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Expansion tank Pipe size...Q"Q ?

Jeff Elston
Jeff Elston Member Posts: 289
I got some newly installed systems all with same issue,  Being the reliefs all seem to weep, but not at the 30lbs.

Some of the techs think the expansion tank line size is to small.  These are new installed Knight boilers not very large not to small.  The tanks are sized properly that's been checked.  The line connecting them to the spirovent is 1/2 copper. 

Anyway pls comment on the expansion tank line sizing. 

Thanks Guys I been lurking for a while now.  Haven't posted in a couple years.


  • Line Size

    Half inch to the expansion tank should be OK. Something else is going on.  Could be:

    1) Pump location.

    2) Expansion tank not pressurized properly or undersized.
    Often wrong, never in doubt.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Expansion tanks

    What is the pressure and temp that the boiler is running at? System volume? Model of xtrol?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Does not matter...

    A 1/2" connection is more than adequate. There is not that much volume being generated at any given time.

    I'd look for dirt beneath the relief valves seats.

    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.
  • clammy
    clammy Member Posts: 2,766
    expansion tanks weeping valves

    I have had this happen it ended up being a plastic  cap which was in the end of the spriovent element .Pull the float head off of the spriovent and also check the length of the element i have had them be a bit tight in the valve body and it seems to block the ablitlity of the 1/2 line to corretly allow for expansion and  ended up with a weep safety .I have not had this happen of reent but a few years ago i ran into it more often then i would have liked .Peace and good luck clammy 
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • Jeff Elston
    Jeff Elston Member Posts: 289
    XTank, Pump, Spirovent..>_>

    Alright so the pump location is by the book, 

    As far as the air side pressure of the tank we matched the system operating pressure when it was installed,

    Greg I am not sure of these,, I did 6 or 7 boilers rooms this past winter all of em were twin Knights with indirects,   5 boiler rooms developed a weeping relief.  The tanks I am sure are slightly over sized,  And i would guess that the temp was around 160-180 for making the DHW, Not sure of the system volume all the systems are baseboard fin-tube in apartment buildings.

    Clammy I did have a vent on 1 of the spirovents do the dribble thing immeditly upon start up I pulled that unit apart did see anything that was alarming  I had to replace it no dirt on notin just a leaky vent.. 

    I am thinking the gauge we were using to set the air side of the tanks is now being heavily scrutinized,  as well as the method. 

    It was always my understanding that the 1/2 was appropriate. 

  • tim smith
    tim smith Member Posts: 2,465
    Re: expansion probs

    How many units in bldg, what size are the old mains at boiler, what size boilers. 

     We have ran into pressure probs over the yrs on change outs and have kind of formulated a method of sizing. Odd but if you think a x will do it, go to x + 1 size. Sometimes being halfway accurate on volume of water is a little iffy. Tim

      Most of our 20 unit bldgs get a Amtrol sx60v. Floor mount so nice to work with. Good luck.
  • Jeff Elston
    Jeff Elston Member Posts: 289
    gathering info

    I am gathering some of that info when i get it together i will post up. 

  • clammy
    clammy Member Posts: 2,766
    spriovent ?

    Jeff may be i explained it bad ,but i would check on how tight the mirco scrubber element sits in the spriovent body top to bottom aside from a oassional plastic plug in the 1/2 copper inset ,just check to make sure ther is enogh room for expansion on the element and the inside housing dimension .I have had it so tight that after the system came to temp (on heating not dhw prodution) the safety would weep .This did drive me crazy the first time i ran into it and i went over everything and double checked it finaly i took the spriovent apart ut the element down about 3/8 of an inch and that was the end of the promblem .Since originally running into this about 5 years ago i have maybe seen it another 5 or 6 times since but evey time it originally drove me nuts.On one side note have you ruled out a leaky PRV just asking .Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
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