Hot water extraction
Further information: Hot water extraction
Although there is an actual steam cleaning industrial process, in the context of carpet cleaning, "steam cleaning" is, in fact, hot water soil extraction cleaning, which is professionally known as HWE. The hot water soil extraction cleaning method uses equipment that sprays heated water, sometimes with added cleaning chemicals, on the carpet. Simultaneously, the water is vacuumed up, along with any dislodged and dissolved dirt. Many carpet manufacturers recommend professional hot water extraction as the most effective carpet cleaning method which also provides a deeper clean. Actual steam could damage man-made carpet fibers and change the characteristics as they are usually set using heat. Natural fiber carpets such as wool can shrink, velvet-piled carpets and Berber carpets will become fuzzy which is known as pile burst.
Hot water extraction equipment may be a portable unit that plugs into an electrical outlet, or a truck mount carpet cleaner requiring long hoses from the truck or trailer. Truck-mounted equipment may be used where electricity is unavailable, but may be unsuited to premises distant from a driveway or road, and require hoses to pass through windows to reach the upper floors of a building. The hoses needed for truck-mount and professional portable carpet cleaning may present a trip hazard, and allow pets or children to escape through doors left ajar. Heated or air conditioned air can also be wasted when doors are left open for hoses. Truck-mounted carpet cleaning equipment minimizes the noise in the room being cleaned, but may cause noise and air pollution offensive to neighbors, and could violate anti-idling bylaws in some jurisdictions. However, truck-mounted cleaning is much faster than portable equipment, and the extra heat will dissolve more spots and stains, and more vacuum suction power will reduce drying times.
A Rug Doctor rental carpet cleaning machine
A common process of hot water extraction begins with preconditioning. Alkaline agents such as ammonia solution for synthetic carpets, or acidic solution (such as vinegar solution) for woollen carpets, are sprayed into the carpet, then agitated with a grooming brush or an automatic scrubbing machine. Next, a pressurized manual or automatic cleaning tool (known as a wand) passes over the surface to rinse out all pre-conditioner, residue, and particulates. If an alkaline detergent is used on a woollen carpet, use of a mild acetic acid solution will restore neutral fiber pH. The acidic rinse thus neutralizes the alkaline residues, and can contribute to softening cleaned fabrics.
The hot water extraction method is the preferred method of many carpet manufacturers as it removes more dust and abrasive particles resulting in less wear and pile abrasion.
Extraction is, by far, the most important step in the hot water extraction process. Since the hot-water extraction method uses much more water than other methods like bonnet or shampoo cleaning, proper extraction and air flow are critical to avoid drying issues such as mold growth & browning of wool fibres. Drying time may also be decreased by extra use of fans, de-humidifiers, and/or outdoor ventilation.
Older surfaces, such as double jute-backed carpets and loose rugs with natural foundation yarns, could shrink after a wet treatment, leading to suppositions that wet-cleaning could also remove wrinkles. However, this notion is antiquated and this method could also occasionally tear seams or uproot strips. Newer carpets, such as with synthetic backing and foundation yarns, do not shrink, and they smooth easily; in such carpets, wrinkles indicate an underlying problem, such as delamination where the secondary backing becomes unstuck from the primary backing, that may need a certified carpet inspector to determine.
Wet-cleaning systems naturally require drying time, which may lead to concerns about very slow drying, the risk of soiling returning during drying as the moisture evaporates bringing the soils from deeper within the pile to the surface, as well as odors, bacteria, fungi, molds, and mildews. Carpet cleaning specialists try to find a balance between rapid drying (attributable to lower flow rate through the cleaning jets of a spray system) and the need to remove the most soil (attributable to higher flow rate).
Pre-treatments similar to those in dry-cleaning and "very low moisture" systems are employed, but require a longer dwell time of 15 to 20 minutes, because of lower amounts of carpet agitation. Ideal pre-treatments should rinse easily and leave dry, powdery, or crystalline residue that can be flushed without contributing to re-soiling.