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X-Tank Blind

Have you ever had a hot water system where the boiler doesn't seem to "see" the expansion tank? I have one. The x-tank is in the perfect location: on the outlet of the boiler, attached to the air eliminator, just before the circulator. The pressure increases when the boiler is on, starting at 12 psi and going up to 30 psi. We have increased the size of the expansion tank, changed the pump (it was oversized) and turned off the fill valve, all to no avail. The boiler is a Laars Mini-Therm and I seem to remember having the same issue years ago on another Laars.

Have any of you had the same experience?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab

Comments

  • Steamhead
    Steamhead Member Posts: 16,728
    Check the input rate to see that it's not over-firing. Also make sure your new circ is actually working- you never know.............
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • hot_rod
    hot_rod Member Posts: 21,866
    Stuck bladder? Probably not on two different tanks however.
    Do you have an accurate gauge to pre-charge the exp tank? Try a 15 psi RV type pressure gauges for accurate, low pressure adjustments.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • delta T
    delta T Member Posts: 884
    I have see the connection on the bottom of an air separator get clogged up with rust to the point to where the exp tank could not access the system, but not often. What kind of air sep?
    kcopprick in Alaska
  • kcopp
    kcopp Member Posts: 4,393
    delta T said:

    I have see the connection on the bottom of an air separator get clogged up with rust to the point to where the exp tank could not access the system, but not often. What kind of air sep?

    I had that happen to me 1 or 2 times... they were both spirovents.
  • I checked the firing rate, circulator, deflated and re-charged the x-tank to 15 psi and stuck some solder up the Spirovent and felt the stainless steel mesh.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • delta T
    delta T Member Posts: 884
    @kcopp Yep me too, seems to happen mostly to Spirovents for me, not at all common though.


    Any chance its undersized? how hot does the boiler have to get before it hits 30 psi? If the connection is clear, pressure is correct, firing rate good, pump working, feeder eliminated, bladder not stuck....not sure what else it could be.

    Once I saw feeder go bad and the ball valve before it also leaked by slowly, but that was a total fluke and seems highly unlikely.
  • Delta T: No way the tank is undersized. The existing system had a 4.4 gallon tank and we installed a 7.6 gallon tank on a 3,300 sq. ft. runtal radiator system. The boiler temperature gets to about 150° as it hits 30 psi.

    The original problem was that the x-tank was rusting and leaking and the owner said the system wasn't heating properly. We replaced the x-tank and increased the boiler set point. I'm pretty certain that the thermal expansion problem was there originally and that raising the set point exacerbated the increase in pressure.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • wcs5050
    wcs5050 Member Posts: 131
    edited November 2015
    Weird. What's the BTU input rating?
  • unclejohn
    unclejohn Member Posts: 1,833
    I guess it's possibly that the copper tubes are blocked up and the pressure is rising rapidly and contain in the heat exchanger.
    Mark Eatherton
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    edited November 2015
    Testing testing, 1,2,3... (FEEDBACK...) :wink:

    I (and a million other people) have had the diaphragm stick to the face of the tank. You can test it by taking it out of the system, and couple a drain cock to the bottom. Then pressurize it with street water pressure, being careful not to exceed the tanks capacity. Then check the air charge pressure. If it is working correctly, the pressure on the air side should be nearly equal to the pressure on the water side. Then disconnect the tank and see how many gallons it has taken on and compare it to the tanks acceptance capacity.

    A good manufacturer puts talcum power inside the tank before it is assembled to avoid the possibility of the diaphragm sticking to the face of the tank.

    Even though your solder test confirmed the tapping was open, it might be worth the time to completely disassemble the MBR, remove the coalessing aglomulating (not my invention...) material and clean it up. It is possible that it has trapped enough solids to the point that it has blocked the water path to the diaphragm.

    (EDIT) While you have the tank out, and still connected to the automatic washing machine hose, reconnect it to a drain cock someplace within the system (preferrably upstream of the biggest circulator) and see if the system reacts differently. Also, if you have a high head pump, AND it is pumping towards the PONMIPC (point of no mechanically induced pressure change, formerly known as the PONPC) AND the makeup is connected to the inlet side of the pump AND the pressure drop through the system is high, as soon as the pump turns on, it drops the pressure as sensed by the makeup regulator, and it starts immediately dumping water into the system, thereby causing it to hit the relief valve threshold, and do its thing. Easy enough to test. Shut the make up water off, and if the pressure remains stable, you've found the bug. Now kill it.

    Let us know what you find.

    ME
    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.
    SWEI