2 recirculation loops

Discussion in 'Tankless Water Heater Forum' started by Noconew, Dec 29, 2020.

  1. Noconew

    Noconew New Member

    Joined:
    Dec 29, 2020
    Location:
    Northern Colorado
    Finishing up install on a Navien NPE240A with option to use internal recirculation or external.

    Hot water tees off right after WH. Can I loop furthest fixture on both sides of house back to recirculation inlet with a tee at the WH?

    One side would be 20 feet and the other about 40. Would only occasionally be used at the same time.

    Is this possible?
     
  2. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    I'm not a hydronics expert, but I think what will happen is that most of the flow will take the shorter path (out 20 feet and back) and less of the flow will take the longer path (40 feet out and back). So with just a tee, it would work, but depending on how the pump is controlled by the unit, you might only get the benefits on the short loop. Or if the pump runs on a timer basis, it might work fine; or if it runs until the return water temperature reaches a certain temperature, then the lesser return flow from the long loop might depress the return water temperature sufficiently that it recirculates until both loops are hot.

    Two things you can do to reduce the imbalance problem. One is to install a balancing valve (just a ball valve, say) on the short return loop before the tee. That way you can throttle down that return side so its effective length is greater. If you match effective lengths, the hot water will still return faster through the short loop, so perhaps you'd want to throttle more to match return times.

    The other option is that your unit's manual indicates it supports an external pump. So you could install a small booster pump on the longer line to balance the two returns. But this is probably the last choice option, because of the extra complexity.

    Cheers, Wayne
     
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  4. fitter30

    fitter30 Well-Known Member

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    Peace valley missouri
    Should work would put in a globe valve in the shorter run that way gives you a way to balance the loops.
    When using external recirculation mode with the built-in pump, observe the following maximum recirculation pipe lengths including fittings (3/4” pipe is recommended):-1/2” Copper Pipe- 100’ (30m) of equivalent length-3/4” Copper Pipe- 500’ (150m) of equivalent length.
    90s 1/2" = 1' equivalent length
    90s 3/4" = 2 '
     
  5. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    A globe valve is better than a ball valve for throttling, because you get finer control over the pressure drop you are adding?

    Those limits are for a single loop, and I take it they are based on the internal pump curve and ensuring a certain minimum flow through the pump and heater. With two parallel loops, it's certainly enough for the longer loop to be under the length limit.

    [In fact, for the case of two identical loops, I think it would fine for each loop to be a bit over twice the single loop limit, since each loop would only need to handle half the flow. So each loop's pressure drop per unit length would be well under half the single loop limit, which means each loop could be over twice as long as the single loop limit.]

    Cheers, Wayne
     
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    A recirculation loop does not need much power or head. The pump in mine only draws 9W, and it works fine pumping water from the basement up to the second level without issue - it's not like it has to prime the loop...it's already full of water.

    THere's at least a few ways to do recirculation. One is to have the pump run most of the time, and use a thermostatically controlled valve at the end of the loop (or loops). That will open or close as it senses hot water at that point. That would use one pump back at the heating source.

    Or, you can use multiple pumps, one on each loop. Everything in my home branches off of one line, so I can get by with one pump and one sensing device (in mine, they're combined, but could be separate). It just so happens that I use one at the furthest point rather than near the WH. It has a built-in aquastat, and an internal timer so only runs during the normal use times. It turns itself on when the inlet water temp is below about 85-degrees, and turns off when it gets to 92. They make a version that is adjustable. Some of the others use a thermostatically controlled valve and most of them shut down the flow when it reaches 105-degrees or so.

    With the system I have, the first time it comes on in the morning, it might take a bit over a minute to get warm water there (the shower is closer to the heater, so it's hot), and then, it only runs somewhere between 3-6x per hour for about 45-seconds or so.

    The copper institute says you should never run hot water through copper pipes faster than 5fps, and that's only about 4gpm on a 1/2" line, or 8gpm on a 3/4" one. If you DO run it faster, expect several deleterious results: water flow noises, potential erosion of the pipe, and, your WH won't like it, either. Plus, if you size your pump for fastest results, you'll be using more power, and the pump will cost more, too. Keep in mind that any pump will need either SS or bronze, which ups the costs a lot over a pump designed for say hydronic heating...potable water will contain some oxygen, and that will rust out a CI pump body or impeller. In a heating system, once filled, the oxygen reacts with the iron, and then, stops since there's none left, so a CI pump can work.

    You don't really want a large pump. Head isn't an issue, since water is just going around a loop, so whatever you push up is being aided by the same amount falling back on the return side, sort of like spinning a bicycle wheel...you only have to overcome friction, not gravity.

    YOu could use a motion or a manual control to turn the system on, but in the system I have, I just let it run most of the day, and the timer disables it when I don't expect to be home or am asleep.
     
  7. Noconew

    Noconew New Member

    Joined:
    Dec 29, 2020
    Location:
    Northern Colorado
    Thanks for all that info guys, much appreciated.

    Still debating on using the internal recirc system (has a buffer) since lines are relatively short and run through conditioned space... Drywall next week so now or never...
     
  8. Bannerman

    Bannerman Well-Known Member

    Joined:
    Mar 19, 2014
    Location:
    Ontario, Canada
    You will need a check-valve in each loop to prevent cooler backflow from the return line from feeding the faucets while they are being operated.

    Advisable to install each check-valve and balancing valve at the furthest distance from the WH where the feed line will connect to the return line.

    Since the pipes are currently accessible, insulate the entire run including each return line back to the WH.

    Instead of continuous or scheduled circulation, suggest installing the Navien Hot Button Accessory to activate the pump on-demand using a button located in each bathroom, powder room, kitchen and laundry room. Activating the pump for a pre-set time when entering the room should ensure hot water will be immedicably available when hot water will be actually utilized without needing to circulate continuously.

    https://www.navieninc.com/accessories/hotbutton
     
  9. Steve_A

    Steve_A New Member

    Joined:
    Jul 20, 2021
    Location:
    Saratoga, NY
    what did you end up doing? I have the same WH with two separate trunk lines right off the WH. how's it working?
     
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