# How Circulators Work 301



## Scott K (Oct 12, 2008)

Now if I told you that a properly placed circulator was the most important air removal device in a hydronic system right now you'd probably laugh at me and tell me that the air seperator does that. 

In this chapter I'm going to hopefully put it all together so you can understand the relationship of the important "things" in a hydronic system and how you can make a Circulator work FOR you. 

First off - some definitions:

1) Circulator - provides flow of water to overcome head/friction loss of piping by creating a difference pressure at a required design flow rate

2) Expansion tank - helps maintain static fill pressure of the water in a hydronic heating system at it's connection point (e.g. tee) to the system.

3) Pressure reducing valve (feed water make up) - reduces the water supply pressure from domestic pressure to the system static fill pressure; the setting on your heating system PRV (e.g. 12 PSI) should be the exact same as your expansion tank air side setting (again, 12 PSI). 

4) Air Seperator or purger (e.g. Microbubble resorber such as a Spirovent) - removes air pockets and mircobubbles as water is circulated through it. 

5) Heat Source - e.g. Boiler

6) Heating distributin and emitters - the heating mains (supply/return) 
that move water to the heat emitters (e.g. baseboards, radiant floors). 

7) Pressure relief valve - a valve designed to discharge water if the system static fill pressure or local dynamic pressure moves above it's discharge rating (e.g typically 30 PSI). 

To be continued tommorrow (it's late).


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## Scott K (Oct 12, 2008)

1) Expansion tank/Feed make up water (heating system Pressure reducing valve) relationship. 

Because the job of the feed water prv is to supply water at a static fill pressure at say 12 PSI, and the expansion tanks job is to try and maintain that pressure as water undergoes some thermal expansion as it is heated up in the system, it only makes sense that you connect them to the system at the same point so they can work together to try and maintain the static fill pressure. 

Now this point - the tee that ties them into the system, or perhaps its the tapping at ther bottom of your air purger or seperator is called the "point of no pressure change." If you have to remember anything I've told you - this is IT. The "point of no pressure change." What does that mean? The pressure where the expansion tank and feed water valve tie into the system can NOT change. That tee and surrounding area will always be 12 PSI. 

2) Now, here's the next concept I'll throw at you - the Circulator and point of no pressure change relationship. Where do you put your pump relevant to ther point of no pressure change? Well if that point can not change (12 PSI) and my pump creates a difference in pressure (adds pressure to the inlet/upstream pressure to create more pressure downstream to induce flow by overcoming the friction loss of the piping) where do I install my pump in relationship to the point of no pressure change? 

Well lets think about some things here first. If the pump creates a difference of pressure, and the area at the point of no pressure change can not change pressure, how would my pump create flow if I was pumping towards the point of no pressure change? Oh it still can, but it creates flow by dropping the pressure upstream of the pump. So instead of 12 PSI upstream of the pump (static fill pressure), since you're pumping at the area that will always remain at 12 PSI, you'll now have 12 PSI downstream of the pump, and there will be reduced pressure UPSTREAM of the pump. The suction or vaccuum of the impellers will drop the pressure upstream of the static fill pressure to help the pump achieve flow. So instead of 12 PSI upstream of the pump, you might have 6 PSI. This 12 PSI downstream (PONPC) and 6 PSI upstream is what will allow the pump to create flow.

So what is wrong with this picture? Why dont' you want to pump towards the point of no pressure change? Well in certain instances, if you have a higher head pump, (say one that produces more than 12 PSI/28 ft/hd), it can actually drop the suctin pressure to a negative pressure. You'll have a vacuum on the inlet of the pump. And what do vacuums do? 

Well if you're thinking about air - by now you're realizing that a vacuum - pressure below atompsheric - could actually cause air seperators and air purgers to suck air into the system instead of remove air. And do we want air in a heating system? ABSOLUTELY NOT!

So now lets pump away from the point of no pressure change. Voila - the static fill pressure just upstream of the pump - perhaps where my air purger or seperator is located as well as I tied my PONPC into it on it's tapping - is now 12 PSI. And my pump adds it's head (i.e. 6 PSI) and you now have 18 PSI downstream of the pump due to the pump creating a difference in pressure, and voila - the pumps head overcomes friction loss in the piping and flow is achieved. 

3) Now this is the 3rd big concept in this chapter - why is a pump an important air removal tool? 

When you have mircobubbles and air pockets in a freshly fired up system that still need to be removed (because air is the enemy), what happens when you pressurize water containing air? Well the air pockets collapse and sort of "integrate" themselves more with the water. No the molecules don't alter the chemistry of the water, but the pressure of the water does allow the water to contain the air better. I'm not going into chemistry or physics, this is on a strictly need to know basics. So if you have 12 PSI static fill pressure, and your pump adds it's 6 PSI as an example you can now work with the pumps added head to help pressurize the water a bit more and push the air through the system. The air pockets that were sitting there when the system was stagnant at 12 PSI are now entrained with the flow of more pressurized water created by the pump and pushed to the point of no pressure change where hopefully your air purger or seperator is also located, where they will now come out of suspension into this lower pressure area and get removed. 

4) Now velocity - velocity is another relationship that requires formula's to figure out. Generally speaking, there are velocity versus flow rate charts that area available, that as you start to do this stuff more and more, you'll probably remember off of the top of your head. Or perhaps there is a John Siegenthaler article that points this out (I know it's in his book). But suffice to say in a certain size pipe at a certain flow rate (gallons per minute) the water will be moving at a certain velocity. Generally speaking, gurus and engineers in ther industry spec 2 to 4 feet per second as the magic velocity that seems to work best at helping push air through a system. This does not mean that if you have velocity below 2 feet that air can not become entrained and pushed out, nor doeds it mean above 4 feet per second it won't get removed out. This is just the relaying of information. Suffice to say I know off of the top of my head that 3/4" M' pipe, when flowing 3.2 GPM is about 2 feet per second, and at about 6.5 GPM is pushing 4 feet per second. But that's off of the top of my head. 
Don't get too caught up on this though - you're not an engineer or designer (well some may be, but you get my point).


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## Epox (Sep 19, 2010)

:blink: I was on the phone, could you repeat that ^^^^? jkkkkkkk this is very interesting. Dumb question here. So once you have the system installed how are you knowing the exact flow you have achieved.


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## DIZ (Nov 17, 2010)

Enjoying this


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## plumber666 (Sep 19, 2010)

Oh come on! You're making that up!!:jester:


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## sigshooter71 (Dec 8, 2010)

Just read dan holohans book pumping away.


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