How to Size a Tankless Water Heater This video will walk you through the preliminary steps in helping you in size for a tankless water heater. Updated on April 10, 2021 Tagged: sizing Was this article helpful? Yes No Related Articles What size pump should I use on my recirculation system? How to Install an External Remote Connecting a 6 Unit Internal System Controller How do I raise the temperature above 120F? What if there is a power outage? What maintenance is recommended?