# Circulator zoning.. S or R?



## whitey

Ive been doing plumbing/heating for more years than I care to remember and have built boilers using the circs on the supply and return but in most cases where the circulators were put on the supplies it was mainly just because that's how it was spec'd as well as being IFC circs. Usually it was on condensing boilers like the burnham alpine or munchkin when ive installed them on the supplies. 

Now I have a customer/friend who wants me to install a burnham V8 in his sons house but went out of his way to get me to order the material to install the circs on the supply, which honestly I could care less either way but in this application it would be easier for me to put them on the returns well as look better.

My question is, is it more than a just preference? Is there a reason its better to put the circs on the S or R or is it application specific? I actually felt kinda stupid for a second because as long as I've been doing this, I really had no answer if it was a better or not.


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## Radium

I am not the best heating guy in the world, but i thought pushing the flow into the expansion tank resulted in loss of pressure to the system. As for supply or retirn on the boiler i am the same way, just go with manufactures specs.


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## OldSchool

It doesn't really matter if its one the supply or return... its a closed loop system...

There is one good reason and that would be a circ pump can not move air...

If the pump is on the supply and air gets in there then nothing is moving...

If it is on the return at the bottom of the boiler then there is a greatly likely hood that the pump will pump even if the line gets air locked


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## CTs2p2

I like the pumps on the supply pumping away from the extrol tank, this is the spot in the system where there is no pressure change... There is many supporting articles on the topic (probably threads too). There are definitely swap out applications (mono flow systems, split return ) where it just makes sense to keep them on the return (to me anyways)... One of the other reasons that installers used to put them on the returns is because they believed the pump would last longer moving with the cooler return water.


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## futz

Pumps on supplies *always*.


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## OldSchool

futz said:


> Pumps on supplies *always*.


Please explain this.... why supply always....


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## Mxz--700

CTs2p2 said:


> I like the pumps on the supply pumping away from the extrol tank, this is the spot in the system where there is no pressure change... There is many supporting articles on the topic (probably threads too). There are definitely swap out applications (mono flow systems, split return ) where it just makes sense to keep them on the return (to me anyways)... One of the other reasons that installers used to put them on the returns is because they believed the pump would last longer moving with the cooler return water.


You said it. Supply is the way to go. Hell u can put a spirovent on and circs on feed and it self purges.


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## OldSchool

Spiro will take out the majority but not all of the air..... Also every time you add new water to the system eventully the air entrapped in the water is released in the boiling or heating process of the water..

Pumps on the supply ... I can see on an injection type of system... Primary loop would not matter.... IMO


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## CTs2p2

I usually only use the spiro on radiant jobs because of the cost, otherwise a regular air scoop with a high vent works fine.. Pipe water feed in-between the expansion tank and air eliminating device.

Those older 3 piece circulators moved a lot more water than the cartridge circs we use now.. Newer circs create a pressure difference to compensate. Those little micro-bubbles stay suspended and are easier for the pump to "move" when they are on higher pressure side (not saying the pressure dif is large but it is there).


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## futz

OldSchool said:


> Please explain this.... why supply always....


Makes the system purgeable, both when new and for service. A pump can push air through the circuit (with water) to a purge valve or air vent. But when the pump is on the return it's pulling water through the circuit. The instant the pump sucks an air bubble it stops pumping and you're finished. 

I've spent many, many frustrating hours on service calls trying to purge systems built with pumps on the returns. Takes forever, and seems like the air bubbles never stop coming.

On my systems, with supply side pumps, I just boost the feed pressure a bit, turn the pump to top speed (if multi speed), open the purge valve a little (sometimes more) and let the pump blast all the air out. On multi loop radiant circuits I'll rough purge one loop at a time. It bubbles vigorously for a while, gradually tapering down to finer bubbles. When it smooths out to just the occasional fine bubble I adjust to normal settings and let the air eliminator do the rest. Easy.


