Circulating or recircpumps have a very small motor typically 1/120 to 1/20 horsepower. The can't produce much head (pressure) and as a result, end up with very low flow rates. Most circulating pumps will not even turn on a tankless water heater.

Hot Water Recirculating System is a pump and valve combination that delivers hot water in an instant to any faucet or bath in your home.  It uses the pump at the water heater along with the patented under-sink sensor valve installed furthest away from the water heater.  This combination creates a pressure differential that allows the cold and cool water in the hot water supply line to "by’ pass" (at a low volume) into the cold water supply line through the thermostatically controlled sensor valve, keeping the water in the hot water supply line at a "no-wait" temperature throughout the home.  The timer on the pump makes it more efficient during downtime.