If I was to get one now I would go for the 9kw. Part of the problem I had with the 5kw from PF is the batteries they supplied were not up to the job, though I still think it was underwhelming anyway.
I would not go near Pure Freedom for a hot system again.
A Thermo Top C diesel heater is rated at 5.2kw. Unfortunately about half of that goes as waste heat out of the exhaust and heat loss through the casing, pipe work and heat exchangers.
I tested a 5.2kw many years ago and was only able to raise the water temp from 9 degrees C to 35 degrees C at a flow rate of 1.5lpm. To raise that temperature of my water took about 2.65kw.
So a Thermo Top C isn't going to do anything more than raise the temperature to make the hose supple.
We have seen a cleaner in the south who has fitted his Thermo Top C to rather heat the water in the vans tank. It will work better than a 2kw electrical immersion heater element but not a good as a 3kw element.
Heating on demand is the most efficient way but a Thermo Top C just isn't man enough. It is brilliant for the job it was designed for - an engine preheater is very cold climates.
I can tell you that this is not right spruce my 5kw heater when working and it would work well for weeks at a time before it played up due to the battery problems,the water is still proper hot but it cycles gets hot then cools until it does another cycle. The 9kw heater gives consistency with heat once it’s hot stays hot throughout the time you are running it,as for saying the 5kw is only enough to make the hose supple is just not true.
That was just my findings which seemed to support the number of cleaners who complained the 5kw Webasto heater didn't put out enough heat.
I didn't spend hours experimenting - such as preheating the internal water circuit before I started. I did it long enough to see that it wouldn't work for me as a heat on demand for both of us working off my van. (In my calculations I didn't account for the heat lost through the exhaust etc. I felt the heater would support both of us at the same time.)
If I remember those early days you were the only one who seemed to get a higher heat output from your heater. There was always the question of whether we were just hearing complaints from those whose heaters weren't working and not from any cleaners whose heaters were working.
I used a second hand Thermo Top C. It could well be that the heater has a lot of carbon buildup inside the heat exchanger which was reducing efficiency. It could well be as it came from a narrow boat. In those days red diesel sold on the canals seemed to create a lot of problems for these heaters as the quality of the diesel was rather poor.
The experiment I did was good enough to tell me that the 9.1kw Webasto series was a better heater to use in our application, even as a single operator.