Combustion of hydrocarbons always produces water vapor. Air provides O2 oxygen and other stuff which combines with the CH chains to make CO2, H2O, and whatever else. When the heater operates there will always be water vapor present in the exhaust duct; whether or not it condenses to liquid depends on many factors. A significant factor is the efficiency of the heater: the more efficient the heater is at putting combustion heat into the domestic water the less heat remains in the exhaust gas. Exhaust gas heat warms the walls of the exhaust duct. If there's enough heat in the exhaust to warm the duct above the dew point of the exhaust then there will be very little condensation in the duct. But if the duct is cold, and/or the heater has high efficiency, it can get a fair trickle of condensate.
Where condensation is a concern there are two approaches. One is to run the duct horizontally with slope toward the outdoors. Any condensate in the pipe runs to the outdoors and drips out the end. The other approach is to slope the duct toward the indoors and provide a condensate drain. This is the approach used when the duct goes vertical as yours does.
I don't read a word of Dutch but I looked through the installation manual anyway for the water heater you linked. This text and illustration from page 54 mention cleaning a siphon and a condensate trap. It looks to me as if this heater is designed to receive condensate drain-back, and therefore you should assemble the exhaust duct with the expectation that it'll need to carry condensate back to the heater.
The installation section on exhaust is number 4.8. It describes arrangements using room air or outdoor air for combustion, duct sizing according to length, and I saw something in there about condensate too.