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Head pressure and the longest circuit
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mferrer
Member Posts: 33
As I replay Holohan's Classic Hydronics Seminar DVD, I enjoy the analogies about drinking beer and cross country running, to selecting a centrifugal pump with enough head pressure to overcome the longest circuit of a hydronic piping system.
However, I sometimes struggle with trying to understand the total pressure drop of a hydronic piping system. According to Dan, if I can run a marathon I should have no trouble with running a 5K. That makes perfect sense. And if I have a HW heating system with, let's say, a main that feeds 5 circuits, my circulator needs to have enough pressure head to overcome the pressure drop of the longest circuit. It would therefore be able to circulate water through the other 4 circuits. That makes sense to me.
Here's where I get confused; If I think of pressure drop as friction loss from piping, fittings, valves, heat transfer units. etc., then I imagine that when all 5 circuits are open and running, wouldn't I have a lot more pressure drop as compared with just the longest circuit's worth of pressure drop?
If I am looking to size a pump according to the total pressure drop, shouldn't that include ALL of the piping, valves, fittings, etc.?
What am I missing? Am I looking at this in my minds eye in the wrong way? I appreciate any clarification. Thanks in advance!
However, I sometimes struggle with trying to understand the total pressure drop of a hydronic piping system. According to Dan, if I can run a marathon I should have no trouble with running a 5K. That makes perfect sense. And if I have a HW heating system with, let's say, a main that feeds 5 circuits, my circulator needs to have enough pressure head to overcome the pressure drop of the longest circuit. It would therefore be able to circulate water through the other 4 circuits. That makes sense to me.
Here's where I get confused; If I think of pressure drop as friction loss from piping, fittings, valves, heat transfer units. etc., then I imagine that when all 5 circuits are open and running, wouldn't I have a lot more pressure drop as compared with just the longest circuit's worth of pressure drop?
If I am looking to size a pump according to the total pressure drop, shouldn't that include ALL of the piping, valves, fittings, etc.?
What am I missing? Am I looking at this in my minds eye in the wrong way? I appreciate any clarification. Thanks in advance!
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Comments

oops. Fat fingers. That last phrase should read "less resistance to flow"Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 
The pressure drop will not increase as you add shorter loops. This is because they are in parallel . Maybe picture holding a syringe in each hand and pushing them both at the same time.The one one the left was not harder to push because you were also pushing the one on the right. You did move twice the volume of fluid.
If you have significantly shorter loops, you may need to add balancing valves (to add resistance), remember water is lazy like ME's brother in law. It will take the shorter path if you let it.
This screen shot shows flow rates of 50, 100, and 200 foot loops of 1/2" pex in parallel."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 
Zman,
Thank you! Your explanation makes sense to me. Parallel circuits are not adding pressure drop. The syringe analogy helps a lot.
When I look at the drawing I see 3 circuits. I am assuming the longest, 200 ft. circuit has the largest flow rate. The GPM of the system is the sum of the GPM of all 3 circuits. Is the pump head on the system gpm or the longest circuit?0 
Jamie Hall,
Did you post something previous to your last post?0 
The loops are in ascending order. The shortest being first. Because there is less resistance in the shorter loops, more water can be moved with the same amount of force.
If even flow was a requirement, you would install balancing valves on the first 2 to add resistance."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 
Yeah, I did. But no matter  @Zman came up with just as good an explanation, if not better...steamfitter said:Jamie Hall,
Did you post something previous to your last post?
But  in general, the longest loop will have the least flow rate, not the greatest, as its flow resistance will be greater.Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 
And technically speaking there is no such thing as head pressure, although the two words often get used together.
https://www.pmmag.com/articles/101174siegenthalertheresnosuchthingasheadpressureBob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 
@mferrer you have a good question. I don't think I have a good scientific answer for you though. I may be wrong, but I'm sure it has to do with it being a closed loop system. As soon as that pump comes on, water begins to move in the entire system. Since there can be no magic "empty space" if water in a closed loop is leaving the pump, water coming back from that loop must be entering it, therefore you only need enough head to overcome the highest resistance loop. Lets say you have three loops, one with a friction loss of 10 ft, one with 20ft and one with 30ft. If the pump can push enough water to overcome that 30ft friction loss, it will also be pushing hard enough to get water through the other loops.
I'm trying to think of a good analogy. Lets picture this. You have one closed loop. The loop goes straight up, over, and back down into the beginning at the bottom. A big rectangle. Since it is a closed loop, the water that goes up is balanced by the water that comes down, like a Ferris wheel. You don't have to lift the water, you just have to circulate it. Imagine using your hand to push that water upwards. Let say you have to push with 2 pounds of force to overcome the friction and circulate that water with your hand. Now, add a second circuit to that like a rung on a ladder. Now you have a rectangular loop with a rung in the middle which is the second circuit. Do you think you would have to lift any harder to circulate water through the longer run now that there is a second loop in the middle? You would still be applying the same 2 pound force to the water to get it to flow through the longer circuit.
That made sense in my head but I have no idea if it actually makes real sense.Never stop learning.0 
The pump must overcome the piping circuit that has the most RESISTANCE. This is usually but not always the longest loop.
Example:
you have a long loop of baseboard for the first floor of your home. That's one zone
Your second zone is an air handler on the 2d floor the actual pipe run to and from the AHU may be shorter but don't forget the AHU coil has a lot of tubing in the coil and a lot of turns.
In this example like all others don't guess. Calculate everything as best and as accurately as you can. If you do this it WILL work 100% of the time. I have fixed many many jobs over the years just by putting in the necessary time on this. No shortcuts.
One job had a HW zone off the steam boiler using the tankless heater as the heat exchanger. The thing never heated, they had at least 6 contractors look at and work on this problem. no one could make it heat.
All they had to do is open there eyes. The solution was obvious. Most tankless coils have 1/2" connections. There is a mile of 1/2" tubing inside that coil.
A larger pump with more head fixed it1 
Thank you all for your valuable input!
Discussion about flow resistance and flow rates helps clear things up for me.
The part about the shorter circuit/loop having less resistance and a higher flow rate helped me to see what I was missing before.
Thanks again!0 
The ferris wheel analogy is perfect to describe the difference between a circulator in a closed system and a pump in an open system. Dan has a wonderful way of explaining details.0

Longest loop is not always highest pressure drop... But pump must generate adequate pressure differential for highest PD. And, you are supposed to add the pressure drop of the larger bore manifold and near boiler piping based on its length and expected flow. So its not just about the longest loop...
METhere was an error rendering this rich post.
1 
Thank you Mark!
I will try to remember that very important information as well.
So the longest loop doesn't necessarily produce the highest pressure drop. Shorter loops can have higher resistance based on the type and amount of fittings, valves, heat emitters, etc.
Interesting and very useful info.
I appreciate it!0 
Think like water!0

There are a few steps involved to determine the head loss in a circuit. We do an example in Idronics 16.
Basically add up all the pipe and devices in the flow stream, come up with a single number called EL equivalent length.
Type of fluid and temperature needs to be entered.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0
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