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## ZL700

futz said:


> Makes the system purgeable, both when new and for service. A pump can push air through the circuit (with water) to a purge valve or air vent. But when the pump is on the return it's pulling water through the circuit. The instant the pump sucks an air bubble it stops pumping and you're finished.
> 
> I've spent many, many frustrating hours on service calls trying to purge systems built with pumps on the returns. Takes forever, and seems like the air bubbles never stop coming.
> 
> On my systems, with supply side pumps, I just boost the feed pressure a bit, turn the pump to top speed (if multi speed), open the purge valve a little (sometimes more) and let the pump blast all the air out. On multi loop radiant circuits I'll rough purge one loop at a time. It bubbles vigorously for a while, gradually tapering down to finer bubbles. When it smooths out to just the occasional fine bubble I just adjust to normal settings and let the air eliminator do the rest. Easy.


All the right reasons and more, confirmed by all hydronic experts besides.
Circulators produce pressure differential, it's best to use the increase to push air which is decreased in bubble size under higher pressures versus expanded under lower pressures. These pressure differentials are lost in the mass of the boiler. Air elimination on inlet side of circ, where lowest pressure of system is measured between boiler and circ(s) is best placement of air elimination, fill and expansion tank.


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## trick1

You can go to the Bell and Gossett Web Site and surf around their technical literature for a while.

There was an engineer that worked there in the 50's...his name was Gil Carson. 

He came up with the "Point of no Pressure Change" theory. It explains why the pump should always be on the supply side of the system.

Dan Holohan, who provides seminars and training on old heating systems wrote articles on the guy and he was a genius for his time.

Read through some of the stuff at the B&G website and it will save you a lot of headaches when it comes to air entrapment issues.


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## mtfallsmikey

trick1 said:


> You can go to the Bell and Gossett Web Site and surf around their technical literature for a while.
> 
> There was an engineer that worked there in the 50's...his name was Gil Carson.
> 
> He came up with the "Point of no Pressure Change" theory. It explains why the pump should always be on the supply side of the system.
> 
> Dan Holohan, who provides seminars and training on old heating systems wrote articles on the guy and he was a genius for his time.
> 
> Read through some of the stuff at the B&G website and it will save you a lot of headaches when it comes to air entrapment issues.


Get Dan's book "Pumping Away"...it explains it all.


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## bikefitter0

Trick 1 is right. Point of no preasure change is the key. Old school plumbers did pump on return because thats where it came mounted for ease of shipping. Think about it. If dealing with circ on return line watch the preasure guage bounce the split second the circ kicks on. Than watch on when circ on the feed, dosen't happen


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## vinpadalino

Pumping away is the best. But it's not pumping away from the boiler it's pumping away from the expansion tank. Something to do with better pressure.


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## OldSchool

futz said:


> Makes the system purgeable, both when new and for service. A pump can push air through the circuit (with water) to a purge valve or air vent. But when the pump is on the return it's pulling water through the circuit. The instant the pump sucks an air bubble it stops pumping and you're finished.
> 
> I've spent many, many frustrating hours on service calls trying to purge systems built with pumps on the returns. Takes forever, and seems like the air bubbles never stop coming.
> 
> On my systems, with supply side pumps, I just boost the feed pressure a bit, turn the pump to top speed (if multi speed), open the purge valve a little (sometimes more) and let the pump blast all the air out. On multi loop radiant circuits I'll rough purge one loop at a time. It bubbles vigorously for a while, gradually tapering down to finer bubbles. When it smooths out to just the occasional fine bubble I adjust to normal settings and let the air eliminator do the rest. Easy.


The only reason the air went through the pump is because you purged it. Try getting that to work with out purging with pump on supply.


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## futz

OldSchool said:


> The only reason the air went through the pump is because you purged it. Try getting that to work with out purging with pump on supply.


What??? I don't understand what you mean by that.


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## OldSchool

Purging a system you need pressure. By adding pressure from the boiler feed. Don't even need a pump running (if the system is designed properly.


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## futz

OldSchool said:


> Purging a system you need pressure. By adding pressure from the boiler feed. Don't even need a pump running (if the system is designed properly.


I've heard people say that for years (purge without the pump), but I've never found it to work worth a damn. If it works for you then I guess it works for you. Doesn't for me, or I guess I just never bother trying it, because whenever I did try it (long ago) it didn't work.


