Ask how the unit must be installed and who is responsible for doing it. If it must be done by a professional, ask whether the unit's purchase price includes installation Some units are installed under the kitchen sink and treat all cold water going into the tap. Other ones only treat water diverted from the cold water line and deliver it to a separate faucet. Some units are mounted on the faucet, while others rest on the counter top. To be effective, water treatment units must be maintained properly.
Some units require more maintenance than others.
Before you buy, ask about the unit's maintenance requirements and, if possible, review the owner's manual or manufacturer's recommendations. After using a water treatment unit for a time, be aware of changes in sediment, water pressure, and taste in your water. Such changes may indicate that the filter should be replaced. If a filter comes with a written warranty, take the time to read what parts and costs are covered under the warranty, and, if you have problems with the unit, whether you can get a replacement or a refund. Ask where repairs would be done.