You'll be there a looooooooong time on a big commercial job using a duo
forget that idea in my opinion, it's useless on commercial carpets anyway.
You'll need a bigger machine like envirodri or host if you want a crb machine for that sort of work. I use a rotary with polyprop brush, either 15" or 19" depending on how much room there is to swing it round in. The rotary is more versatile than a crb, in that you can use it also for bonnet cleaning and floor scrubbing too.
The duo is brilliant when you want more agitation than you can get from a hand broom, but dragging a rotary or big crb machine upstairs etc is too much hassle.
You can keep the duo clean without taking it apart! After you finish with it, start your extraction machine, turn on the duo and stick it upside down. Use the hose cuff from your machine to clean all the crud away, pushing it firmly along the brushes until the crud stops flying off. Takes about 60 secs and leaves no mess cos you suck up all the bits
