question

I have been doing exactly what you are trying to achieve, using a 4-noks power reducer. it has been running for about 4 years and has not yet damaged my Victron Multiplus (5000/48). 4-noks sell it to work in conjunction with the Elios4You but you don't have to have the Elios; the Power Reducer will work with anything that can produce a 0-10V control signal (so if you want a 2100 W immersion heater to draw just 1300 W, simply feed 6.19 V into the Power Reducer control signal input (6.19 = 10 * 1300/2100). (If you just want a fixed diversion percentage you can use the 10V reference signal from the Power Reducer and scale it down to 6.19 V with a simple resistor bridge).

There is a very good article on this at https://www.renewableenergyworld.com/2013/09/04/the-importance-of-pv-immersion-control/

In particular, note where he says: following rigorous testing, 4-noks have found PWM to be the most suitable in altering the energy diverted to the load, whilst causing the least strain on the AC sine wave. With the inclusion of specific filters, harmonic emissions can be contained to acceptable levels, enabling the solution to meet the stringent CE and EMC compliancy for the domestic class.

When I take the front off my 4-noks Power Reducer and have a look inside I see that it has a really large coil. I think this is the main component in "smoothing" the demand, so that although the output to the immersion heater is "pulsed" (which does it no harm since it is simply a passive resistor), the input does not suffer from these pulses, which might otherwise harm the inverter.

From a brief glance at the United Automation CSR2-15E I would be doubtful about whether this would work. It clearly doesn't have the smoothing ability of the 4-Noks machine. I am attaching a picture of the inside of the 4-noks - as you can see it is in a different league from the CSR2-15E.

I only really know "what works for me" - I can confirm from actual experience that my set-up works but I don't have the knowledge to say what else might work. I looked up "continuous voltage dropping fan controller" in the usual place and it seemed to be all about the tiny DC-powered fans that are used to cool PCs etc. It does seem to me that anything that pulses the output is going to pulse the input too, unless there is some serious smoothing components inside it. In the case of immersion heaters, they have to be big to store enough energy between the on- and off-pulses.

There is possibly a much simpler solution if you simply want to reduce the wattage of your immersion heater. My immersion heaters have 3 parallel elements and it is simple to wire them so that only one (or two) elements are in use (giving you, obviously 1/3 or 2/3 of the full output).