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## Scott K

*Circulators, not pumps.*

First off, if you don't understand why you put the circulator on the supply side of a system, as was already mentioned, you should buy Dan Holohan's book Pumping away - it's worth every penny (I think it's only $10 if memory serves). You can buy it off of his website on heatinghelp.com. It explains it to you in plain easy to ready language that anyone can understand. 

Circulators do NOT create pressure, but they do create a difference in pressure. If your static fill pressure is 12 PSI and the circulator creates 6 PSI of differential pressure, there will now be 12 PSI upstream of the circ, and 18 PSI (12 static + 6 differential) downstream. The difference in pressure between the inlet and oulet of the circulator as it does it's magic is what drives its flow. This is the basics of how circulators in a closed hydronic system work. Remember though - they do not CREATE pressure in this application, they only create a difference in pressure. This is a critical detail that Dan Hololhan elaborates more on, in his book. 

Now to elaborate further - the amount of flow that a circulator actually makes depends on how much differential pressure it creates. If there is a LOT - say it adds another 20 PSI, then there will be a lot more flow. The differential pressure it creates however, does not add pressure to the system; instead, as water flows, the differential pressure dissipates as the water flows through the various elbows, piping, and compenents as it does work. The less friction and things that dissipate this pressure differential, the more water flows. The more friction, the less water flows. Now you're getting into the pump impeller design - which is way over most of our heads, myself included. 

When you look at water that contains air - air tends to seperate from water in 2 areas - areas with less pressure, and in warmer water. As soon as you pressurize water, it tends to collapse those water bubbles. This is why it's a good idea to put your circulator on the supply side. If you have air bubbles in a static filled system at 12 PSI, then you turn your circulator on, the differential pressure it creates will help to collapse some or all of those bubbles and move them to lesser pressure areas of the system i.e. your air seperator. This is why it's always a good idea to put an air seperator downstream of your boiler on the supply side (warmest water) and upstream of your circulator (least pressure zone). These allow your air sepeator to do it's best work in removing air and avoiding call backs. 

Now the next question is where do you connect your expansion tank. No matter what - the point that you connect your expansion tank to the system (whether it's a tee or to your air seperator tapping) can NOT change in pressure, typically. If you set your expansion tank air side at 12 PSI, and your fill valve at 12 PSI and connect them to the same tee or to the air seperator tapping, that point that they connect to the system is the point of no pressure cha ge. It is what it says - the pressure there can not change. So, this is one reason why you ALWAYS put your circulator downstream of the point of no pressure change typically or if you have it on the supply of your primary/boiler loop, you put your circulator on the return pumping into the boiler where it can dissipate all it's pressure differential into the often high pressure drop zone of the heat exchanger of many condensing boilers. If you place the circ d/s of the point of no pressure change, it will add it's 6 PSI or whatever to the static fill pressure (i.e. 18 PSI total) and push air around until it gets to the reduced static fill connection at the point of no pressure change where the nice warm water and reduced pressure will let that air seperator kick the air out and then replace it with water from the feed water valve. 

Now if you decided to not follow this advice which is proven and pump towards the point of no pressure change - well remember one thing - the circulator doesn't care how it creates flow, it's always going to create a pressure differential no matter what. So how it does this pumping towards a place that pressure can not chagne is it drops the pressure on the inlet and it just pumps towards the point of no pressure change. So you may have 12 or maybe at best 13-14 PSI downstream of the circ, but you'll have maybe 6-8 PSI upstream of it. All fine and dandy - flow is created still as there is a difference in pressure. But lets pretend we put a high head pump in there (one that creates a lot of differential pressure). That pump could drop the inlet into a vacuum condition - in the negative pressure zone. This is the point where you start sucking air in through your air seperator and air vents and not removing it. Further to this - you don't have favourable conditions in the system to help push air - instead of adding pressure differential, you're subtracting static fill pressure to create a differential pressure, which is not favourable to collapsing bubbles and moving them to your air seperator.

