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Surely the answer is to buy a quality fan.
I can highly recommend the Spal range. Not exactly cheap, but they just work as they should. So under £60 gets a quality bit of kit.
Also be aware, having tested teh current flow of a couple of cheaper fans, the power quoted was on start-up. So a 120Watt fan did indeed pull 10A, but within a second or two was down to about 5amps, so 60watt when running.
https://www.t7design.co.uk/catalog/prod ... tegory/33/
Not sure it's a surprise that it draws more power while starting than steady state - some 120 Watt ones do mention 7 Amps draw, presumably as continuous current, from a 12-13 Volt supply.
My main concern about the budget fans is the waterproofing, which I worry won't really be the full IP68 level.
The problem I have is that if I knew what CFM value I need and what current/power that relates to, I'd be able to evaluate fans on specification and check what I get against that. As said, I want to do this with the smallest power fan that does the job, unless I can add speed control (and there are complications in that option). I certainly don't automatically believe that bigger is necessarily better anymore than I accept that "you get what you pay for" is a real axiom. But as I don't know what CFM/current/power that is, I can only really suck it and see. And while in that mode, I will do what I can to save costs and try to characterize what does the job.
All I can say, so far, is that a fan drawing 4 amps continuous with the transverse curve of the blades the right way round - low pitch against the rad - is a bit short of what's needed when stuck in traffic in high summer. However, it's not too much current draw with the engine idling with the lights, wipers, and demist fan all on.
As to the fan shown at that link, the one pictured is the wrong way round to go behind the Dolomite's radiator on both counts of the curvature's of the blades (and it's listed as a 24 Volt unit). Also, there's definitely room for a 12" fan rather than an 11". I know it's not a huge difference, a 30 cm vs 28 cm fan gives 17.5% more swept area, assuming the same 11 cm hub, but I think it's worth that extra. Also, if we're looking at quality not cost, is it being a brushed motor, rather than brushless, not a concern?
Graham