Because foam fills the recovery tank with air rather than water, collapses vacuum lift and can reach the vacuum motor.

An extractor recovers solution into a tank under vacuum. Foam occupies volume without carrying soil away, so the tank appears full while very little liquid has actually been removed - and the carpet is left wetter than it should be.

Once foam reaches the float shut-off or the motor, the machine either stops working or is damaged. Vacuum motors do not tolerate liquid.

This is why general-purpose or shampoo products must never be used in an extractor. They are designed to foam.

Defoamer added to the recovery tank controls foam generated by residues already in the carpet from previous shampooing, and is a standard item for anyone cleaning carpets with unknown history.

It attracts soil, so a carpet cleaned with a residual product re-soils noticeably faster than before.

Surfactant left in the pile stays tacky and slightly hygroscopic. Dry soil that would have vacuumed out instead sticks to it, and traffic lanes reappear within weeks.

This is the mechanism behind the widespread belief that cleaning carpet 'ruins' it. The cleaning did not ruin it; the residue did.

Two practices prevent it. Use products formulated to rinse out or to dry to a crystalline non-tacky residue, and finish with a clean-water or acidic rinse pass that removes what the cleaning pass left behind.

An acidic rinse also neutralises alkaline pre-spray, which matters for wool and for dye stability, and it leaves the pile at a pH closer to its manufactured state.

High alkalinity cleans faster and damages wool and some dyes; wool and delicate fibres need mild pH with heat, dwell and agitation doing the work instead.

Alkalinity saponifies oily soils and swells fibres, which releases embedded particulates. On nylon and polypropylene commercial carpet, a strongly alkaline pre-spray is normal and effective.

Wool is a protein fibre and is damaged by high alkalinity - it yellows, loses strength and can felt. Products for wool work at near-neutral to mildly acidic pH.

Dyes are the other risk. Some are pH-sensitive and bleed or shift colour under alkaline conditions, which is why colourfastness testing on an inconspicuous area is standard before any strong chemistry.

Where an alkaline product has been used, an acidic rinse restores the pH balance and reduces the risk of both browning and accelerated re-soiling.

Pre-spray is the professional method: apply chemistry, let it dwell, agitate, then extract with a rinse solution rather than more detergent.

Chemistry needs dwell time and agitation to release soil. Running detergent through the machine gives it only the instant between injection and recovery, which is not enough for anything beyond light soil.

The standard sequence is: vacuum thoroughly, apply pre-spray, allow the stated dwell without letting it dry, agitate with a brush or rotary tool, then extract with a rinse - typically an acidic rinse rather than a detergent.

This also puts far less chemical into the carpet overall, because the pre-spray is applied where it is needed and the extraction step removes rather than adds.

Running detergent through the machine is reasonable for light maintenance cleaning of lightly soiled carpet, and it is what most single-tank machines are designed for.

As little as possible - over-wetting causes browning, delamination, odour and shrinkage, and it accounts for more failed carpet cleans than anything else.

Extraction works by injecting and immediately recovering. Where recovery is poor - a blocked filter, a full tank, worn vacuum shoes, foam, or simply too many wet passes - water is left in the backing.

Consequences follow within a day: cellulosic browning wicking up from a jute backing, adhesive failure and delamination, microbial odour, and shrinkage in natural fibre carpet.

The controls are dry passes after wet passes, keeping the recovery system in good order, and moving air over the carpet afterwards with air movers.

Drying time is a legitimate specification item. In commercial premises the requirement is usually that the area is usable within a stated number of hours, and that drives the method as much as the soil does.

Alkaline for synthetic commercial carpet, mild and wool-safe for wool and blends, solvent-boosted for oily soil, and enzyme products for protein soil.

Nylon and polypropylene tolerate alkaline chemistry well and respond to it, which is why most commercial carpet cleaning uses it.

Wool and wool blends need products certified as wool-safe, at mild pH, with an acidic rinse to finish.

Olefin and polyester repel water and hold oily soil, so solvent-boosted pre-sprays outperform straight detergents on them.

Protein soils - food, body fluids - respond to enzyme products given enough dwell time at the right temperature; enzymes are denatured by very hot water and by high alkalinity, so they are used differently from ordinary detergent.

Always identify the fibre before choosing. Testing a hidden area for colourfastness and fibre reaction takes minutes and prevents the failures that cannot be undone.

Generally yes for synthetic fibres and detergent chemistry, with clear exceptions for wool, some dyes and enzyme products.

Heat accelerates chemical reaction and lowers the viscosity of oily soil, so hot water extraction removes more per pass than cold on synthetic commercial carpet.

Wool is the main exception - excessive heat combined with alkalinity and agitation risks felting and damage, and manufacturers usually specify a maximum.

Dyes can bleed at elevated temperature, particularly on older or hand-dyed carpet and on many rugs.

Enzymes are proteins and are denatured by heat above a certain point, so a hot rinse over an enzyme pre-spray defeats it.

Practical approach: hot for synthetic commercial carpet with alkaline chemistry, warm for wool and delicate work, and follow the enzyme product's stated temperature range where enzymes are used.