The fan controller controls 3 speeds.
1 & 2 from the rad switch and 3rd speed from the black sender in the thermostat housing. The fan control unit sends power to one of the three cables from the signal it gets. That’s why you can join the three wires together near the fan plug without getting current bleed going somewhere else that connects to the new fans. I use the two signals from the rad switch and the black switch unplugged so I can’t go over temp and bring that signal in. It would have easier and less complicated to just use the rad switch for just the first two signals direct from the fan switch like the Phenix fan set up.
mine don’t change speed.
the slim fan come on a 95 Celsius and both come on at 105 Celsius.
that shroud seem basic for the money and the fans aren’t the best either.
My bent plate cost £40 I think and I cut it out with a jigsaw. Look on the Spal fan website to determine fans and flow rates.
it’s the fans, relays and wiring where the money is.