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Slugs of air from indirect hot water

Bill01568 Member Posts: 1
edited March 2021 in Domestic Hot Water

We had our heating system updated in a condo that we plan to rent out, but which is only used infrequently. (We're using it as storage for now.) New system has an oil-fired burner. Boiler. Indirect hot water tank. Used existing Slant-fin baseboard. All of the copper around the boiler was replaced. Previous boiler was tankless, so all the copper to the tank is new too, as is the indirect tank. Used existing oil tank.

If we don't use the hot water for a few days, then turn on the water in the kitchen sink all the way to hot, we'll get a little water, then some chugs of air, more water, more air, ... this keeps going for about 30 seconds before it calms down.

If we turn on hot water in other locations we can also get some air out. Multiple times I've gone to all locations to let air out. After a week the air is back.

HVAC person used ProPress. Obviously air is entering the system from someplace. It must be a place where there is a slight inward pressure? Not sure how to locate an inbound air leak in a system like this. I asked the HVAC person and he suggested that the anode in the water tank had already started dissolving after only 6 months, and that was somehow causing the air. We're on town water. Some chlorine. I'm not sure I'd believe that but I haven't called him back to check out the anode.

How do I find this inbound air leak? It's frustrating.

P.S. One zone pump for first floor, one zone pump for second floor, and one zone pump for indirect hot water tank.

Bill in MA USA


  • Jamie Hall
    Jamie Hall Member Posts: 23,428
    If all of your system pressures are normal you can't have an inbound air leak. Think about it. Your domestic water pressure is at least 30 psi -- and may well be quite a bit higher. Your heating system -- which isn't directly connected to the domestic water anyway -- is probably around 15 psi.

    I'd be more concerned about electrolysis taking place in that tank somehow. Is everything -- I do mean everything, pipes, tank, anode rod, boiler, etc. etc. -- thoroughly bonded and grounded?

    Either that, or if you are on well water it is conceivable (though it shouldn't happen) that there is a problem with the well; there are a few odd things that can go amiss with submersible pump wells which can do that, but I'd expect to see air in the cold water then, too.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 7,574
    Are you turning off the water main when you are not there?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Bill01568
    Bill01568 Member Posts: 1
    We're not turning the water off when we leave. Condo is 1 of 4 in the same building. There are several other condo buildings; 6 in total. Condo is on city water. Our washing machine is in the basement. Normal water fixtures in first floor and second floor. Nothing in unfinished attic. We didn't have this problem in the 10 years we lived there before the heating system was replaced. I'll check with the person in the adjacent condo to see if she's seen any issues like this. Can't the Bernoulli effect cause negative pressure if water speed is sufficient, slowly drawing in air bubbles?
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    edited March 2021
    ProPress has nothing to do with your problem.
    Might not be air, might be hydrogen gas. The awesome @Larry Weingarten could speak more about it

    There was an error rendering this rich post.

    Larry WeingartenRobert O'Brien
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,331
    Hi @Bill01568 and thankyou @STEVEusaPA ! Is the "air" just in the hot side plumbing? And, Is the tank a glass lined one? If yes to both of those, you likely have hydrogen gas being generated in the tank. This would be most apparent during the first hot water draw in the morning, after gas has had a chance to build up. Two ways to fix this are to put an air vent in the line above the tank, so gas is vented out as it's generated. The hot piping needs to make a trap so gas cannot just float past the vent. The other and probably better long term approach is to install a powered anode rod. These don't seem to make gas and have the side benefit of getting rid of the rotten egg odor in tanks that sit unused for periods of time.

    Yours, Larry
    PC7060Robert O'Brien
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,060
    edited March 2021
    Do you have an expansion tank on the domestic water? They have a lot of air pressure in them. If there is a pinhole in the bladder then it will put air in the system as you described. But that will end as soon as the tank runs out of air.

    I'm thinking Hydrogen Gas is more likely in the vacant building. If you could find a way to collect it, you might be able to sell it to the Hindenburg Company. I hear that get all fired up about that stuff.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Bill01568
    Bill01568 Member Posts: 1
    It's a Heat-Flo indirect tank.

    Neither my wife nor I have noticed any smell in the burped-up air and we're both fairly sensitive to smells. But H2 wouldn't smell anyway. I guess I could try to capture a small amount of the gas then expose it to a small flame. If it goes POOF! I'll know that it's probably H2. I'll also have to check re: bonding. Is the thinking that I've got electricity splitting H2O into H2 and O2 because of some voltage difference in the system?

    Would it be best if the hot water circulator didn't run at the same as either of the 2 baseboard zone circulators? I believe it's not set for hot water priority. I've always figured that if the house has gotten cold due to an extended power outages that heating up the house should have priority over heating the water. I've considered adding a little "off/auto" switch to the tank's hydrostat.

    Yes, just the hot water has the "air".

    I'll try to get a photo next time I'm there including the plumbing, size of pressure tank, etc.. Is there anything else I should include in a photo?