For the best results in a domestic situation your best bet is to engage the services of a local company who use a truck-mounted steam system which is driven by a petrol or diesel powered engine. This is also known as HWE or Hot Water Extraction and is the system most often recommended by carpet manufacturers themselves.
In brief the operator will first vacuum your carpets and then pre-spray with a soapy substance that will help to emulsify the stubborn soil attached to the pile of the carpet - the truck mounted system is then employed to spray at high pressure a detergent suspended in a very hot mixture onto the face fibre of the carpet and then extract it with the powerful vacuum almost as soon as it is spread on the carpet (steam can usually be seen jetting from the end of the metal wand tool used by the operator). Some operators use electric brush attachments attached to a truck mount solution hose with vacuum hose from the truck mount and some use electric driven machines for carpets in high blocks of flats etc. where truck mounts cannot reach and electric machines are also commonly used to clean upholstery and curtains.
Most carpets in the USA are cleaned using the HWE truck-mount system and many of the machines used in the UK were developed there. But HWE is by no means the only system as there are LM or Low Moisture machines and DF or Dry Fusion methods as well Encapsulation systems that can give equally good results depending upon the degree of soiling in your carpets and the age and severity of any stains. Each operator will vouch for the benefits of their particular system and each system has many hours of research and development behind it in terms of technology and chemical testing. Some systems were developed for particular applications such as hospital or nursing home commercial grade carpets and may not be suitable for domestic grade carpets. Do ask you operator which system they use.
When you engage the services of a professional carpet cleaner you should try to establish during the survey or quotation visit if all the marks and stains in your carpets will be removed - if you have long-standing stains caused by (say) ink, blood or shoe polish they may be permanent and no system currently available can remove all permanent stains in wool mix carpets - sunlight and your own remedies or shop-bought treatments that have not been properly rinsed out may have made the problem worse. You may have wear marks in high traffic areas and these parts may not clean as well as other less worn areas because of abrasion to the fibres.
Most operators will provide a free quote and you can ask for two operators to quote on the same job to see what the difference is between them. Some systems are franchised by equipment operators who also supply training and chemicals and allocate territories in the UK and Ireland. Some operators are independents running one or two vehicles using the same or very similar systems. As with many trades who visit your property you will usually get the service you pay for so beware of very cheap prices and very expensive prices - a reasonable cost for a decent job and an agreed outcome is what you should be looking to achieve.
Just remember that all the dust, dust mites, allergens, human skin particles, pollen, pet dander, odour causing dirt and soil that ends up attached to your carpet fibres that cannot be removed by regular vacuuming (once a week) should be tackled annually or even every six months using another more powerful system and that is where professional carpet cleaners come into the equation.
If you just have rugs on hard floors then the best remedy is to lift and air and beat those rugs (on the back) outside you house and then vacuum them outside and again when you get them back in the house. Alternatively you could get a professional in to do that for you and to steam clean the rugs as well.
Reputable operators will be fully insured for treatment risks and will give you their full company address details - resist the temptation to get your carpets cleaned by the company who just pushed a card through your door with a mobile number and an offer of 'whole house' carpet cleans for very low prices. They may be excellent or they may also be what is known in the trade as 'splash and dash' merchants who use light, dry-cleaning shop rental machines and leave you carpets only marginaly better than when they started.
If you are serious about getting your carpets into tip-top condition, have them deep steam cleaned and dry in 2-3 hours by calling in a professional who uses a truck-mounted steam system - avoid the DIY machines you rent from the dry-cleaners as well as they are woefully short on either true cleaning ability or drying (vacuum) power.
We run a truck mount steam carpet cleaning company in Hampshire and rarely travel outside the Hampshire area but can usually point you in the right direction if you cannot find a suitable local operator in your part of the world. Contact us through Ebay.