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System has both types of expansion tanks?

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TR1JR2
TR1JR2 Member Posts: 14
Newbie here, My residential boiler has an old overhead expansion tank an airtrol tank at the bottom of the circulator pump. Why?
I have to replace the pump so the question do I need both? And any advice on draining and refilling with 2 tanks?

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  • neilc
    neilc Member Posts: 2,711
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    can you post pictures of how each is piped to the rest of your system ?
    and are there any auto air vents there anywhere,
    it is unusual, and likely unnecessary to have both,
    let's get a look at what someone else thought was needed
    known to beat dead horses
    EdTheHeaterMan
  • leonz
    leonz Member Posts: 1,165
    edited January 2023
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    UGH, the steel compression tank is now flooded,
    and you have no point of pressure change and
    if you have a TACO circulator all you may need
    is a pump cartridge.

    A picture will help and plumbing surgery is needed to
    get rid of that bladder tank and the automatic air vents.
  • TR1JR2
    TR1JR2 Member Posts: 14
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    Ok standby for pics btw this system has operated fine for the last 20+ years.
  • TR1JR2
    TR1JR2 Member Posts: 14
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  • TR1JR2
    TR1JR2 Member Posts: 14
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    Note that the airtrol tank is not connected to the water line, it’s below the pump
  • hot_rod
    hot_rod Member Posts: 22,327
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    Your system has a Spirovent air purger and a diaphragm expansion tank. No need or place for the compression tank. If it is waterlogged it is basically a storage tank for boiler water? I would move the new diaphragm tank upstream of the new circ that you install

    I would also connect the fill valve at the expansion tank connection..

    When the system is drained split open the spiro, inspect and clean out.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TR1JR2
    TR1JR2 Member Posts: 14
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    So would this area suffice for the new expansion tank? It’s below the spirovent .
  • TR1JR2
    TR1JR2 Member Posts: 14
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    This is where the old tank connects to the system.
  • hot_rod
    hot_rod Member Posts: 22,327
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    Nope, unless you move the circulator to the right of the Spiro.

    The pic is too dark to see all the piping at the circ. The circ is fine pumping into the boiler, and the spiro is in the best, hottest, fluid.
    The relationship of the exp tank to the circ could be better

    The expansion tank should always be at the inlet of the circ, as close as possible. It establishes the point of no pressure change in the system, as such that is where the fill should attach also.

    Here are 3 piping options that would improve the system, fill line ties in at the tank in any of them

    Have you had any air problems, noise in the pipes, uneven heat? Plenty of systems are piped like that, it can work okay, if you are getting into the system, make some upgrades. Air removal will be a cinch when you pump away from expansion tanks. The spiro should do an excellent job of air removal. Assuming it is not plugged up😳
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TR1JR2
    TR1JR2 Member Posts: 14
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    Thanks for the reply Rob, here’s a clearer pic, and no I haven’t had any air/ noise issues.
    I’m going to replace the tank and PRV valve, the Pressure regulator valve was replaced about 8 years ago.
  • TR1JR2
    TR1JR2 Member Posts: 14
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    Also i ordered the 0010 pump
  • TR1JR2
    TR1JR2 Member Posts: 14
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    Bob your diagram on the right side would be doable but it’s similar to my existing system no? X = Expansion tank correct?
  • TR1JR2
    TR1JR2 Member Posts: 14
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    I also saw an expansion tank connected directly to the Spirovent. Thoughts?
  • pecmsg
    pecmsg Member Posts: 4,909
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    TR1JR2 said:


    So would this area suffice for the new expansion tank? It’s below the spirovent .

    A couple of RED flags stick out
    Use of a bladder tank and a compression tank!
    It appears the mains are large 1/2" 2" that's a lot of extra water to get hot and a lot to cool down after! All that extra volume may require a larger x-tank.
    We still need a few shots showing everything in 1 or 2 pictures, but it might be better to wait and re-pipe in the spring pimping away with a properly sized piping and x-tank.
  • TR1JR2
    TR1JR2 Member Posts: 14
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    Circulator pump is leaking with a drip out of the weep hole every 3/4 seconds hence the need to replace. I’ll be gone the whole month of April and the boiler is for my upstairs tenant. I’m in PGH btw.
    Not to further complicate but there is a second boiler next to this one so piping pics may get confusing , I’ll post a couple more pics. ( I live in an up/down duplex.)
  • TR1JR2
    TR1JR2 Member Posts: 14
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  • hot_rod
    hot_rod Member Posts: 22,327
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    This graphic may help explain the expansion tank location in a closed loop system.

    Your system is a bit on an anomaly if both expansion tanks are still connected and working? You have two PONPC point of no pressure change.
    If the steel compression tank is completely filled with water, it is no longer an expansion vessel. With only that small tank functioning I would expect some high pressure when the boiler is hot??

    Basically when you pump away from the tank (PONPC) the circulator adds pressure differential to the system delta P, ∆p.

    This added energy, aka head, is what allows flow, it overcomes the resistance to flow in the piping. Head is the mechanical energy in the fluid

    When you pump at the tank, as you are doing with the small diaphram tank, the circ tries to take the ∆p, but now from the suction side of the circ. So you see the pressure drop.

    Not an easy concept to wrap your head around, I know. The book Pumping Away available in the store here does a great job of explaining the concept. Also an introduction to nothingness in hydronics :)

    It depends on the circulator and the actual piping as to how much that pressure can be lowered on the inlet. The 3rd graphic shows how you could pull negative pressure in a system with either low fill pressure, or a circ that develops higher ∆p

    Bottom line, always pump away from the tank location as @pecmsg mentioned, you may need a larger diaphragm tank, once the compression tank is abandon. You could put multiple tanks together, 2- # 30 tanks for example.

    Really no reason the tank needs to be below the air purger, it is sometimes convenient to do so, is all.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TR1JR2
    TR1JR2 Member Posts: 14
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    Thanks Bob, fyi the old tank feels empty but not sure, the valve leading to it is definitely on though, again this system had no issues(pressure etc) for 20+ years. I’ll ck your additional info, thanks again!
  • hot_rod
    hot_rod Member Posts: 22,327
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    Typically you wouldn't have an air purger on a system with a compression tank, as it could deplete the air bubble in the tank over time.
    Often times those tanks have a sight glass at one end so you can just drop in and see what condition the condition is in :)

    I'm curious why the small tank at the pump was even added. I doubt it alone has the expansion capacity that your system needs.
    So I could believe the compression tank is still in play.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TR1JR2
    TR1JR2 Member Posts: 14
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    I have to think that small tank was added after the update from an old gravity type system, the very large pipes call for a lot more volume, maybe??
  • pecmsg
    pecmsg Member Posts: 4,909
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    TR1JR2 said:
    I have to think that small tank was added after the update from an old gravity type system, the very large pipes call for a lot more volume, maybe??
    Or the installing contractor didn’t know what they were doing?
    hot_rod