To understand why you may want to pump into a condensing boiler, look at what a circulator does. In this application, most condensing boilers have some, or a moderate to high amount of friction in their heat exchangers that a circulator must overcome to move water through it. This is very typical of your modern water coil type heat exchanger like that found in Viessmann boilers or boilers that use the Giannoni heat exchanger.

In these heat exchangers, they are fairly low mass (low water content) with thin stainless steel walls (in most, some have aluminum heat exchangers) that are designed to maximize heat transfer and extract as much heat from the flue gas condensation as possible. These heat exchangers are not friendly to pockets of air or low water or low flow conditions, where steam could form if you are not careful due to the proximity of the exchanger walls to the main burner. A good method to protect the heat exchanger is to pump into it where you pressurize the water in the exchanger to help push those air bubbles out to the air seperator and get rid of air quickly. Since there is significantly more fricition in the heat exchanger travel for the circ to over come, compared to the piping between the outlet of the air seperator and the inlet of the boiler/primary circ, you could argue you are still technically pumping away from the point of no pressure change.


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## gtmechanic

*cavitation*

In my opinion, pumps must be installed in the point of the system with lowest temperature, because boiling point of the water depends on water pressure, and at the back side of impeller blades is the zone of low pressure right on the impellers surface, were water starts to boil , creating water hammer and eating away impeller material.that is why pumps should be installed on return. as per pumping away, it is pumping away from the expansion tank, not from boiler.


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## OldSchool

gtmechanic said:


> In my opinion, pumps must be installed in the point of the system with lowest temperature, because boiling point of the water depends on water pressure, and at the back side of impeller blades is the zone of low pressure right on the impellers surface, were water starts to boil , creating water hammer and eating away impeller material.that is why pumps should be installed on return. as per pumping away, it is pumping away from the expansion tank, not from boiler.


I agree on the above statement*.*

* I dont think any one took pump cavitation into account.... it would be more likely that pump cavitation would occur at higher temperatures. *


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## plumber666

Scott K said:


