If there is anything left on the carpet that is made out of metal, then it may leave behind rust that cannot be removed by anyone but a professional.
If you set your system too low below 15Hg you may not have enough lift for extraction, set it too high above 15Hg and you run the risk of doing damage to your system. Any carpet cleaner that doesnt have insurance where they have caused such damage is nothing more than a common rip off artist.
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