Whole-house hot-water circulation systems use a pump to move cooled water in the hot-water supply line back to the water heater. Closed-loop systems have dedicated return lines.
There are also some open-loop systems with special manifolds that allow the cold-water supply to be used as the return line. The pumps shown here are equipped with timers that limit operation to specified periods of demand.
On-demand hot-water kits are small enough to fit in a bathroom vanity cabinet.
This kit includes a wireless switch for mounting on the counter and a remote receiver for installation in the cabinet. When retrofitted to existing plumbing, the pump is located at the fixture farthest from the hot-water heater.
A pair of tees installed behind the angle stops create an open loop between the hot- and cold-water lines. When the pump is activated, the slug of cool water in the hot line is pushed back to the water heater via the cold-water line. The pump runs only a few seconds, until a sensor detects a rise in the temperature in the hot line.
When possible, the author prefers to install the on-demand circulating pump at the water heater in a closed-loop configuration. The pump can be activated by any number of manual switches or motion detectors located near fixtures on the hot-water line.
Pump controllers include simple wireless switches, like this one installed on a vanity top.
A hard-wired push-button switch is installed next to a kitchen sink.
This wireless motion-detector switch is attached to a bathroom backsplash.
Unlike traditional whole-house circulation systems, on-demand pumps can be used with tankless water heaters without voiding the manufacturer's warranty. The author uses a 3/4-inch hot-water return line and increases the pump size to make sure there is enough flow to activate the burner and to account for friction losses within the heater itself.