When you run a water heater JUST for the pond, and you don't take off hot water from your home water heater, you have to create a closed system. You see, some people hook up their residential water heater to the exchanger and it keeps your bathtub and your pond supplied with hot water. This is not ideal. When you have the water heater dedicated to the pond, it stays hotter, and uses less energy. But, you don't have fresh water in the heater all the time so it creates a special challenge to blow out the last bits of air and get the system primed.