Design/Theory question: dP pump on primary and secondary loop

ThatF_inGuyInNY

If a primary loop has a dP pump and the secondary loop has a dP pump, and they are both set to maintain the same constant pressure, will the primary pump always match the flow of the secondary loop?

I think the answer is yes, but want to make sure Im not missing something. Does this have implications for hydraulic separation (i.e. its not needed, or less is needed).

EdTheHeaterMan

If piped correctly, the primary pump should have no effect on the secondary loop and the secondary loop should have effect on the primary loop. So your question, although a good question, really has no basis for discussion. So I believe the answer is no. But @hot_rod has earned the Hooligan-Carlson award, he should know.

Jamie Hall

Np. Why would they? The primary pump is seeing the head loss around the primary loop. The secondary loop pump is seeing the head loss around the secondary loops. Each will adjust its own flow to produce the head loss which they are set to maintain.

Let's suppose, for example, that someone has throttle the secondary loop way down -- say one loop 800 feet long of 3/8 PEX. Meanwhile the primary loop -- pump, manifold, boiler, and back -- is wide open. The secondary loop pump will throttle back to maintain its set delta P. Very low flow. Meanwhile, the primary loop pump willl be happily churning away maintaining its delta P.

ThatF_inGuyInNY

EdTheHeaterMan said:

If piped correctly, the primary pump should have no effect on the secondary loop and the secondary loop should have effect on the primary loop. So your question, although a good question, really has no basis for discussion. So I believe the answer is no. But @hot_rod has earned the Hooligan-Carlson award, he should know.

Yea, after I read through it, I started thinking it wouldnt matter since the P/S are separated by nature.

I guess the question I am really asking is if its not P/S and two dP pumps are in line, would they match flow rates?

And if separation is inadequate, could a dP primary adjust to accommodate it

hot_rod

Series P/S as in fig. 5-8?

Jamie Hall

If you have two pumps in series -- that is, the outlet from one pump goes more or less directly to the next, with no additions or subtractions, of necessity they will have the same flow rate. However, two delta P pumps connected that way will fight each other. How badly will depend on the damping in the whole rest of the system and in their own controls, but they will fight. One will sense a low delta P and start to ramp up. The other will then sense a high delta P, and start to slow down. This may be stable, with one pump doing all the work and the other doing nothing, or it may oscillate back and forth -- but unless the pumps are fantastically well matched, and nothing disturbs the system, they will do one or the other.