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Thread: Check valves in hot water recirc loop

  1. #1
    Electrical Engineer jdf405's Avatar
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    Default Check valves in hot water recirc loop

    Hi all,

    About 2 years ago I implemented a recirc system in my house that uses a small Taco pump and a third recirc leg. We're really happy with how it works for the most part. Check the schematic... I installed a pair of check valves on each of the "input" legs to the pump to prevent cold water from somehow sneaking back into the hot leg, and they do work.

    My question is regarding types of check valve... At the time I just used some plain old swing checks that I got from a plumbing store. They are not sprung, but rather rely on reverse flow / pressure to shut the valve. Now, after a couple of years of use they make a loud bang when the hot tap is opened. The valve is slamming shut. Obviously they're supposed to be shutting, but I started to research and I'm finding that there are dozens of different kinds of check valves. Some advertise silent operation, some have springs, some have seals made of buna, HDPE, etc...

    I'd like your thoughts / recommendations / advice on what to look for in replacements. They need to be rated for drinking water as it is conceivable someone will drink the hot tap. Obviously "quieter" operation is what I'm looking for here. Silence would be nice but I'll be reasonable. Also, since they are in the crawl space below the first floor, long life is desirable... I am not looking forward to servicing this!

    Much appreciated!
    Jonathan

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    Forum Admin, Expert Plumber Terry's Avatar
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    When I used to use swing checks, I would drill a hole in the flapper to let it drop down smoother.
    Now I use spring checks.
    On issue with combining two recircs, is the balancing of the two. If you have pretty even friction loss, that may be okay.

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    Electrical Engineer jdf405's Avatar
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    Terry,
    Thanks much for the info. I actually have two valves, one in each leg, that I used to balance. Didn't show those for clarity. Any particular types of seat material to go for or avoid on the spring checks?

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    Moderator & Master Plumber hj's Avatar
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    Using two check valves and two balancing valves introduces a new "dynamic". The two sides will have to be adjusted so that there is enough flow to operate the check valves, AND also enough total flow to NOT "starve" the pump and create cavitation, (control valves on the inlet to a pump can always create problems which is why they should only be on the pump's discharge, and is impossible with your installation). Your real problem may be to find out WHY the pressure fluctuates so greatly that it "overwhelms" the pump's flow and closes the check valves.
    Last edited by hj; 02-03-2014 at 05:39 AM.
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    Electrical Engineer jdf405's Avatar
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    So there is no problem whatsoever when the pump is running. The system is balanced just fine and works exactly as designed and desired from a flow perspective.

    The bang of the check valves occurs only after when pump is off and then a hot tap is opened. If you look at the schematic and envision the path of least resistance for hot water, you'll notice that its actually from the cold going towards the HWH through the check valves, then to the hot leg. This is why the check valves shut when a hot tap is opened, and this is the exact situation they were designed into the system to prevent - sneak of cold back through the recirc leg into the hot leg. The issue here is the noise of the valves, not the function of the system as a whole.

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    Moderator & Master Plumber hj's Avatar
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    You are missing the point. The check valves should be CLOSED when the pump is off, so therefore a pressure drop could NOT "close" them. And, in a normally operating system, there should NOT be that great pressure drop in the hot water system to "close" them, even if they were open.
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    Electrical Engineer jdf405's Avatar
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    How do you propose the check valves be closed when the pump is off? (Type / design of valve, gravity, etc...?) I am posing my question looking for advice on a self closing check valve... So I am still not clear what your point is - perhaps to explain to me that the system doesn't work or is fundamentally flawed? It works extremely well with the exception of the existing check valves banging shut when I open a tap and the pump is off. This simple fact indicates the valves are not closed, as you state they should be - and I do not disagree! If they were closed the noise would not occur. So instead of debating what should and shouldn't be, lets address the reality that it is what it is, and maybe there is something that can be done to improve upon the existing design. Terry indicated simply and clearly what he does...

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    Moderator & Master Plumber hj's Avatar
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    quote; I am posing my question looking for advice on a self closing check valve.

    ALL check valves are "self closing" by definition, in fact "closed" is their normal position. FLOW is what opens them then they close when the flow stops. THEREFORE, your check valves should ALWAYS be closed, unless the pump is running, (and if it is running it should create enough flow that opening a faucet would NOT close them), and also, therefore, they could NOT be "slammed closed" by opening a hot water faucet. ERGO, you have some other problem that you have not diagnosed yet.
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    Electrical Engineer jdf405's Avatar
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    I understand what you're saying. Lets talk about the type of check valve I have in there and how it closes. These have no spring or self closing feature; I inspected them prior to installation. They require gravity (or reverse flow) to close the flapper / check. Therefore, I contend that it would take a very small amount of forward flow to keep the valve ever so slightly ajar. Another possibility is that when the pump stops the valve flapper slowly descends towards its seat. It has to "push" the water out of its way as it drops into place. The viscosity of the water slows the descent. The water goes around the unseated flapper on its way down. If the valve is not fully seated, due to either situation, I posit that the slightest back pressure on the valve will finish the job of seating it, causing the bang.

    Thoughts?

    In the process of diagnosing the location of the noise to the valves, I have gone under the house and pump my ear against the check valves while someone opened and closed a tap. I can hear them knocking consistently. If I move to other parts of the system I hear the noise, but it is much quieter.

  10. #10
    Forum Admin, Expert Plumber Terry's Avatar
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    Quote Originally Posted by jdf405 View Post

    Thoughts?

    In the process of diagnosing the location of the noise to the valves, I have gone under the house and pump my ear against the check valves while someone opened and closed a tap. I can hear them knocking consistently. If I move to other parts of the system I hear the noise, but it is much quieter.
    That's why I've been using the spring checks. I think the "swing" checks can be noisy.

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    Moderator & Master Plumber hj's Avatar
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    quote; alve flapper slowly descends

    Unless you are flowing molasses, the "flapper" descends IMMEDIATELY, and it takes more than a "minimal" flow to open it, which is why some plumbers drill a small hole so the convection currents will allow the water to flow through it. You are NOT convincing me of anything, yet. You have still NOT told me WHY there is a pressure drop when a faucet is opened.
    Licensed residential and commercial plumber

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    Electrical Engineer jdf405's Avatar
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    Thanks again Terry! Gonna get me some spring checks and be done with it!

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