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Doc F has target flow rates for given volumes and duties - fans are sized on meeting that target, plus the pressure drop in the length of ducting, plus other margin factors. So, the right size fan pushing air through a correctly sized and installed duct, with sufficient inlet air via leaky door, will do a very good job of extraction - it's just most tradespeople will say - "bathroom, 4" fan, loop a bit of flexi-duct over the rafters (where it'll sag and become a blockage to efficient flow), external grille, jobs a good un"
rive_gauche
Howard
Back story is that the builder has suggested fitting an extractor + flu
I'm not sure he's done his homework. I worry that I'd be paying for something that a) I don't need and b) doesn't work.
If it could be shown to me that it will work, I would pay for it. I just don't know the question to ask. Until now. So is the question/s: 'is it designed to meet building regs? can you prove it?'