# How Circulators Work 101



## Scott K (Oct 12, 2008)

First off, while it is not technically uncorrect to call a circulator a "pump," and many, myself included, use both terms freely at times, it is more correct to call a pump in a closed hydronic heating or cooling system a "circulator." 

So how do circulators work? How do they create flow? Lets not worry about the internal mechanics right now- I'll keep it simple for you and tell you what they don't do. They do NOT create pressure. But what they create is a DIFFERENCE in pressure. This difference in wording, while subtle, is critical to understanding how they work, and how they can work FOR you (and well, your customer, and their hydronic system). I'll dig a bit more into how you can make them work for you later. 

If I have 100 feet of pressurized pipe full of water, and there is 12 PSI at one end, and 100 feet away the pressure is 20 PSI, what do you think will happen? Yup, you got it - the pressure will try to equalize itself as the area of higher pressure will flow to the area of lower pressure. This is very much how circulators create flow, and it also explains the sublte difference in a circulator creating a difference in pressure (and not creating pressure). 

So imagine you're standing in front of a hydronic system next time and you charge it up to 12 PSI. You bleed all the high points as best you can, perhaps do a power purge, and you're confident most of the air is out. Then you flip the power switch and that circulator starts humming away. Now what is happening? Upstream of the circulator the static fill pressure (12 PSI) still prevails, however downstream of the circulator there may be 18 PSI, or 20 PSI, hell even 14 PSI. The bottom line is there is a difference of pressure downstream of the circulator, and this difference in pressure is what drives the flow of water to try and equalize it. As long as the difference in pressure is created by the circulator, water will always flow. How much will flow? Well that depends on how much of a difference in pressure is created by the circulator (on top of the design of the impellar). Bottom line is if you can wrap your head around this simple concept for hydronics, then you'll have no problem with 201.


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## ckoch407 (Sep 30, 2009)

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