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 common process of hot water extraction begins with preconditioning. alkalinity 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 scale. 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. Newer carpets, such as with synthetic fiber 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).
Pretreatments 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 (action). Ideal pretreatments should rinse easily and leave dry, powdery, or crystalline residue that can be flushed without contributing to re-soiling.
Heavily soiled areas require the application of manual spotting, pretreatments, preconditioners, and/or "traffic-lane cleaners", (commonly sprayed onto carpet prior to the primary use of the dry-cleaning system) which are detergents or emulsifiers which break the binding of different soils to carpet fibers over a short period of time. For example, one chemical may dissolve the greasy films that bind soils to the carpet, and thus prevent effective soil removal through vacuuming. The solution may add a solvent like d-limonene, petroleum byproducts, glycol ethers, or butyl agents. The amount of time the pretreatment dwells in the carpet should be less than 15 minutes, due to the thorough carpet brushing common to these "very low moisture" systems, which provides added agitation to ensure the pretreatment works fully through the carpet. The benefit of dry carpet cleaning, over wet solutions, is that dry chemical compounds don't attract dirt, like dried shampoo.
A 98% biodegradable or others,Heinrich Waldhoff, Rudiger Spilker, ''Handbook Of Detergents, Part C:'' P. 26, CRC Press, 30 November 2004 slightly moist absorbent cleaning compound may be spread evenly over carpet and brushed or scrubbed in. For small areas, a household hand brush can work such a compound into carpet pile; working like "tiny sponges", the attracted cleaning solution dissolve dirt, dirt and grime is attracted/absorbed to the compound, after a short drying time (the cleaning solution which is attracted to the compound must evaporate), it will be removed with a vacuum cleaner, the drier the better, leaving carpet immediately clean and dry. But it's very difficult to remove all residues, the residues can cause allergies and biological compounds may cause discolourations on carpets. For commercial applications, a specially designed cylindrical counter-rotating brushing system is used, without a vacuum cleaner. Machine scrubbing is more typical, in that hand scrubbing generally cleans only the top third of carpet.
Tea leaves and cut grass were formerly common for floor cleaning, to collect dust from carpets, albeit with risks of stains. Ink was removed with lemon or with oxalic acid and hartshorn; oil with white bread or with White pipe clay; grease fats with turpentine; ox gall and naphtha were also general cleaners. Ammonia and chloroform were recommended for acid discoloration. Benzine and alum were suggested for removing insects; diatomaceous earth and material similar to cat litter are still common for removing infestations. Candle wax is removed by placing a towel over the affected carpet area and applying steam from a clothes iron until the wax absorbs into the towel. Some traditional methods of stain removal remain successful and ecological. Caution should be used when treating natural fibers such as wool.
The longer the stain material remains in the carpet, the higher the chance of permanent color change, even if all the original stain material is removed. At times pets urinate on the carpet and this results in a bad odor especially when it is hot and humid. The carpet or rug is usually taken outside and immersed in water to remove such stains. Immediately blotting (not rubbing) the stain material as soon as possible will help reduce the chances of permanent color change. Artificial food coloring stains are generally considered permanent stains. These may be removed by professional cleaners or deep cleaning rental machines with heat-transfer stain-reducing chemicals, but carry risks of burning the carpet. Stain removal products can be combined with anti-allergen treatments to kill house dust mites.