For an indirect heated hot water tank, the hot water comes from a boiler.

Why are there two output ports on my tank?

I have appended below a diagram of an HTP/Superstor tank to use as a reference:

enter image description here

A - emergency relief
B - outlet 1
C - thermostat
D - input to boiler
E - output to boiler
F - outlet 2

So, from the diagram you can see that there are two outlets. Does the hot water make a circuit and one is an outlet and one is an inlet?

2 Answers 2


It's because inside the tank there's a coil in which circulates hot water from boiler (without mixing with clean DHW). The boiler is connected in a 'closed loop'.

The 'loop' connections are on DE, clean water enters at F and hot water exits from B.

I think they call both 'outlet' because you could use the lower one also as outlet when you drain your system. Also input to boiler sounds little weird to me.

Another option is if that thank is used as 'cold' puffer for AC: heat pump cools water inside then water is 'recirculated' trough the fan-coils. In this case return from fan-coils should be plumbed on B and cold output to F

top to bottom:
1> hot water outlet (B)
2> input FROM boiler (D)
3> output TO boiler (E)
4> clod water inlet (F)

also check thisenter image description here


With indirect tanks the heated water from the boiler is pumped in high on the tank and cooled return goes back to the boiler. The cold domestic water goes in low (even if it enters at the top, inside the tank it goes directly to the bottom) and heats up and exits the top. This is called counterflow. This way the hottest water from the boiler can still transfer heat to the hottest domestic water in the tank. There are however, a great deal of configurations and some tanks are not separated internally.

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