# How Circulators work 201



## Scott K (Oct 12, 2008)

Now if you feel confident you understand How Circulators work 101, here is the next level - 201. 

So Circulators create a difference in pressure and this is what drives flow. 
Now, the difference in pressure that is created between the static fill pressure upstream of the circulator (say 12 PSI as an example) and the pressure immediately downstream of the circulator is what drives the flow. But how much flow occurs? 

The amount of flow that occurs is dependant on the design of the circulator and it's impellar first off, but lets not get involved in that because we don't make circulators, we just buy 'em and install them. The amount of flow that occurs also depends on what you are trying to push water through. I'm talking the piping, elbows, components, accessories - anything where this difference in pressure has to move through to create flow. The friction encountered versus the difference in pressure created by the circulator is ultimately what determines how much water will flow. 

So this forms the basics of selecting pumps with pump curves. Now let me warn you first and foremost that this CAN get quite technical. There are all sorts of math formula's once you get into bigger systems to determine friction loss (also called "head loss") and all that. But in residential systems it doesn't have to get THAT technical. When you look at a typical pump selection graph, it will have two headings. Typically the vertical line will be head loss, often in feet of head (which is easy enough to convert to PSI - 1 PSI = 2.31 feet of head or 0.433 PSI = 1 feet of head), and the horizontal line will be the flow in US Gallons per minute. You have to determine how much flow you need first off, and then determine what you are flowing it through. Then you can determine your head (friction) loss (this requires a bit of research on your part). If you know that the Base boards produce this many BTUH's at 1 GPM and this many BTUH's at 4 GPM and your base board circuit is 100 feet long, you can quite easily work with a head loss table. You pick your flow rate, and figure out the head loss of your circuit, and then select the appropriate pump curve in the table that matches your head (friction) loss and flow rate as close as possible. 

This is the basics - at the very least I hope that you now at least understand, or at a minimum respect where pump sizing and selection comes from. It would be so easy to select a larger pump, and flow more water than needed in most cases. However you are quite honestly wasting electricity, increasing the cost of the installation, and potentially altering efficiency and/or reliability of the heat source.


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## Epox (Sep 19, 2010)

Moving water too fast can also deteriorate copper fittings as explained to me by an engineer I called for consult on apartment complex's system that was having problems with the end of line units having luke warm water.


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## plumbpro (Mar 10, 2010)

mpsllc said:


> Moving water too fast can also deteriorate copper fittings as explained to me by an engineer I called for consult on apartment complex's system that was having problems with the end of line units having luke warm water.


Water velocity should not exceed 8 fps in cold water systems, 5 fps up to 140*, and 2 fps over 140*. Higher velocities cause erosion in copper piping. Not sure if it would apply to plastics.


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## klempner (Mar 4, 2011)

Thanks for your help, but I'm still lost. Let's say it's an open system, one-story. supply and return both run underground. What is the head loss, just from elevation, not counting friction? 4 or 5 feet?

Let's say it's a two story, open system. supply underground, return between floors. Head loss? 12' pr so?

I called Bell and Gossett, and they simply could not tell me.

I sometimes hear that since rise and fall equal out, there is no head loss from elevation. But sometimes I hear that that only applies to a closed system.

Does an open system become a closed system with a check valve?

Would love to get a definitive answer.


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