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Circulator pressure differential question

The question is what would be the system pressure while the circulators are running both on the mains and their individual circuit?

The very top of the diagram are the circulators on the return and the numbers represent each pumps possible pressure that they can produce. Being that they are pumping towards the expansion tank and fill, I know that they will reduce the available system pressure through each circuit. I'm just not quite sure what happens to the system when all of them are on.

4- boilers input (400,000 each) 1,200,000. Output 970,000, ibr output 860,000

3" heating mains

The other question is at what pressure does air break free from the water molecule ? I know we want 12 psi or more to keep air dissolved in water.

Thanks for your help in advance.

Ps. I know the system is installed wrong but I want to prove it to them so they can imagine being a BTU in the water which has air that can break free and that I can help solve these issues and more.


  • icesailor
    icesailor Member Posts: 7,265
    System Pressures:

    In my (obvious) minority opinion, a circulators add absolutely nothing to system pressures. The system pressures are controlled by the fill valve pressure settings. And nothing else.

    Circulators add pressure locally and not globally. What a circulator "sees' is restriction. It develops enough pressure to overcome resistance in the system. The system pressure only needs to be high enough to push the fluid to the highest point in the system above the fill, nothing more except maybe a small cushion of pressure to overcome unplanned situations. When the circulator "sees" resistance and overcomes it, you do not add the system pressure because the system, when say at 12#, causes equal pressure on both sides of the circulator when off. If gauges are installed on either side of the circulator, they will both show 12#. If the circulator starts, and the outlet pressure rises to 15#, the return should drop to 9#. Which would equal 6# or 13.86' of head pressure. If the pump is shut off, the pressure reverts to 12#. If you could flip the whole system into a horizontal position (like flipping a ferris wheel on its side) the pressure would still be 12#. If the circulator starts, it will show the same 6# or resistance as it did before. The closed system maintains pressure through the expansion or pressure tank. The fill valve/PRV can be turned off.

    As a plumber, a pump installer and a well pounder, when I first read the explanation in the IBR heat loss guide on system head pressure, it took me a while to wrap my brain around the concept. Which IBR went out of their way to explain. Consider this.

    If you had an open 2" pipe, 100' into the air, with a pump connected to the bottom with a never ending supply of water to draw from, the pump would need to develop 44#  (43.29#) of pressure to make the water flow over the top of the 2" pipe. If you dropped the pressure to 43#, it wouldn't flow over the top. If you close the system and connect a 2" return pipe so it becomes a "closed" system, it will still take 44# (43.29#) to fill and keep the pipe full. Assuming that the 2" circuit has little or no restriction, the pump sees no "head" pressure. Both gauges at the pump will read 44# "head" pressure when the system is static, not pumping. The pressure at the top of the 100' pipe is zero. At the bottom, it is 44#. If you drop the system pressure or the level in the pipe by 10' or 90' above the ground, the gauges would both register 39# (38.96#) with the pump stopped. If the pump starts, it will need the additional 4.33# to push the water over the top of the pipe. If the pipe is still connected as a closed circuit, the gauges will show about 5# more pressure on both gauges because there is no restriction of the large 2" pipe. The pump only adds pressure to overcome the height restriction.

    If you have a massive municipal water system, with a standpipe on the top of a hill, the same rules apply. If the top of the tank is 100' above the pump, and water is being pumped to keep the tank full, the head pressure stays the same. The discharge pressure is such that it keeps the tank full while users draw water. You can go to a municipal water pumping station. There will be a gauge on the wall showing pressure at that location. They can be pumping 1,000,000 gallons a day into the system. The pressure on the gauge stays the same. Multiply that pressure by 2.31 and it tells you how high the water is in the tank. It doesn't change unless the water in the tank goes up or down. That's how pressure switches work.  
  • icesailor
    icesailor Member Posts: 7,265
    Breaking Wind:

    Air pressure & water.

    The greater the pressure on the water, the more air can be compressed into the water.

    Look at it in this way. Water boils at sea level at 212*.

    Water boils at 244* at the Dead Sea, below sea level or at 12# PSIG.

    Water boils in Denver CO at 194 degrees.

    Gas, Nat. gas or LP Gas has a certain BTU content at sea level. In Denver CO, it has a lower content. Not because the gas has less available energy, its because the lower atmospheric pressure has less available oxygen content PER CUBIC FOOT OF AIR.

    Professor Tim McIlwaine can tell me I'm wrong, but there is more available oxygen at the Dead Sea because the air is compressed. A cubic foot of air contains 80% nitrogen and 20% oxygen (app.) at sea level. More at the dead see and less in Denver, because of the change in pressures.

    Anyone ever wondered how jet aircraft engines get enough air to combust fuel at 35,000'? Notice that the front of the engine is a massive fan. It takes massive amounts of air and compresses it so that the engine can mix Jet-A kerosene and burn it for thrust. To the tune of over 700 PS!, all compressed. So they take some of that compressed air after it is compressed and before it is mixed with the Jet-A, and they use it to pressurize the cabin. 700# PSI gauge compressed air that started at -40, is still quite toasty. So, they cool the air and extract the heat through a HX. They heat the cabin with hot air mixed through a heat recovery unit. They maintain a cabin pressure equal to living and breathing at 8,000'. The outside pressure is less, the inside pressure is higher. As they go down in altitude, the pressure increases. Your ears pop with the higher pressures. If the airframe develops a small crack and the pressurization system can't keep up, the cabin pressure drops and there is less oxygen available. Same air, just taking up a larger space. That's why a decompression event in an aircraft is deadly. The plane doesn't have to blow up, just be unable to maintain pressure until the aircraft can get to a lower altitude.

    When water rises in a system, the pressure drops and the gasses that are compressed, now expand. The higher you can maintain the system safely the less air/gas will be released from the system water.

    If you take a case of air elimination devices and pipe them in a long series row, and they are venting air from a system after it has been running for a day or week, something else is wrong. Where is the air coming from? Not enough pressure or bad piping design.

    Water is 1 part hydrogen, 2 parts oxygen. Heat it up and you get steam. Which has different gasses. But it still came from the same source. Where's the air coming from?
  • RobG
    RobG Member Posts: 1,850

    That type of installation should be piped primary secondary. Since the water flows through all the boilers all the time in reverse return, If those are zone pumps, any one of them needs to provide minimum system flow through all of the boilers. If that is the case, when all of the pumps are running you will be over pumping and subjecting the system to erosion. It needs a re-pipe. I would also suggest a good controller (if not already installed) as that system is meant to be stepped fired. I have worked on allot of the Hydrotherm MR series boilers, when installed correctly they are a decent boiler.

  • icesailor
    icesailor Member Posts: 7,265
    Certainly are:

    They certainly are. And you are 100% correct.

    Those work well on gas. Not on oil if they are ODR and not have any boiler protection. The sections are horizontal and if you can't get the exhaust out of the top, you can't clean them and they will plug up and fail. The seal breaks between the block and the firebox. But that has nothing to do with the need for Primary/Secondary. Which will even work satisfactorily if the crossover bridge loops are reversed.