Filter the air or move it
Filtering machines clean the air and return it to the room: a purifier suits a home or office, while an air scrubber is built for the dust and spores of building, mould and fire damage work. Fans and fume extractors move air instead, taking fumes, heat and moisture outside or bringing fresh air in. Filters cannot remove gases such as carbon monoxide from engines and fuel-burning heaters, so those always need ventilating to the outside.
Sizing by airflow and room volume
Almost every machine here is sized by airflow, quoted in cubic metres an hour or CFM (cubic feet per minute), where 100 CFM is about 170 cubic metres an hour. Multiply floor area by ceiling height to get the volume, then multiply by the number of air changes an hour the job needs, typically four or five for allergies or dust control. Long ducting and tight bends reduce what a machine actually moves, so allow some margin.
Power, venting and filter changes
The machines here are portable and plug in, with 110 volt versions of many for building sites, though a few large fans need a 400 volt three phase supply, which a qualified electrician has to install. Anything that extracts has to discharge outdoors, clear of open windows and doors. Running costs are mostly electricity and, for purifiers and scrubbers, replacement filters, which last longer when the pre-filter is kept clean.
