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circulators and the PRI/SEC concept

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Member Posts: 296
just musing, but something came to me that may be common knowledge and Ive been clueless or its a real consideration. knowing there should be no, or very little flow in the common pipe between the "closely spaced tees" this question came to me. if there is no flow in the common pipe then for all practical purposes they are no longer tees but effectively ells. and following that logic this puts the circulator on the primary loop in series with the circulator on the secondary loop. should this be considered in circulator selection as now the developed head is increased? (sorry about no paragraph breaks, user error using this laptop)

• Member Posts: 8,166
edited January 2023
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There is flow in the common pipe. Unless the pump on the primary loop is pumping the exact same amount of water as the secondary loop. If the boiler is the secondary and is pumping say 10 GPM and the primary is moving only 8 GPM then the water in the common pipe is moving in reverse to add heat to the return. If the Primary is moving 10 and the boiler is only moving 8 then there is 2 GPM moving in the common pipe.

The concept is to allow the secondary to operate with some Hydraulic separation so the system can operate separately from the secondary. If by coincidence the primary pump and the secondary pump move the exact same amount of water, that does not interfere with the separation and need not be calculated differently.

Or does it? Maybe there is the beginning of a black hole in that common pipe where nothing can exist.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 296
edited January 2023
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yes Ed I know there will always be some flowing the common pipe. my mistake for not clarifying this is a hypothetical. more to the point of this rambling is two circs in series causes the circs to add there heads yet the flow remains the same. I'm not questioning hydraulic separation
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ch4man said:

yes Ed I know there will always be some flowing the common pipe. my mistake for not clarifying this is a hypothetical. more to the point of this rambling is two circs in series causes the circs to add there heads yet the flow remains the same. I'm not questioning hydraulic separation

I know that 2 head are better than one, But pumps in series just don't add up that way.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

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And in some cases there is two direction flow between the tees

I built some clear plastic closely spaced tees to do some demos of that concept. It’s in one of the videos around here somewhere.

pumps in series do double, or nearly double head
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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If the two pumps are in fact moving the exact same gpm no flow between common tees. Pumps in series the head capacity of the pumps is additive. The GPM will not be additive if the pumps sometimes are in series and sometimes in parallel with changing flow in either circuit.
• Member Posts: 296
edited January 2023
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hot_rod said:

And in some cases there is two direction flow between the tees

I built some clear plastic closely spaced tees to do some demos of that concept. It’s in one of the videos around here somewhere.

PUMPS IN SERIES DO DOUBLE, OR NEARLY DOUBLE HEAD

thanks Bob. case in point is where a early Munchkin was replaced with a NTI FTVN. installer did nothing to the near boiler piping design other than connect to existing. the Munchie had the pump on the return, NTI has a built in pump and relief just on the other side of the heat-exchanger.

I've had a few with cronic weeping or just plain opening up the relief valves. NOW BOTH OF THESE WERE PIPED DIRECTLY NOT P/S one was solved buy cranking down the onboard pump to minimum, one yesterday we just pulled the existing pump and let the NTI take over.

so Bob will a hypothetical situation of no flow in the common pipe essentially link circulators in series?, how much head loss needs to be between them to make this a moot point?
• Member Posts: 23,635
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If you have the system head demand curve available, and the pump curves for the two pumps also available, you can determine the total flow for either a configuration for two pumps in parallel or two in series rather easily. You will find that two pumps in parallel almost double the flow at increased pressure -- but not quite -- and two pumps in series almost double the head at increased flow -- but not quite.

However, if you throw a couple of connected loops in there with a pump on each loop -- which is hydraulically what primary/secondary piping does -- the computations, while still possible, become remarkably difficult. Depending on the head demand curves of the two loops and the head/flow curves of the two pumps, you may find that water flows one way through the connecting pipe, or the other -- or not at all -- and any change in any of the three curves may change the flow. It gets interesting...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England