> First off, if you don't understand why you put the circulator on the supply side of a system, as was already mentioned, you should buy Dan Holohan's book Pumping away - it's worth every penny (I think it's only $10 if memory serves). You can buy it off of his website on heatinghelp.com. It explains it to you in plain easy to ready language that anyone can understand.
> 
> Circulators do NOT create pressure, but they do create a difference in pressure. If your static fill pressure is 12 PSI and the circulator creates 6 PSI of differential pressure, there will now be 12 PSI upstream of the circ, and 18 PSI (12 static + 6 differential) downstream. The difference in pressure between the inlet and oulet of the circulator as it does it's magic is what drives its flow. This is the basics of how circulators in a closed hydronic system work. Remember though - they do not CREATE pressure in this application, they only create a difference in pressure. This is a critical detail that Dan Hololhan elaborates more on, in his book.
> 
> Now to elaborate further - the amount of flow that a circulator actually makes depends on how much differential pressure it creates. If there is a LOT - say it adds another 20 PSI, then there will be a lot more flow. The differential pressure it creates however, does not add pressure to the system; instead, as water flows, the differential pressure dissipates as the water flows through the various elbows, piping, and compenents as it does work. The less friction and things that dissipate this pressure differential, the more water flows. The more friction, the less water flows. Now you're getting into the pump impeller design - which is way over most of our heads, myself included.
> 
> When you look at water that contains air - air tends to seperate from water in 2 areas - areas with less pressure, and in warmer water. As soon as you pressurize water, it tends to collapse those water bubbles. This is why it's a good idea to put your circulator on the supply side. If you have air bubbles in a static filled system at 12 PSI, then you turn your circulator on, the differential pressure it creates will help to collapse some or all of those bubbles and move them to lesser pressure areas of the system i.e. your air seperator. This is why it's always a good idea to put an air seperator downstream of your boiler on the supply side (warmest water) and upstream of your circulator (least pressure zone). These allow your air sepeator to do it's best work in removing air and avoiding call backs.
> 
> Now the next question is where do you connect your expansion tank. No matter what - the point that you connect your expansion tank to the system (whether it's a tee or to your air seperator tapping) can NOT change in pressure, typically. If you set your expansion tank air side at 12 PSI, and your fill valve at 12 PSI and connect them to the same tee or to the air seperator tapping, that point that they connect to the system is the point of no pressure cha ge. It is what it says - the pressure there can not change. So, this is one reason why you ALWAYS put your circulator downstream of the point of no pressure change typically or if you have it on the supply of your primary/boiler loop, you put your circulator on the return pumping into the boiler where it can dissipate all it's pressure differential into the often high pressure drop zone of the heat exchanger of many condensing boilers. If you place the circ d/s of the point of no pressure change, it will add it's 6 PSI or whatever to the static fill pressure (i.e. 18 PSI total) and push air around until it gets to the reduced static fill connection at the point of no pressure change where the nice warm water and reduced pressure will let that air seperator kick the air out and then replace it with water from the feed water valve.
> 
> Now if you decided to not follow this advice which is proven and pump towards the point of no pressure change - well remember one thing - the circulator doesn't care how it creates flow, it's always going to create a pressure differential no matter what. So how it does this pumping towards a place that pressure can not chagne is it drops the pressure on the inlet and it just pumps towards the point of no pressure change. So you may have 12 or maybe at best 13-14 PSI downstream of the circ, but you'll have maybe 6-8 PSI upstream of it. All fine and dandy - flow is created still as there is a difference in pressure. But lets pretend we put a high head pump in there (one that creates a lot of differential pressure). That pump could drop the inlet into a vacuum condition - in the negative pressure zone. This is the point where you start sucking air in through your air seperator and air vents and not removing it. Further to this - you don't have favourable conditions in the system to help push air - instead of adding pressure differential, you're subtracting static fill pressure to create a differential pressure, which is not favourable to collapsing bubbles and moving them to your air seperator.
> 
> To understand why you may want to pump into a condensing boiler, look at what a circulator does. In this application, most condensing boilers have some, or a moderate to high amount of friction in their heat exchangers that a circulator must overcome to move water through it. This is very typical of your modern water coil type heat exchanger like that found in Viessmann boilers or boilers that use the Giannoni heat exchanger.
> 
> In these heat exchangers, they are fairly low mass (low water content) with thin stainless steel walls (in most, some have aluminum heat exchangers) that are designed to maximize heat transfer and extract as much heat from the flue gas condensation as possible. These heat exchangers are not friendly to pockets of air or low water or low flow conditions, where steam could form if you are not careful due to the proximity of the exchanger walls to the main burner. A good method to protect the heat exchanger is to pump into it where you pressurize the water in the exchanger to help push those air bubbles out to the air seperator and get rid of air quickly. Since there is significantly more fricition in the heat exchanger travel for the circ to over come, compared to the piping between the outlet of the air seperator and the inlet of the boiler/primary circ, you could argue you are still technically pumping away from the point of no pressure change.


I was wondering when you were going to chime in with the best answer for this one. I even guessed the reference to "Pumping Away". One of my personal favorites.


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## Scott K

OldSchool said:


> I agree on the above statement*.*
> 
> *I dont think any one took pump cavitation into account.... it would be more likely that pump cavitation would occur at higher temperatures. *


Most modern wet rotor circulators today have fairly high temperature ratings. If memory serves me correct (please correct me if I'm wrong), a basic 3 speed Grundfos 15-58 can tolerate in the 225-228 degrees fahrenheit range max operating temperature. 

Also, presence of air on the other hand, is a large contributor to pump cavitation. What is the most ironic thing however, is most would assume that your air seperator or air vent/scoop would be the most important devices for air removal, but what is just as critical is the location of your circulators and the point of no pressure change. You DO want some seperation between the outlet of your air seperator and the inlet of a pump however (some recommend 6x pipe diamaters to I've heard as high as 12x) to reduce inlet turbulence to your pump.


