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How to diagnose a bad expansion tank?

Foregone Member Posts: 5
So my boiler began to burp fluid out of it's overflow valve last week. As I understand it this is often a sign of a bad expansion tank, but my expansion tank isn't waterlogged, I can tap on the bottom half or so and it's empty air, while the top half is filled with water. However, when I tried to measure the air pressure only water came out of the the air valve at the bottom.
Is the expansion tank bad, or should I look elsewhere for the problem?


    JUGHNE Member Posts: 10,801
    If you get a fair flow of water out of the air check valve you most likely need a new tank.

    Removing it you will find it heavy with water that will not dump out.

    A new tank needs to have a pre charge BEFORE installing, to match your system pressure, probably 12-15 PSI of air.

    Check your system pressure now with the boiler cold.

    You want to isolate all parts of the system as much as possible and drain the part where the tank is connected, before removing the tank. Or you will have to bleed air from all parts drained.
  • Jamie Hall
    Jamie Hall Member Posts: 21,553
    I only water came out -- it's bad. The bladder is shot. The reason you feel air at the bottom is that air is there -- a little -- trapped between the bladder and the tank wall.

    Replace it. Be sure to remember to pressurize it to the correct cold system pressure before you connect it to the system!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,079
    edited November 2020

    Hello @Foregone , It looks like you'll be replacing your tank. Here's a photo of one I just did for domestic hot water, but the application is the same. Look at the shut off valve and see that it has a waste built in under the handle. This lets you service the tank without removing it.

    Yours, Larry
    Dave T_2
    EBEBRATT-Ed Member Posts: 13,834

    If you get water out of the air connection the tank is shot, and check the air pressure before installing the tank as others have mentioned.

    Install a valve as @Larry Weingarten mentioned with some type of drain between the valve and the tank.

    Also be very cautious removing the tank it will be VERY heavy if waterlogged
  • rick in Alaska
    rick in Alaska Member Posts: 1,429
    Larry. You really need to have a drain valve between the tank and that shut off valve, otherwise shutting the valve just traps the pressure in the tank.
  • heathead
    heathead Member Posts: 226
    Rick, I was thinking the same thing in the split second before I saw the Cap Bleed screw to unscrew to blead the pressure. Larry I have used those before but never though of using it in this application. Thank you Larry. Great valve application to remember in the future.
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,079
    Hi, Someday, I might figure out how to insert an arrow into a photo. :p

    Yours, Larry
    rick in Alaska
    JUGHNE Member Posts: 10,801
    Stop and Waste design.
    Old school use was for outside wall hose bibs....supposed to close and drain back every fall, usually never happened.
  • Foregone
    Foregone Member Posts: 5
    Thank you all so much for your expertise! I finally have an expansion tank in hand and following all the tips here feel fairly confident in replacing it.
    There's a drain valve placed near the expansion tank so it's no problem to relieve pressure.

    The issue now is that in replacing the expansion tank I'll have added air to the system.
    When heating and cooling guys have been out they purge air from the system with the help of an electric pump. Basically they drain the system into a bucket and pump water from the bucket back into the system (having sealed off the loop they wish to purge with valve closures).
    I saw on youtube some guys purge air from their systems using the feeder valve to run fresh water in, but I don't want to do this since it loses too much antifreeze.
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,079
    Hi @Foregone , How close is the shut-off valve to the expansion tank? If it's not far away, you wouldn't be introducing much air into the system, and vent(s) on the system should be able to deal with it. Maybe some photos of the plumbing would be good for us to look at. ;)

    Yours, Larry
  • Foregone
    Foregone Member Posts: 5
    Here's some photos. The supply side valve is near the expansion tank. The other side is to the left, around in a loop to the right and then all the way down near the boiler. All the zone valves are of course closed for this job.

  • Foregone
    Foregone Member Posts: 5
    Also, down near the floor there's a drain valve a bit upstream of the valve next to the expansion tank. I can use it to relieve pressure or completely drain the isolated loop.

    Could I just relieve pressure and then quickly swap in the new tank before too much water leaks out?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,131
    edited December 2020
    It is difficult to actually see in the picture angle you have posted, but I can see a yellow handle ball valve on the verticle supply pipe just below the air vent and to the right of the expansion tank. It appears closed in the picture. GOOD

    there are 6 or so red handle valves on the pipes behind the heater. If you close all 6 of those valves, the expansion tank will be isolated and you can release pressure in only that area by slowly removing the air vent on the top of the air scoop. when you get near the end of the threads water will start to leak out. You will be under pressure and will lose about 1 gallon or less of water/antifreeze this way. Hold the air vent in place and let the pressure out slowly. Put a large bucket under the tank to collect what comes out. Once the pressure is released, Screw the air vent back on. Make sure the cap on top is sealed.

    At this point, if the water pressure does not seem to be dropping at the air vent, with a gallon of water released, then replace the air vent and call a Pro.

    Get a 1/2" close nipple and a 1/2" ball-valve with waist ready to quickly screw in the opening. be sure to have pipe sealant already applied to the valve and nipple and the valve is closed.

    Now you can remove the expansion tank. As previously stated the tank will be very heavy. It is filled with water. There will be spillage and be prepared for that. You will want to quickly install the valve once the tank drops. once the valve is in place and the chance of air leaking into the system is eliminated, you can attend to any water on the floor and set the expansion tank so the water stops pouring out.

    The hard part is over. Now screw on the new tank and open the valves to the normal operating position and bring the water pressure back up to the normal operating pressure by adding a little water.

    With the failed tank removed, there will be a substantial amount of water/ antifreeze missing from the system. You may want to check the specific gravity (Percentage of antifreeze) of the boiler water after it circulates for a day or so.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • hot_rod
    hot_rod Member Posts: 19,942

    Hi, Someday, I might figure out how to insert an arrow into a photo. :p

    Yours, Larry

    A stop and waste ball valve.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Foregone
    Foregone Member Posts: 5
    Thanks Ed, I finished the job with minimal water loss and the boiler is working GREAT! When I repressurized the system it seemed like the air vent above the expansion tank bled out most of it.
    Boiler pressure is now stable, but I may need to replace the feed valve. It's a 12 psi feed valve but to get 12 psi in the system (cold) I needed to manually work the valve.

    I'll check the specific gravity next. With the bad expansion tank it overflowed a LOT of water so I'm a bit worried about the antifreeze content.

    One last bit that I haven't talked about - I did have a heating and cooling guy out and he spent two days on the system and finally said I needed a new boiler. After the first day I did some reading and tried to check the expansion tank pressure but only got water out of it. When I told the repair dude he tapped the tank and told me it was good because it wasn't filled with water. That sounded fishy to me so I ran it by you guys and you all saved me a LOT of money!