Air lock, or why can you not use logic to explain to your wife?

Discussion in 'Water Heater Forum, Tanks' started by LLigetfa, Feb 27, 2011.

  1. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    OK, here's the scoop. My water is aerated with a micronizer for iron removal. That is the only point where air is entering the system. Cold water comes up through the floor, into the top of the tank. Hot water comes out the top of the tank and goes back down through the floor. It's been that way since I built the house over 12 years ago. Now, I will 'fess up that I should not have taken the cold supply to the WH off of the side of a Tee, as the straight through wins out every time. (Maybe that is sage advice to others, looking to save their marriage).

    When this aerated water is heated in the tank, some of the entrained air is released causing an air lock in the plumbing. When drawing warm water, I always draw just hot at first to purge the cold water (and the air) from the line. When, and only when it runs hot, I mix in cold to achieve the desired temp. Been doing it this way for 12 years.

    Enter the wife... she has a large soaker tub... lemme backup 12 years... When spec'ing the tub filler, I made sure to use one that ran full bore, knowing that a slow filling tub is a PITA. When the cold tap is full open, it steals pressure from the cold supply to the water heater and with the air lock, no hot water flows despite the hot tap also being open.

    Now, the wife knows the ratio of hot to cold what the tub usually requires, and she goes about her way, first setting the cold, then the hot, and fills the tub, with... get this, cold water. At this point, I'm in hot water or the doghouse or (insert fav euphemism here). Telling her to run the hot to purge the air seems simple enough but she's not buying it. Trying to explain to her why it does what it does in technical terms and how to prevent it buys me no brownie points. You'd think that in 12 years, that this sort of thing would have manifested long ago. By her logic, my logic must be flawed since this is only the second time in 12 years (twice this month to boot) that she stepped into a soaker tub full of cold water. What are the odds?

    Redoing the supply line to the WH is not an option. Remembering to purge the air every morning just in case it happens again, is not practical. BTW, she has also complained that it takes way too long for the water to run hot at her sink. So... I start to think, maybe I can solve both problems with a recirc pump, however I have no way to run a return line.

    Enter the Grundfos Instant Hot Water Recirculation Kit with the crossover valve at the point of use. I am trying to understand how it works. I'm guessing that it provides a few PSI boost on the hot water line, just enough to create flow to the crossover with a temperature regulated valve that opens when cold and closes when warm. With the weight of 5 feet of water, it would need at least 2 PSI just to overcome the airlock. I cannot cut the recirc into the high point of the hot water line as the pump would lose its prime. Does the recirc have to be on the hot side or could it be cut into the cold supply?

    If that won't work, should I just cut in an automatic air vent as is used in Hydronic heating systems? Help save my marriage.
    Last edited: Mar 1, 2011
  2. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    OK, I showed her how to purge the air and we are in marital bliss.

    Back to my Q... did I stump the panel?
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,817
    Location:
    New England
    How about one of those 'top-hat' type air eliminators plumbed into the top of the WH? It would need to be replaced periodically as it would corrode eventually (but maybe not before the WH died). As I understand it, it has a float in it. WHen water is pushing up on it, it is closed to the atmosphere, but if air accumulates, the float drops, it opens itself up, the air is purged out, and then fills with water which then closes the valve. They are generally used more on a boiler, but would seem to work on the WH as well. Since this one is made of brass and SS, it should work on a potable water system http://www.watts.com/pages/_products_details.asp?pid=664
  4. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Never use logic and 'woman' in the same sentence. Absolutely incompatible.

    Woman are notorious for 'air lock' between the ears, and need a lot of unpleasant maintenance to keep working.

    And as they age, the maintenance becomes very vague - no one knows what it is they want, including themselves.
    Last edited: Mar 4, 2011
  5. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,270
    Location:
    Cave Creek, Arizona
    Your "logic" is faulty. An air bubble CANNOT prevent pressurized water from flowing. The water will just push the air out ahead of the water flow. Therefore, you have some other problem, possibly terribly low water pressure, or a British style water system with a cistern in the attic and water flowing by gravity, rather than pressure.
  6. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    You're not the first person to say that but I stand by my logic.

    True, as long as there remains a pressure differential. The water (and pressure) leaves the iron filter and water softener via a 3/4" copper line. In-line resistance from said filter/softener limits flow to around 6 GPM, so as flow increases, pressure decreases.

    The water heater competes with the cold water line for this pressure. When the cold water valve at the tub filler is wide open, it steals pressure from the hot water tank. The flow restriction at the tub filler can provide a slight back-pressure to the hot side of the mixing body at the tub filler.

    Ups and downs in a water supply generally cancel themselves out as long as there is no air. Take a syphon for example... the weight of the water going downhill will pull the equavalent weight of water going uphill. Introduce air in the top of the tank and now there needs to be slightly more pressure differential to push the air uphill until it breaks the crest of the hill and gravity assists.

    Add to all of this my plumbing screw-up. The water heater gets its supply from the side of a Tee. Water prefers to go in a straight line. Given a 3/4" supply into the Tee and two, 3/4" outlets from the Tee, the straight-through will win out as there is no remaining pressure for the side take-off. The side takeoff could even get a slight negative pressure, like a venturi, and suck instead.

    So, as you see, there can be a culmination of several factors that when combined, conspire to create marital discord. Now this all sounds like incredible odds and in 12 years has only happened twice. Purge out the air and initiate the flow by first opening the hot valve full and then opening the cold, and all is well. This is why I was considering adding a circ pump like the Grundfos Comfort system or Watts 500800 but with the propensity of entrained air collecting on the hot outlet of the tank wondered if the pump could be mounted on the cold side. I don't know if there is any temperature sensing logic in the pump or not. Looking at installation manuals, I see it being inserted on the hot side. If it has to be on the hot side, an air bleeder like Jim suggests would solve the issue albeit at a slightly higher cost and concern about whether it is rated for potable water use.

    Another thought I had was to tap into the high point of the line with saddle valve like what's used for supplying humidifiers and run a very slow drip to the drain as a trap primer.

    Also, I have a mixing valve that blends hot and cold for the toilet supply to combat tank sweating and if I had known then what I know now, I would have plumbed it above the hot water tank to bleed off the air. That, of course would only work if the toilet was flushed first thing in the morning before the tub is drawn. Anyway... the wife now understands that opening the hot before the cold mitigates that issue, so the remaining *plumbing* impediment to marital bliss is that it takes so long for water to run hot at her lavatory basin upstairs. I say "her lavatory" because I use the downstairs lavatory which shares a wet wall with the hot water tank.
  7. oreo

    oreo New Member

    Messages:
    6
    Location:
    Georgetown, Ontario
    I agree with HJ. An air bubble, cannot stop water flow, regardless of use of T's or whatever.

    I'm thinking your mixing valve (that connects the hot water to the cold water supply), is the problem. It sounds like it's opening if you turn on the cold water first.
  8. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    There is no "magic" in the mix, no pressure balance, no temperature sensing, no anti-scald. They are just two low tech full bore valves that come to a Tee and then out the Roman tub filler. There is no marked difference in flow out the spout with one valve open versus both valves open, meaning the restriction is on the end of the Roman tub filler. When this soaker tub is being filled, it steals so much of the pressure from the rest of the house that the taps downstairs will barely produce a dribble.

    Anyway... I don't care to debate it further. What I was hoping for was an answer to my question about putting a Watts or Grundfos booster pump on the cold water side versus the hot side.
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