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## OldSchool

Scott K said:


> Most modern wet rotor circulators today have fairly high temperature ratings. If memory serves me correct (please correct me if I'm wrong), a basic 3 speed Grundfos 15-58 can tolerate in the 225-228 degrees fahrenheit range max operating temperature.
> 
> Also, presence of air on the other hand, is a large contributor to pump cavitation. What is the most ironic thing however, is most would assume that your air seperator or air vent/scoop would be the most important devices for air removal, but what is just as critical is the location of your circulators and the point of no pressure change. You DO want some seperation between the outlet of your air seperator and the inlet of a pump however (some recommend 6x pipe diamaters to I've heard as high as 12x) to reduce inlet turbulence to your pump.


This is what we are trying to say... about pump location

that with the pump on the hot side of the boiler pump cavitation is more likely as it is more likey for the fulid will reach vapour pressure 

here is the cause of pump cavitation

Pumps and propellers

Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.
As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. *As it reaches **vapour pressure**, the fluid **vaporizes** and forms small **bubbles** of gas.* *This is cavitation.* When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades.
Cavitation in pumps may occur in two different forms:
* Suction cavitation*

Suction cavitation occurs when the pump suction is under a low-pressure/high-vacuum condition where the liquid turns into a vapour at the eye of the pump impeller. This vapour is carried over to the discharge side of the pump, where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look spongelike. Both cases will cause premature failure of the pump, often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.
In automotive applications, a clogged filter in a hydraulic system (power steering, power brakes) can cause suction cavitation making a noise that rises and falls in synch with engine RPM. It is fairly often a high pitched whine, like set of nylon gears not quite meshing correctly.
* Discharge cavitation*

Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller, it must pass through the small clearance between the impeller and the pump housing at extremely high velocity. This velocity causes a vacuum to develop at the housing wall (similar to what occurs in a venturi), which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump housing. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.
Discharge cavitation in joint fluid is thought to cause the popping sound produced by bone joint cracking, for example by deliberately cracking one's knuckles.


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## TX MECH PLUMBER

I do lots of hydronics. And hot water has pump on supply ( supply for AHU, FCU, VAV ) you never pump in to the boiler !! You draw out of it !!! With the heat and added pressure from pumping in to the boiler it can pop off the t&p !!! On chill water pumps go on return ( pushing through the chiller) !!


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## OldSchool

TX MECH PLUMBER said:


> I do lots of hydronics. And hot water has pump on supply ( supply for AHU, FCU, VAV ) you never pump in to the boiler !! You draw out of it !!! With the heat and added pressure from pumping in to the boiler it can pop off the t&p !!! On chill water pumps go on return ( pushing through the chiller) !!


I have been doing boilers even before you needed pumps.... and your telling me that a pump on the return can pop the t&p.... Really

If that is the case you or somebody else is doing something wrong.


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## TX MECH PLUMBER

OldSchool said:


> I have been doing boilers even before you needed pumps.... and your telling me that a pump on the return can pop the t&p.... Really
> 
> If that is the case you or somebody else is doing something wrong.


I'm just saying what I was taught!!! It's what my master p and an engineer told me!! Are they rong?? School me !!!???


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## Scott K

Old School - did you actually read ANY of what you posted? Or did you just read the parts that you think agree with your position but neglect to read the other parts?

I'm going to bed. I'll try and find time to blow your points out of the water tommorrow.


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## OldSchool

Scott K said:


> Old School - did you actually read ANY of what you posted? Or did you just read the parts that you think agree with your position but neglect to read the other parts?
> 
> I'm going to bed. I'll try and find time to blow your points out of the water tommorrow.


Scott I knew that before I wrote it... I highlighted parts so you would not miss it

Cavitation is caused by pressure differential and temperature of a fluild in a pump.....

Every pump has it.... 

The higher the water temp the more likely cavitation will occur and you are getting closer to the point of vapourization of the liquid...

The cavitation is not just regular air in the fluid but the liquid vapourizing at the temperature and pressure drop caused by the pump...

Ever take a pump a part and the impeller is worn out or the body is worn?


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## futz

OldSchool said:


> Ever take a pump apart and the impeller is worn out or the body is worn?


Nope. I just don't see it happening in the hydronic heating systems I've serviced. I take the old junkers apart and they're old and black inside, but the impeller and body are fine. Invariably all that's wrong with them is the motor has failed or the bearings have seized. On bigger pumps the spring couplers and seals fail.


