney renovation is needed because pellets as fuel are very dry and do not produce much condensate. The chimney sweep says that "almost always" a renovation is needed with pellets and that the stainless steel pipe to be installed must also have a minimum length of 4 meters - measured from the connection point of the flue pipe. Since there are only 2.70 meters to the end of the existing chimney, the pipe would then stick out 1.30 meters at the top, which I honestly find ugly. I would rather have a new chimney installed on the outside of the house wall. Does it also have to be 4 meters long? To the roof it is only 2.5 meters. But who is right now? I definitely do not want to start breaking anything up later; the construction site is still open and everything is accessible, so if something needs to be done, it should be now. Can this not be somehow calculated or do you have to be surprised later whether the chimney will become tarred or not? Exhaust gas temperature is at least 100°, 1.60 meters of the chimney go through an uninsulated attic and then 1 meter protrudes at the top. I find it strange that the one who wants to sell me something advises against renovation ("wait and see first"), and the one who would not make any money from it advises to do it.

I just checked. For gaseous fuels, the effective chimney height (height from the center of the flue gas inlet) is at least 4m, for liquid or solid fuels, e.g. wood pellets, at least 5m. If this is not met, it may be that the chimney does not draft.

We have a pellet boiler ourselves, the chimney is new, effective chimney height about 9m.

Regards

The chimney sweep is right because he approves it. The stove fitter cannot release the stove/chimney.

The chimney sweep said that the chimney renovation is not mandatory, he will accept it without it as well. We just need to know if we want to take the risk or break everything up again later... But I think we will build an extra lightweight chimney, it looks better and the two flues of the current chimney will remain for the stoves.