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empty expansion tank acting like aa a waterlogged tank?

ch4man Member Posts: 294
so in 3 decades of doing residential service work last week i found the second or third time where a completely empty tank type ( no bladder) was the cause of great pressure swings just as a waterlogged tank might.
at first i suspected a broken isolation gate valve but that worked fine,
this tank has no airtrol fitting on the boiler connection and the drain valve on the other end was missing the hand wheel but i do not believe it incorporated an air vent, just a drain valve.

once i purged all the air from the tank via the drain and flushed it by filling the system while draining and then repressurizing the boiler system now heated correctly and the pressures where stable.

can someone please explain the physics involved as to why the the water to air interface is in the pipe to the tank due to the tank being so empty that it acts as though its waterlogged?

thanks ... John


  • Jamie Hall
    Jamie Hall Member Posts: 22,147
    What was the air pressure in the tank before you emptied it? If the pressure in their is high enough, it could do that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ch4man
    ch4man Member Posts: 294
    that may be it, it blew air with gusto. i do not remember if the boiler was equally pressurized or not when i purged it.

    now how does the condition happen? there are no auto air bleeds on the system that i could find. 2 zone, up and down, second floor split in two circuits, BB fin&tube. not a bad near boiler piping job except a lack of one critical purge cock by a ball valve.
  • ch4man
    ch4man Member Posts: 294
    x-tank comes right off the top of the boiler, no air scoop. unsure if the boiler section incorporated an internal one or not fwiw
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,377
    Maybe there was crud blocking the pipe somewhere, and when you dropped the system pressure it dropped out. There was a thread on here about a year ago where I responded about a failed bladder tank, tanking it off and seeing black crud blocking the tank opening.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    HOW TO ADJUST HYDRONIC EXPANSION TANKS (old style non diaphragm, ceiling hung type)

    First off, if there is a sight gage on the expansion tank, the standing water level should never be more than 2/3’s of the tanks vertical profile. If it is less than 1/3 air and 2/3’s water, the tank needs serviced. If it is between 50% and 60% full then service is not necessary. If there is no site gage, then erratic operating pressures indicate the need for cushion tank service as follows.

    Shut OFF the boiler burner and makeup water.
    CLOSE tank isolation valve.
    CONNECT hose to the bottom drain valve on the ceiling mounted tank(s)
    OPEN drain valve and drain water into boiler room floor drain.
    Once tank has hit the ZERO pressure point, if there is a vent valve on the bottom of the tank connection, open that connection to allow air into the tank so that the water will drain out. If there is no vent valve, you can install an ice maker tap saddle valve on the piping between the tanks isolation valve, and the tank connection to serve this same purpose of letting air in, and letting the water out. ALL WATER MUST BE REMOVED FROM THE TANK AND INTERNAL PRESSURE EQUALIZED TO ATMOSPHERIC PRESSURE.
    CLOSE off drain valves and venting valves.
    OPEN tank isolation valve and RESTORE makeup water and burner operations.

    As previously noted, if the tanks were properly sized, at peak loading conditions, operating pressure of the boiler should not exceed 27 PSI on a boiler with a 30 PSI relief valve threshold. If it does exceed this pressure, there may be other problems (failed DHW heat exchangers, etc). NOTE: with this style of tank, there can not be ANY automatic air venting valves on the system. All air in suspension in the water is removed from the system at the boiler and directed back to the cushion tank to maintain the air in the upper portions of the tank. An automatic air vent valve will remove this compressible air cushion, thereby causing the tank to become water logged, causing the system pressure to go up drastically every time the boilers are fired, and causing the pressure relief valve to discharge excess pressure.

    If the expansion tank is a newer diaphragmatic style of tank, an automatic air vent valve is REQUIRED.

    This educational information is courtesy of Mark Eatherton and Associates, hydronic heating consultants. 720-375-3107
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    And while on the subject...HOW TO ADJUST HYDRONIC EXPANSION TANKS (newer diaphragm type)

    To begin with, you need to know the required fill pressure of the closed loop system, as previously explained. For this example, we will use 18 PSI.

    Turn boiler burner and make up water to the OFF position.
    ISOLATE the expansion tank with the tank isolation valve.
    CONNECT a hose to the drain valve on the expansion tank and bleed the residual pressure off of the water side of the expansion tank to ZERO pressure.
    Using a tire pressure gage, CHECK the Schrader fill valve, and ADJUST the air side fill pressure to match the required system fill pressure, 18 PSI for this example.
    CLOSE the drain valve serving the expansion tank.
    OPEN the isolation valve connecting the tank to the system.
    RESTORE burner and makeup water to the ON position.

    It is assumed that the pressure reducing valve servicing the system has already been adjusted to the 18 PSI fill pressure. If not, do so before opening the expansion tank isolation valve. Once the boiler fires and all system fluid becomes hot, if the tank has been properly adjusted, the operating pressure should never exceed 90% of the boilers pressure relief valve setting capacity. So for a 30 pound relief valve, the operating pressure should not exceed 27 PSI. If it does, the expansion tank is either undersized, or the diaphragm separating the compressed air from the water has failed, and there is no where for the expanding fluid to go to. This condition (failed diaphragm) is usually indicated by water coming out of the Schrader valve on the diaphragm side of the tank, but not always. A leaking relief valve with a high pressure at the boilers pressure gage may also indicate expansion tank failure, or a water logged expansion tank in the case of the older non diaphragm expansion tanks. (see next section for that style of tank)

    Enjoy, and pay it forward.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    And while on this same topic...HOW TO ADJUST HYDRONIC BOILER PRESSURE

    The required water pressure for a given boiler system is based on the system height above the boilers pressure gage. For example, if the building is 4 stories, and the floor to ceiling/floor height is 10 feet per story, and the boiler gage is 5 feet off the floor, then the total vertical height would be 5 (from gage to ceiling) plus 10 (second story) plus 10 (third story ceiling, which is the 4 story floor) for a total of 25 vertical foot (assuming floor mounted base boards.)

    In order to get the water to the top of the system, you will need to fill the boiler with 1/2 PSI per vertical foot of system elevation over the gage, and then add 5 PSI to this total to ensure positive residual pressure at the top of the system for manually bleeding air out of the top floor convectors. If the building uses ceiling mounted fan coil units, then you will need to add that additional height (10 feet) of the fourth floor to your total height calculation. For simplicities sake, in this example assume hot water baseboards mounted at floor level, for a total vertical height of 25 feet above the boilers temperature/pressure gage.

    25 feet multiplied times 1/2 PSI = 12.5 PSI, plus 5 pounds top end residual pressure equals 17.5 PSI, rounded up to 18 PSI. So for this example, the boiler would need to be pressurized to 18 PSI. Adjustments to the pressure reducing valve should be done in small increments, allowing the pressure to stabilize before making additional adjustments.


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.