An indirect water heater is a tank that gets heated by a heating system. So, on this side it's just like a zone inside your house like a thermostat. So, a heating system sits on the floor. An indirect tank gets heat from that heating system and makes hot water. So as you are drawing water out of the tank, like a zone, it tells the heating system to fire up and give you more hot water and sends hot water through the tank and delivers hot water almost instantaneously.