In the manual for my SSU-119 indirect fired hot water heater, the following text seems to suggest that the circulator points towards the tank:

On the water heater, the boiler supply connects to the outlet of the circulator. The circulator inlet is to be connected to the hot outlet side of the boiler. Be sure that the arrow on the circulator is facing the correct flow direction. (See pressure drop sizing for circulator, this manual.) On the tank, the boiler return connects to the return side of the boiler. The return(s) from heating loop(s) should have a flow check or swing check valve installed before the return pipe from the tank.

However, a diagram later in the same manual seems to show the circulator flowing towards the boiler, not towards the tank:

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So, which is it? Does the circulator point towards the tank or the boiler?

2 Answers 2


More information is needed about the system to accurately answer the question. It depends on what the two manufacturers recommend. Some boiler companies specify return pump. So the boiler circulator pumping into the boiler, others specify a supply pump. The first thing to consider is how the boiler manufacturer wants the system configured. Most of the systems I design use a primary boiler pump on the return side with a heating system pump and a primary indirect pump on the return side. Both primary pumps equipped with check valves. In this case the boiler primary will be denergized on priority, on a call for domestic hot water. On a call for heat the boiler primary energizes.

This is an example of a recent job

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red is common piping, blue indirect, green central heating primary, purple central heating secondary, magenta central heating return

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this is prepped for 3 low temp zones and a high temp potable water zone of the Turbomax (indirect) The low temp are high mass radiant and the high temp is a fan forced hydronic coil

In this case I actually went against the manufacturer recommendations for piping to make everything fit. There's only about 50' height here.

What is important, is that when the DHW circulator is running, it's not causing flow in the heating system and vice versa.

There are other things to consider like where you are tapping your expansion tank, micro-bubbler and dirt separator (if required)

Basically you want the domestic circulator to act on its own loop and not affect the central heating side and vice versa.

This is the simplest way to configure it

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this works well on a single zones system. On a old-school system a "boiler bypass needs to be installed

Something like this would apply if using primary secondary piping though you could remove the injection pump if utilizing a full primary loop

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you must always ensure that your boiler has full flow


Usually circulator are 'pushing' towards the boiler to avoid cavitation. Also in the image above you have a 1-way valve that block the flow if circulator points toward the thank.

My question is: are 2 circulators really needed? wouldn't be better a 3 way valve and a single pump?

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