Ok. I've got one of these same fans and spent quite a bit of time researching a solution for bathroom ventilation and planning for it to be installed in the bathroom. Oh and then installing it...
The following is worth bearing in mind.
i) The fan has a selectable hi and lo speed operation. This is configurable in the static wiring at the connection to the fan. If you are running at normal speed - you could try hi speed?
ii) You are using ribbed flexible ducting - this has more resistance than rigid PVC ducting, which I used and then lagged with airpocket foil insulation.
iii) Air source. You are correct you need a source for replenishment of air.
However, by using two sources you are potentially bridging the airflow circuit. For example, in my bathroom I also have a window but do not open it for use with the fan. This is because the location of the air inlet is close to the window, so therefore using the window as a source offers an airlflow route in the room and straight up and out the vent.
This does not offer the most effective route for picking up humid air. You need to try to get movement across the room. E.g. door one side and vent on other.
You need to use ONE air source or the other. You may find more success closing the window depending on your room layout.
vi) Timer - you need to run the fan for 15 minutes after vacating. Are you using the timer facility?
v) I believe you can also use a couple of Y adapters and use a second fan in parallel for double flow. I think this would be a better alternative to using another in series which doubles pressure. However, it also depends on the max flow rate capability of the flexible 100mm PVC you are running, which may have an effective upper limit flow rate. It may also become unacceptably noisy.
I use the same fan in a bathroom which had no ventilation before (apart from opening a window).
When we moved in, clearly the condensation had been a problem for many years and successive layers of paint applied over mold and delaminated surfaces, was used to hide / repair the problem.
Since using the fan with a 15 min overrun, the room is clear of mold and new ceiling paint holding up well. i did use a soft sheen rather than matt to give a little more resistance to moisture.
The fan pressure is not huge - as you say it will hold a piece of paper but is no where near as strong as the kitchen extractor but I don't think it needs to be. I think you have to be realistic that if you have a power shower with lots of misting at say 40C, you are going to get some steaming. The fan needs to clear it in a reasonable time after finishing so it does not spread through the house and remain in the bathroom causing damage.
In our case, it just about keeps the window and mirror clear (our shower is low pressure rose and temp about 38C) but there is still a hint of condensation on some spots of ceiling after the shower.
However, after the completion of the overun the room feels and smells damp free and most of the spots have dried on their own due to the airflow.
You could try 125mm and it will offer a lot more flow. More than the 25mm would suggest. However, from reading a lot on the subject including these forums 100mm seems to be in widespread use for bathroom and is considered acceptable.
I would also take the flow rate specs of all fans I've ever encountered with a grain of salt - I don't think they ever are close to the presented specs once installed.
Hope some of the comments above helps.
Edits: Typos
