Injector confusion

Spock

I get the "pumping away" concept, but I don't understand how an injector loop between two primary loops can pump away from both of them at the same time. What am I missing?

Ironman

One Primary Loop

In a properly configured system, there can only be one primary loop: it's the one with the PONPC where the expansion tank is connected. All other loops are secondary, tertiary, quaternary, etc.



Every other loop "sees" the entire primary loop as the PONPC.

Spock

Two buildings....

Yes, I get that.



However, I'm wanting to share heat between two buildings. Each has its own primary/secondary system in place.



Are you suggesting that the existing  expansion tanks need to be removed and replaced with just one large tank in one building so that there is only one tank in the system?

Gordy

So you have

Two boilers? One in each building also.

Spock

Two Boilers, and maybe the answer

Yes, there are two boilers: Geothermal in one and electric in the other.



I think I may have figured it out though. The answer is that the injector loop has no effect on either system and so it just does not matter which way its circulator directs the flow.



Each end of the injection loop terminates in a pair of closely spaced T's: Zero pressure change in the common pipe. Zero pressure means no effect.



Thoughts?

JStar

Pressure

You got it!



Keep pumping away, even if not directly from the expansion tank.

Gordan

Not quite.

From the point of view of static pressure, any number of pipes connected at (or about) the same point will have the same effect, and act as a single larger pipe: they will allow for equalization of pressure between the two circuits. If you connected your expansion tank to the system using a pair of closely spaced tees, it would have the same exact effect as it does when connected with a single pipe.



From the point of view of circulation, you're correct; the closely spaced tees allow for hydraulic decoupling so that the injection circulator will not be able to induce flow in either of the two circuits that it's circulating between, it will only be able to induce flow in the injection circuit itself. But that's not what "pumping away" is all about. Pumping away is about making sure that a circulator has the greatest available absolute pressure at its inlet, in order to be as safe as possible from gaseous cavitation. As you stated, yes, the clear case of pumping away would entail having a single place where an expansion tank and a water feed connect to the system, and placing each of the circulators at the point "closest" downstream from the connection to the expansion tank (or a circuit that leads to the expansion tank.) That would put an expansion tank on one side of the injection circuit but not both. This can create issues if you can foresee wanting to isolate the two circuits from each other, as you'd cut yourself off from the expansion tank.



In reality, however, it's probably just fine to locate the injection tees so you can pump away from the injection circuit AND the expansion tanks on either side, and pump away from the closely spaced tees (and into the pipe connecting to the other system) on whichever side of the injection circuit it makes most sense to put the injection circ.

Spock

Got it!

Thanks for all the input and help!



And thanks for the detailed clarification Gordon!