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Community and Q&A

Hot water recirculation loop with PEX homerun system?

jberks | Posted in Green Building Techniques on

Has anyone played with the idea of doing a hot water recirculation loop with a pex manifold homerun system?

My desire for a recirc loop is mostly inspired by Gary Klein and my reading/watching of his work. Although he doesn’t recommend a home run system because you can’t recirculate hot water…. well, I think it might be possible. Maybe I’ll sound crazy here, but what can I say, I love systems design.

I see this as 2 issues:

1. Is it doable?
I imagine the easiest way would be a circulation pump on the output side of the hot water tank and before the viega pex manifold. each selected hot run (showers and faucets) would have a Tee below the fixture and a checked return pex line. then a return manifold on the supply side of the hot water tank completes the loop. (I can draw a diagram if needed)

I don’t see why it wouldn’t work, but I’ve never seen a system like this in action. The idea being this system would allow one pump to handle all the circulation vs an individual pump at each desired fixture.

2. Is it more efficient?
The idea of turning on a shower or faucet and dumping a bunch of water down the drain as we wait for the hot water line to purge (behavioural waste) gets me a little upset. Its inefficient with resources and user experience. Trunk and branch systems are even worse for wait times because the larger diameter water lines and low flow shower head output create a slow hot/cold mixing within the pipe. That being said, I’ve never played with a homerun system before but I’ve heard how a 1/2″ pex line purges water quite quickly due to its low internal volume.

I personally wouldn’t be able to do an accurate model, but my question is along the lines of: Would the small amount of water dumped from a 30′ 1/2″ pex line outweight the energy used of recirc pump + hot water tank running for 10mins once a day. Which would actually be more green?

Any energy and mechanical nerds out there? I would love to hear your comments.

Jamie

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Replies

  1. walta100 | | #1

    If I understand your plan please consider

    1 You will need a check valve for each return.

    2 The pump will be supplying parallel water circuits, so the flow rates of each circuit will be different the circuit with the least length and fewest bends will get most of the flow. The long circuit with lots of elbows will get almost no flow and never get warm. Think of it like a parallel electric circuit, the water will take the path of least resistance.

    3 If your goal is low energy usage recirculation loops are in opposition to your goal.

    4 If your goal is use less water to save money on water bills. This plan likely to fail as most people have access to water so cheaply, that spending money to saving it cannot be justified economically. The motto of my local water company is “delivered to your home for less than a penny a gallon”.

    5 If your goal is be green and reduce water and energy usage without regard cost. Then design your home so all the plumbing fixtures very close together.

    But let’s be honest recirculating loops are a luxury designed to save your time wasted waiting for hot water at the expense higher energy bills and higher install costs.

    I have a recirculating pump loop with electric tank less heater. It works this way, anytime there is a draw of hot water that will turn on the water heater. My control senses the heater current. If the current loop temperature is below the set point of 110° the pump will run until the loop reaches the set point. The pump control will time out after 3 minutes and stop regardless of loop temp. So when I want hot water I turn on any hot water faucet for a few seconds and do something else for a few minutes almost no wasted water.

    I never could see any advantage in home run plumbing systems. All I see are problems highest install cost, highest average wait time and highest average water waste.

    Please consider your goal and design with that in mind.

    Walt

    1. user-1112742587 | | #6

      I'll mention 2 advantages that I like. First is the ability to shut off water to any faucet or appliance without affecting service to others. The second is to RAPIDLY perform the first. Soon after plumbing a home run to my clothes washer, my dad was helping me put up dry wall. You'd think I could have remembered the pipe location for at least five minutes before putting a screw right through it's middle. But, Nooo! I'm not sure Dad knew what was happening at first when I jumped up and made a bee line to the basement and shut off the water at the manifold.

      The good new is that after 20 years that is the only time I remember having a leak in my PEX system (Wirsbo now Uponor pipe with PEX ring clamps). Yet!

      1. josh_in_mn | | #7

        You should have had that PEX at least 1 1/2" from the edge of the stud, then you wouldn't have hit it with a screw. Failing that, there are protection plates that code requires to protect vulnerable plumbing from this sort of thing.

  2. Jon_R | | #2

    You can put valves on the return lines and adjust them such that head loss in each parallel loop is the same.

    A homerun system has much more pipe surface area and therefor even more heat loss (when heated continuously).

  3. mcdillio | | #3

    You might be able to do a hybrid homerun / branch and tee system, with the supply side as homerun and the return side branch and tee. Again, you'd need check valves on each return line.

    For balancing purposes, you would pipe the return side so that the fixture with the highest head (longest run, most bends) would be the first to get returned, and the fixture with the lowest head would be the last.

    Minus the homerun part, this style return is called reverse return; it's sometimes used in large hydronic heating/cooling applications and makes balancing the system easier, although it usually requires more pipe.

  4. Expert Member
    BILL WICHERS | | #4

    1: Probably, but not easily. Chances are, in your home-run system, each run is a different length and some may also be a different diameter of pipe. Longer runs will have more resistance to flow than shorter runs, but longer runs of larger diameter pipe might have the same resistance, or even less resistance, than a shorter run of smaller diameter pipe. This will make balancing a pain, and I doubt the system will work well without at least some degree of balancing.

    You’d need a check valve on each return, preferably a swing-type check valve which offers less resistance to open than does the more common spring-loaded check valve. You’d also need a valve to balance each run. DON’T use gate valves for this. Ball valves are better, but not ideal. “Real” balancing valves are expensive so you’ll probably want to use either a ball valve or a stop valve.

    2: The answer to your question “is it more efficient?” Depends on how you define efficiency. The usual trade off for recirculating systems is less water waste but more energy use. Recirculating water in the pipes essentially makes all your piping act like a long and skinny radiator, losing heat along the way. Your water heater will have to use more energy to maintain the system temperature because of this additional heat loss. Because the recirculating system allows for each fixture to get hot water fast, there is much less “let the water run until it gets hot” water waste. You will absolutely want to insulate ALL of your hot water lines (supply and return) if you choose to put in any type of recirculating system.

    I think you’ll find it easier, assuming this is new construction, to use a tapped trunk type of distribution system if you want to use a recirculating system. It will be much easier to implement a practical recirculating system this way, and you might even be able to avoid the need for a pump if your hot water heater will be in a basement (look into a “thermosiphon” system).

    Bill

  5. Expert Member
    AKOS TOTH | | #5

    I run a recirc on a home run system. Pretty much how Bill describes. There is a balancing valve for each section that gets recirced. Even with insulated pipes it is an energy pig, not a problem in the winter time, but quite noticeable in the summer utility bill.

    If you do install this type of setup, I would recommend you take a careful look at controls. Over the years I've tried a couple of different setups, currently I'm running a timer with temperature controls on the return line (recirc only when I'm at home and only until the water gets warm at the pump).

    One bonus you can do is run the return line underneath the bathroom floors (mine is embedded in the mortar bed under the tiles). If the recirc is running, should keep the floors nice and toasty without having to install a separate floor heat system.

  6. Andy_ | | #8

    A big selling point of the home run system is that it doesn't need recirc pumps. A 1/2" line will purge very quickly unless you are running the line to way over even McMansion lengths.

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