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Thread: Gravity recirc loop strange problem

  1. #1
    DIY Member benze's Avatar
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    Default Gravity recirc loop strange problem

    Hi,

    I just tried putting in a single branch gravity recirculation loop and am running into a really strange problem. I've got a bottom feed hot water tank (ie: the cold water enters at the bottom), and the hot water exits at the top.

    My incoming cold water supply is 1" PEX and my outgoing hot water is 1" PEX. The 1" PEX is branched off to 3/4" PEX which is connected to 1/2" copper to my kitchen sink. I tee'ed into the 1/2" copper and ran a 1/2" pex back to the HW tank. At the tank, I removed the original drain faucet and put in a Tee and connected the return line to that point.

    So here is the weird problem. When I first put this config together, everything "seemed" to work better. I was getting hot water at the kitchen sink within 3-5 seconds vs. the original 40+ secs. However, I soon noticed that the hot water coming out of the faucet wasn't as hot as it should have been. I figured that I was having a problem with reverse flow; that I was getting cool water going up the return line and making the hot water at the faucet less hot. So I added in a check valve on the return line, figuring that would prohibit any cool/cold water from travelliing back up that path.

    However, now the recirc loop doesn't seem to work at all. I'm back to waiting 40secs to get hot water from the faucet.

    I'm really confused at this point as to why it seemed to work without the check valve and fails with it.

    I know that a recirc pump would resolve the situation, but am trying to figure out why it would seem to work better without the check valve than with it.

    I've used a SharkBite check valve, if it makes any difference.

    Thanks for any insights!

    Eric

  2. #2
    Plumbing Contractor for 49 years johnjh2o1's Avatar
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    What type of check valve did you use a swing check or a spring check? A gravity system will not open a spring check.

    John

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    Moderator & Master Plumber hj's Avatar
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    Gravity will often not open a swing check either. We used to install the check valve an angle so that the "flap" hung slightly open to allow the circulation, but back flow would push it closed. A small hole in the flap will do almost the same thing.

  4. #4
    DIY Member benze's Avatar
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    Quote Originally Posted by hj View Post
    Gravity will often not open a swing check either. We used to install the check valve an angle so that the "flap" hung slightly open to allow the circulation, but back flow would push it closed. A small hole in the flap will do almost the same thing.
    I am using a SharkBite check valve, and I'm not 100% sure, but I believe that a SharkBite checkvalve is a spring valve. That's what you get for being lazy. I had both in my hands, and figured for an extra 2$ for the SharkBite, I would just be lazy and save the sweating of a swing valve.

    I guess I'll go grab myself a swing check valve Monday and test it out. Ideally I want to install it on a vertical pipe; will a swing check valve work on a vertical pipe? Will the backflow be sufficient to close the flap? Do I still need a hole in the flapper? It should, in theory, hang open if there is no back flow, correct?

    Thanks!

    Eric

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    DIY Member arfeller's Avatar
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    You do not want to put the check valve vertical. The valve should be placed on a horizontal. I have a gravity system that works very well and I used a swing type check valve on the horizontal with a 1/8" hole drilled through the center of the check valve flapper.

    You may be able to get away with a slightly smaller hole. Good luck!

  6. #6
    DIY Member benze's Avatar
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    Quote Originally Posted by arfeller View Post
    You do not want to put the check valve vertical. The valve should be placed on a horizontal. I have a gravity system that works very well and I used a swing type check valve on the horizontal with a 1/8" hole drilled through the center of the check valve flapper.
    HJ mentioned that he has installed them at an angle; any idea what the max angle would be? Is 45deg too much? Putting it at horizontal would require a bunch of extra effort to make it a nice looking / neat job. If I really need to put it horizontal, can I put 90 elbows right before and right after? Or is the sudden change of direction too difficult for the gravity circ to handle?

    Does your 1/8" hole not cause you backflow issues? What size valve is it? (3/4"? 1/2"?) I realize the flow wouldn't be enormous, but I assume you would still have a little backflow through there? Or is the recirc loop pressure strong enough to counter any water trying to backflow through the hole?

    Tx,

    Eric

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    DIY Senior Member Hairyhosebib's Avatar
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    They do make a spring check for solar hot water heater systems that take very little pressure to open. Maybe only a 1/2 LB PSI.

