Flow is a consideration also, but generally a larger heater will handle a higher flowrate. A bypass, if not required by code, should be installed regardless for full control and ease of maintenance. Over 385,000 BTUH would usually be a large Commercial Swimming Pool. Up to 385,000 BTUH would be a residential Swimming Pool, or commercial Spa. Versa available in standing Pilot or Electric Spark Ignition RP2100 is a fully Electronic Ignition Boiler Electric Heaters are self explanatory.
Heat exchangers can be single or multiple to suit most any application.