Frequent vacuuming is a great way to get rid of surface dirt, but every now and then, carpets need a thorough deep cleaning. In fact, carpet manufacturers recommend professional carpet cleaning every 12 to 18 months. One way to stretch time between professional carpet cleanings is to buy or rent a carpet cleaner. You'll recoup the money you spend after just a few uses.

To help you find the right carpet cleaner, the Good Housekeeping Research Institute put several to the test. Find out how we evaluated them, then read up on what you should know before buying one.

What We Looked for in Carpet-Cleaning Machines:
The GHRI evaluated carpet-cleaning machines for:
• How well they cleaned up dirt and soil
• How much water they extracted from carpet samples
• Ease of filling and emptying the tanks
• Ease of operation
• Clarity, accuracy, and completeness of owner's manual
• UL, ETL, or CSA certification

Twenty consumers also evaluated how easy it was maneuver each machine.

How We Tested Carpet-Cleaning Machines: To see how well the carpet cleaners removed dirt, we cleaned soiled carpet swatches with each machine. The swatches dried overnight, and then we took colorimetric readings of the cleaned carpets, comparing them to similar readings taken on unsoiled and precleaned swatches. These readings measure how much light is reflected off of a surface; the cleaner the surface, the more light is reflected. To determine the amount of water left in the carpet after extraction, we measured the weights of the recovery tanks and the clean tanks prior to and after cleaning.

Before You Rent a Carpet-Cleaning Machine
• Remember to factor in the cost of the cleaning solution. It doesn't come with the machine rental, and you'll have to purchase it separately.

• The machine is probably not brand-new and may not be in the best condition.

• Think about convenience: You'll need to lug the rental home and return it to the store when you're done.

How to Get the Most Out of a Carpet-Cleaning Machine:
• Before deep-cleaning your carpets, vacuum the area thoroughly to pick up any loose dirt.

• When cleaning the entire floor, move furniture out of the way. For pieces that are too heavy to move, place aluminum foil or wax paper under the legs. This prevents wood finishes from staining the carpet as it dries.

• Begin cleaning at the farthest point from the doorway and work back toward it so you can step out when you're done.

• Never close up a room with a wet, newly cleaned carpet; it slows down drying time and can promote mold growth. Instead, keep the air circulating by opening the doors and windows.

• Once the carpet is dry, vacuum once more to pick up any remaining soil.