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    Plumbing Contractor for 49 years johnjh2o1's Avatar
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    Quote Originally Posted by Hairyhosebib View Post
    They do make a spring check for solar hot water heater systems that take very little pressure to open. Maybe only a 1/2 LB PSI.
    Solar systems have pumps they don't run on gravity.

    John

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    Moderator & Master Plumber hj's Avatar
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    1. the return line to the heate should be flowing DOWNWARD, if you expect it to work, and a swing check does NOT work when it is pointing down.
    2. The angle depends on the angle of the "flap" in the check valve. It is installed at slightly more than that angle so the flap is slightly open to allow flow, but still closed enough so that any backflow will close it entirely.
    3. That is why I said the small hole will "do almost the same thing".
    4. A gravity system has basically ZERO pressure so any resistance will stop the circulation. In fact any up, then down, (or vice versa), offset will also stop it from working, which is why few gravity systems will work unless the system was installed with it in mind.

  10. #10
    DIY Member arfeller's Avatar
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    Hopefully I attached the photo correctly. I'm not a plumber and installed this gravity loop myself. So there may be errors...

    Sometime in the future I will be pulling out that plastic nut between my brass fittings and hot water heater. Also adding a T with a quarter turn valve to empty the tank is also something I should have.

    The pex is 3/4" and so are the fittings. I do not have any issues with back flow.
    Attached Images Attached Images  

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    DIY Member benze's Avatar
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    Thanks for the tips HJ. Have a few questions based on your response though:

    Quote Originally Posted by hj View Post
    1. the return line to the heate should be flowing DOWNWARD, if you expect it to work, and a swing check does NOT work when it is pointing down.
    Yes - return line is flowing downward. If the Swing check is installed vertically, the flap would be essentially be normally open (ie: just hanging down loose). I was thinking that if the backflow was strong enough, part of the stream would catch the edge of the flap and push it upwards to a horizontal position thereby shutting the backflow. But would it stay closed, or would the backflow pressure drop enough for the flap to open up again?

    2. The angle depends on the angle of the "flap" in the check valve. It is installed at slightly more than that angle so the flap is slightly open to allow flow, but still closed enough so that any backflow will close it entirely.
    As in point #1, if the backflow closes the flap, would the pressure then not drop thereby allow the flap the reopen? And once reopen, then reclose due to the backflow? Is the flap basically opening/closing/opening/closing cyclically while the tap is on?


    If I find I need to place it on the horizontal with a hole drilled thru the flap, can I put 90s right before and right after so I end up with vertical->90->swing check->90->vertical? Or do I need some run along the horizontal plane to keep the circulation/momentum of the water moving.

    Thanks again for the tips. I'm learning much.

    Eric

  12. #12
    DIY Member arfeller's Avatar
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    Your walking a fine line with hj, I think this is about to get interesting : )

  13. #13
    Moderator & Master Plumber hj's Avatar
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    The backflow would NEVER close an upside down check valve, period. The backflow will close the check valve ANY TIME it occurs, and keep it closed as long as there is a reverse pressure differential, which is what you want to happen. As long as the valve is horizontal, it makes no difference what you do on either side of it.

    Artfeller;
    I would NOT, repeat not, have connected to that water heater drain valve. They are not designed to hold pressure when opened. You should have opened the valve all the way, then while pulling on it rotated it the opposite direction, to remove the plastic piece. Then you would have had a 3/4' brass pipe to screw whatever fittings you need to make the connection and provide a drain valve. Using 3/4" pipe was overkill. you could have used 3/8" or 1/2" and got the same, or better, performance.

  14. #14
    DIY Member benze's Avatar
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    Quote Originally Posted by hj View Post
    The backflow would NEVER close an upside down check valve, period. The backflow will close the check valve ANY TIME it occurs, and keep it closed as long as there is a reverse pressure differential, which is what you want to happen.
    Good point; had forgotten that the pressure on the faucet side of the checkvalve would drop since the tap would be open thereby allowing the checkvalve to remain closed.


    Not quite sure if your definition of an "upside down check valve" is one that is rotated vertical at 90deg, or one that is installed in the wrong direction.

    Thanks again for all your tips. Will try a couple of things out today now that the plumbing store is open and can swap my sharkbite for a swing check.

    Eric

  15. #15
    DIY Member arfeller's Avatar
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    Yeah, I goofed on that assemble. My buddy told me about removing that drain plug after I had it all together. It is on my list... I think I'll move it back up to the top now that you have reminded me : )

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