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## wundumguy

Supply vs Return isn't really the culprit in pump cavitation.

For example, on the Grundfoss UP Series Circulator Pumps, the data sheet provides the maximum water temperature the pump can handle based on the ambient temperature of the room (to keep pump cool). It also provides the maximum temperature - minimum pressure at pump inlet (to prevent cavitation):

- Grundfoss UPS Model 15, for example is:
-- 140F at 1.3 PSI
-- 165F at 1.9 PSI
*-- 190F at 4.0 PSI*
-- 230F at 15.6 PSI

This model is fairly representative of a wet rotor circulator any manufacturer markets as suitable for hydronic heating applications. Meaning, it's operating characteristics is such that it will be happy on either the supply or return of a typical residential or light commercial hydronic heating system..

---------------- 

We can also evaluate the temperature-pressure effect on the rapid vaporization of water (the root cause of circulator cavitation).

At 212F water vaporizes at 14.7 PSIA (0.0 PSIG)
At 180F water vaporizes at 7.50 PSIA (-7.3 PSIG)
At 160F water vaporizes at 4.74 PSIA (-9.96 PSIG)

There is a difference of (7.50 - 4.74) 2.64 PSI between the 180F and 160F to rapid vaporization. It would be a freak accident or a handyhack poor design to encounter a system where a circulator cavitates on the supply but gets by on the return. Look at it this way, to suggest a circulator cavitates on the supply, but will not on the return, is to suggest the same circulator on the return would also cavitate if we dropped the system static pressure a mere 3 PSI. Alternately, if we were to encounter such a freakishly designed system with a circulator on the supply, we could also just increase the system pressure by 3 PSI instead of moving the circ to the return.

------------------------

In second year, we learned how to use the NPSHA > NPSHR calculation to deterimine when a circulator will cavitate and what margin of safety from cavitation exists. This equation can be modified to determine the effect that a specific temperature range has on cavitation. Not my most prefered method, but if one didn't want to read manufacturer's instructions, and forgot all of grade 10 chemistry, one can rely on apprentice training to evaluate the cavitation issue.

NPSHA = Net Positive Suction Head Available (to pump to prevent cavitation)
NPSHR = Net Positive Suction Head Required (by pump to avoid cavitation)

Difference in NPSHA = NPSHA(160F) - NPSHA(180F)
Difference in NPSHA = 6.19 feet of head.

I'm not presenting the actual calculation because nobody wants to look at a bunch of math.

When all else is equal, the Difference between NPSHA at 160F and 180F, is a meagre 6.19 feet of head, or 2.64 PSI. Our result is similar to what grade 10 chemistry gives us. The effect of the 180F (supply) vs 160F (return) location on pump cavitation, for a conventional or typical residential and light commercial hydronic heating system, is somewhere between inconsequential and irrelevant.

Pump cavitation is generally not a valid consideration for the determination of supply vs return location of a circulator on a common, everyday, residential or light commercial hydronic heating system.


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## ]3ones

****** said:


> Ive been doing plumbing/heating for more years than I care to remember and have built boilers using the circs on the supply and return but in most cases where the circulators were put on the supplies it was mainly just because that's how it was spec'd as well as being IFC circs. Usually it was on condensing boilers like the burnham alpine or munchkin when ive installed them on the supplies.
> 
> Now I have a customer/friend who wants me to install a burnham V8 in his sons house but went out of his way to get me to order the material to install the circs on the supply, which honestly I could care less either way but in this application it would be easier for me to put them on the returns well as look better.
> 
> My question is, is it more than a just preference? Is there a reason its better to put the circs on the S or R or is it application specific? I actually felt kinda stupid for a second because as long as I've been doing this, I really had no answer if it was a better or not.


It truly doesn't make any difference as long as your not pumping into the expansion tank. You always want the expansion tank upstream of the pump thats all that matters. 
Engineers will say this and that and blah blah blah. Pumps have been on the return side for 50 plus years and it was considered the only option. I've seen heating systems 50 plus years old and still working great with pumps on the return.


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