A recirculating pump is designed to work as an extension of your water heater to provide constant hot water to all of your fixtures. In order for a traditional recirculating pump to work properly you need to install a return line that runs from your furthest fixture. It is possible to add a recirculating pump to an existing plumbing system but it usually works best as part of a renovation or new build.
There are a few products that offer alternatives to traditional recirculating pumps by utilizing the cold water supply line as your return. This type of system is usually cheaper and easier to install. The disadvantage is that the cold water line will dump out some hot water when it’s first turned on.
There are also gravity fed recirculating systems that can usually works best on a ranch or single story home. The gravity fed system works based on the principle that hot water will rise. By installing a return line off the boiler drain for your water heater you can get instant hot water on all your fixtures.