Why "pump away"?

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  #1  
Old 10-23-09, 01:17 PM
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Why "pump away"?

I've been reading in a lot of places that zone circulators should be installed so that they're pumping away from the boiler and the primary loop. Why is that and how important is it? The reason I ask is that my zone circulators are pumping toward the boiler. Is that bad or is it just a "best practice" kind of thing?

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  #2  
Old 10-23-09, 03:49 PM
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that is a pretty deep subject.
Old timers always pump towards the boiler.
Simply because the older circs could not handle the water temps.

Pumping away from the boiler puts the air ellimination and expansion tanks in the best possible position.
There is paragraphs on paragraphs as to why pump away. Not hard to find that info.

Interestingly though, many (most) Mod Cons need to be pumped into the boiler for a number of reasons, pressure drops etc.
 
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Old 10-23-09, 03:57 PM
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The wise a55 in me wants to simply ask; 'Why not?'

Let's see if I can keep this concise...

For various reasons, in the past, circulators were placed on the return side.

Newer materials improved the heat resistance of the pumps, and now they can be placed pretty much anywhere in the system.

The single biggest reason to 'pump away' is to improve the process of removing air from the system.

When system pressure is higher, air has a greater tendency to stay in suspension in the water.

The point in the system that the expansion tank is connected is called the "Point Of No Pressure Change" or PONPC.

The system pressure is HIGHEST at the DISCHARGE side of the circulator pump, and LOWEST at the SUCTION side.

When a system is configured such that the pump is pumping TOWARD the PONPC, then when the pump is running, it SUBTRACTS pressure from the rest of the system, and vice versa, when it is pumping AWAY from the PONPC, it ADDS pressure to the rest of the system.

The AIR ELIMINATION DEVICE should ALSO be placed at the PONPC, so that when pumping away, the air elimination device is at the lowest pressure point in the system. And the rest of the system is at a higher pressure.

The net result is that the air in the water more easily travels through the system, remaining dissolved in the water (or smaller bubbles because they are 'squeezed' more by the higher pressure), and are more easily 'caught' by the air elimination device at the lowest pressure point in the system.

Also, HOT water has less capacity to hold air. If the PONPC also happens to be on the HOT SUPPLY side of the boiler, air elimination becomes another bit easier to achieve.

Not concise really, but hope it helps!
 
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Old 10-23-09, 03:59 PM
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I was writing at the same time as TO...

Also, if you Google the term (use quotes around the term), you will find that there is even a whole durn book written on the subject.
 
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Old 10-23-09, 05:03 PM
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Just to say it a little different but the same as Trooper stated "air elimination". Let's not confuse the issue. As far as flow and pressure the pump and air elimination device location has nothing to do with the boiler. Also as Trooper stated hotter water is easier to eliminate the air, so there is a benefit to heating the water before air elimination. The rule is always pump away from expansion tank connection. The reasons see Troopers post. The only correction I have is the inlet side of the pump is not the lowest pressure point in the system. It is the lowest pressure point in the near boiler piping. The lowest pressure in the system is at the end of the highest heat run where the water heads back to the basement. For every 2.31 ft (28") above the fill point you lose one psi. To operate properly and eliminate air out of the high point of the system, you must always have at least 4 psi at the highest point even while the pump is running to keep air in solution.
The change came about in the 50's, some still have not learned, to move the pump to pump away from the PONPC. The difference that brought about the change is the old 3 piece pumps moved plenty of gallons the air got pushed out. The newer wet rotor pumps require adding pressure to help get rid of the air.
 
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Old 10-23-09, 05:59 PM
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'static' vs. 'dynamic' pressure

rbeck, does it make a difference if we're talking 'static' or 'dynamic' pressures?

By 'static' I mean the pressure that the system is actually pressurized to, relative to atmospheric, and by 'dynamic' I mean the pressure (or perhaps more correctly 'head') added to the system by the pump.

The pump would still add 'head' to even the highest point in the system when pumping away, correct?

And by NOT pumping away, you subtract 'head', and in many cases decrease the pressure at the highest point to below atmospheric... which REALLY sucks the air out of the water.

Am I understanding this right?
 
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Old 10-23-09, 06:20 PM
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A good and timely article on the topic by Siggy,

Pumping Away From What?
 
