Selecting the correct size recirculation pump is a fairly involved process and there is not a “one size fits all” solution. Your entire hot water plumbing system needs to be taken into consideration to make sure the pump is able to create not only enough flow to activate the tankless unit but to efficiently circulate the water quick enough to minimize temperature loss. Bigger and more complicated plumbing systems will require a larger and more powerful pump than a smaller simple plumbing system.
A pump should be sized to overcome the head losses of the water heater and frictional piping losses. It is recommended that the pump be sized to provide 2 GPM of flow based on 10 feet of head loss plus additional losses. Aquastat and/or timer should be used to maintain warranty of product. Please also refer to Plumbing applications in the installation manual.
Below are some pump manufacturer’s who can properly assist you with sizing your pump:
Grundfos – https://www.grundfos.com
Taco – https://www.tacocomfort.com
Bell & Gosset – https://bellgossett.com
Aquastat
Thermal switch that is used in recirculation systems; shuts off the pump when it senses the return water has reached a predetermined set temperature
GPM
Gallons per minute
Timer
In a recirculation system, a timer is used to turn a pump on and off within a programmed window of time.
Head Loss
the measure of energy lost from a fluid as it flows through a pipe; typically expressed as “feet of head”