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# point of no pressure change

Member Posts: 484
Anyone have a more understandable description (example ) of the point of no press. change in hydronic pumping
Thanks RJ

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• Member Posts: 2,338
In closed system a pump raises pressure. Does not lower it. So a little upstream..... That's how it's explained. But with some sort of expansion tank????
• Member Posts: 15,885
It's one of those things you just have to accept.

Wherever the expansion tank is connected to the system is the PONPC. The expansion tank contains a cushion of air. The air is pressurized to whatever pressure the system needs to maintain 4-5 psi at the highest point in the system. Rule of thumb is 2' of rise =1psi of pressure.

so a 10 story building with the expansion tank in the basement would have to be pressurized to about 55 psi assuming 10'/floor + 5psi at the top. so when the system is filled with no pump running you would have 55psi in the basement and 5psi at the top of the building. If the expansion tank is in the basement it would be pressurized to 55psi. The air side of the bladder tank would also be pressurized to 55 psi with a tire pump or air compressor.

Same system with the ex tank at the top would work at 5psi, 5psi of air pressure in the tank.

The air pressure in the tank has some control over the system pressure....they have to match.

That being said, if the ex tank is properly sized you will get some rise in pressure as the water is heated and it will drop as the water cools
• Member Posts: 9,546
Here is two pictures of how a system responds to PONC. One is pumping into the PONPC the second is pumping away from the PONPC. Which is the correct way. You want to add the pump differential pressure to the system.

• Member Posts: 23,746
I suppose that one might be tempted to wonder why it makes a difference. After all, one might say, either way the pump adds its pressure increase and all is well.

The reason is that all pumps -- but especially the centrifugal types we deal with in heating systems -- require a certain minimum absolute (not gauge) pressure at the inlet to the pump. If the inlet pressure is less than that, they will pump poorly, if at all -- and if the inlet pressure is low enough they will cavitate, which will destroy the pump in a remarkably short period of time (even if the noise doesn't bother one). The minimum absolute pressure is dependent on the temperature of the fluid and the kind of fluid; the higher the temperature, the higher the inlet pressure must be -- and it may well be above 0 gauge pressure (15 psi absolute) for hot water, such as is found in a heating system. Therefore if the point of no pressure change is just upstream of the pump inlet, and is set high enough to avoid cavitation, it will stay that way whether the pump is running or not, whereas if the point of no pressure change is after the pump, the inlet pressure must be lower, as @Gordy 's diagram so nicely shows. One might get lucky and have it still high enough to avoid problems. But... one might not, too, or something in the system might change and increase the circuit resistance and lower the pressure.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,338
Jamie's point is important. An excellent idea for high buildings is to put HHW heater on roof so that it doesn't have to handle high pressure. But circulator should be lower,preferably at least two floors. The business that in closed circuit it doesn't matter may be true when there's no expansion tank.
• Member Posts: 15,885
I have seen a commercial job where they were pumping toward the ex tank. The system had enough resistance that they were pulling air into the system through the automatic air vents.

@jumper, I have seen a lot of office buildings with the boilers, pumps, ex tank are on the top floor so they are not subjected to high pressure. Works fine
• Member Posts: 4,469
Don't take this wrong, but, how does that water get back up from the basement?
• Member Posts: 23,746
Paul48 said:

Don't take this wrong, but, how does that water get back up from the basement?

Keep in mind, @Paul48 , that the pressure in the basement is the pressure at the top, plus the elevation difference. All the pump has to do is overcome the circuit resistance. Assuming, of course, that you've got all the air out...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 4,469
I know....And what's the benefit of putting the boiler on the roof?
• Member Posts: 5,853
RJ said:

Anyone have a more understandable description (example ) of the point of no press. change in hydronic pumping
Thanks RJ

A pump is nothing more than a pressure differential machine. Whether is creates all positive pressure or all negative pressure depends on whether it is before or after the expansion tank connection. If it is pumping away from the expansion tank ( also known as the point of no pressure change, or also known as the point of no mechanically induced pressure change ) it will present all of its pressure as positive. If it is before the PONMIPC it will present it pressure as negative pressure. The above statement is only true on a closed loop system. An open systems PONMIPC is its connection to the atmosphere... Which is what an expansion tank mimics in a closed loop.

The best drawing to visualize this phenomena is the one that Gordy posted, which was originally developed by Gil Carlson I believe...

The term point of no pressure change is incorrect, because the pressure does change at that point due to fluid expansion, hence the introduction of the term PONMIPC.

Simple enough? If not, then just know that if all pumps pump away from the expansion tanks connection to the system, air problems will go away and the system will work much better.

ME

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• Member Posts: 2,338

@jumper, I have seen a lot of office buildings with the boilers, pumps, ex tank are on the top floor so they are not subjected to high pressure. Works fine

Depends on how system is pressurized? If glycol is simply topped up, that's only 35'. Similar situation occurs when wrong pump is bought for cooling tower.

Do you know how long pump lasts?
• Member Posts: 22,544
Another visual in color. Pumping away, pumping towards, or low fill pressure condition (or high head circ)

Low system fill pressure shows how sub-atmospheric conditions can occur. That could allow air into the system, depending on vent type and locations.

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 9,546
Paul48 said:

I know....And what's the benefit of putting the boiler on the roof?

Then the boiler is not subjected to the static pressure of the entire building elevation. It could be any floor location which keeps the boilers pressure rating in exceptable limits.
• Member Posts: 9,546
edited April 2017
Also remember that locating the circ, and the PONPC on the supply side of the boiler eliminates the chance of the pressure differential of the circulator popping the relief valve on the boiler in certain situations. This could happen if the circ, and PONPC are on the return side of the boiler. It's still pumping away however now the boiler sees the highest system pressure differential created by the circulator. Usually when high head circs are used, and or high elevations of the system.
• Member Posts: 15,885
as @Gordy said putting the boiler at the top of the system and a 30psi relief valve is fine. That means a non ASME expansion tank is fine, that means standard pumps and circulators are fine.

Not the case if located on lower floors. Also the expansion tank size will change due to the higher static pressure if located on a lower level