On demand hot water systems offer a solution to the problem of turning on a faucet or shower and waiting for hot water. Instead of relying on pressure in the line, a recirculating pump brings hot water quickly to the faucet when it's needed. The pump is installed between the hot and cold water lines at the faucet furthest from the water heater. It's activated by push buttons connected with low-voltage wire or via a wireless remote control.
Once activated, the pump rapidly pulls hot water from the water heater down the hot water line. At the same time, it pushes the cold water back through the cold water line to the water heater. The pump comes on only when it's needed. Once hot water has filled the line, the system shuts off.
A variety of products are available, but as a specialty item you may not find these systems in builders hardware stores. A web search using “on demand hot water system” or “recirculating hot water system” should provide a list of manufacturers and products.