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Old 10-24-09, 11:13 AM
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Wow, thanks guys! It sounds like the biggest reason to pump away is to aid in the elimination of air. My system seems to be piped as a bit of a hybrid since the circulators are pumping toward the boiler, but the spirovent and expansion tank are the first hop after the boiler, so they're still getting the hottest water.

I have no doubt that the installer just left the zone circulators pointing the way they were with my old boiler because it was easier than swapping them around.
 
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Old 10-25-09, 08:58 PM
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Originally Posted by generaltso View Post
Wow, thanks guys! It sounds like the biggest reason to pump away is to aid in the elimination of air. My system seems to be piped as a bit of a hybrid since the circulators are pumping toward the boiler, but the spirovent and expansion tank are the first hop after the boiler, so they're still getting the hottest water.

I have no doubt that the installer just left the zone circulators pointing the way they were with my old boiler because it was easier than swapping them around.
That can be troublesome, if the highest friction loss of the system is your boiler than things should be fine.
If you boiler is a cast iron or copper fined boiler with low loss, then you circulator will be pumping into the expansion tank which is the "place of no pressure change" and this will actually drop the pressure on the suction side of the pump.
Now add to that a high loss system piping and all of a sudden your boiler circu is cavitating and starving the system for flow.

A very wise man from Burnham explained the theory of pumping away to me one day and I have never forgotten it.
 
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Old 10-26-09, 05:48 AM
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Originally Posted by TOHeating View Post
That can be troublesome, if the highest friction loss of the system is your boiler than things should be fine.
If you boiler is a cast iron or copper fined boiler with low loss, then you circulator will be pumping into the expansion tank which is the "place of no pressure change" and this will actually drop the pressure on the suction side of the pump.
The boiler is a Triangle Tube Prestige Solo 110 mod/con, which is low loss from what I understand. Is my current configuration a big enough problem that I should ask the installer to change it?
 
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Old 10-26-09, 04:28 PM
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Trooper you are correct. Circulators add pressure to static pressure if pumping away from the PONPC and can increase head. Head and pressure changes with temperature and flow.
http://www.comfort-calc.net/pump.html
 

Last edited by rbeck; 10-26-09 at 04:46 PM.
  #12  
Old 10-26-09, 07:44 PM
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Originally Posted by generaltso View Post
The boiler is a Triangle Tube Prestige Solo 110 mod/con, which is low loss from what I understand. Is my current configuration a big enough problem that I should ask the installer to change it?
My concern would be piping a mod con with 3 zones with out any kind of hydraulic isolation. I sure hope the smallest of the zones meets the Solo's minimum flow rate, which is in the installation manual.

A zoned system should either use a low loss header, or close spaced tee's to isolate the flow rates during the operation of different zone valves. There could be the potential to underflow or over flow the boiler depending on how many zones are open.

The Solo is about the best boiler to have used to pipe this way, as the "low loss" heat exchanger forgives a lot of errors.

I cannot advise you if things need changing or not with out knowing each zone's loads and flow rates.
If the boiler does not short cycle, and the delta T is not crazy high then your likely ok the way it is. The air elimination is not in the most idea spot, and will be less effective. Will it remove air, u bet.
 
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Old 10-26-09, 08:13 PM
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I don't actually have any zone valves. There's a primary loop that uses the circulator built into the boiler and two heating zones, each with its own circulator. If I understand correctly, flow rate through the boiler shouldn't be a problem with this type of primary/secondary loop configuration.

Assuming that's the case, are there any other concerns about the way the system is piped other than the air removal system not being in the ideal spot?
 
  #14  
Old 11-10-09, 05:39 AM
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Originally Posted by rbeck View Post
Trooper you are correct. Circulators add pressure to static pressure if pumping away from the PONPC and can increase head. Head and pressure changes with temperature and flow.
http://www.comfort-calc.net/pump.html
Based on the the chart that rbeck supplies in teh link would it be safe to say that if you are pumping towards the PONPC that these numbers would be the amount that you are subtracting from the highest point before return to the boiler? So the calculation of the pressure at the highest point based on how many feet above fill and the above would give you you approx. pressure at the highest point? Is this logic correct? Or does the calculation only work on head pressure and can't be used in reverse?
 
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