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Community and Q&A

Recirculating Pump vs. Shorter Runs / Narrower Pipes for Hot Water

orange_cat | Posted in General Questions on

New consturction, copper supply lines L (which I understand come is flexible rolls too). Wondering if it is necessary to have a recirculation hot water pump or is it enough to rely on short(er) runs with narrower hot water supply lines?  Zone 5A. In-floor heating throughout (does it hamper installation of hot water supply?) and the stacks are along exterior wall, so avoiding running supply lines through the horizontal outermost dimension.

(Objective: hate waiting for hot water, keen for as immediate hot water as it gets at sink in the kitchen, and the two upstairs bathrooms sinks and showers. Other bathrooms and sinks not concerned. Not keen on recirculation pump because of the inefficiency of running it frequently – do not have the most predictable pattern of use – and just “one more thing to break/monitor/service.” I like it simple.).

I am attaching my diagram (not the plumbers or the architect, so please bear with me if it is hard to read) of the two plumbing stacks. Everything (with one exceptions) is along a single side of the house, within a 3-story rectangle that is 8.5 feet deep and 35 feet long, with 2 vertical stacks described from the bottom (Stack A: mechanical, garage sink, 1/2 bath, full bath, Stack B: full bath, kitchen, full bath).  One exceptions – there is a washer (which heats its own water if needed) on the top floor separated from the top floor Stack A bath on the opposite wall of the house.

The upstairs most isolated hot water intakes in the top floor bath Stack B is what makes the rectangle large. Ignoring Bath faucet and double sink faucet in that bathroom makes the 8.5 by 27 floor rectangle for all supplies (again, forgetting the laundry on the top floor and now the freestanding bath and double sink in the top floor bathroom. I do not even care for the bath itself – the water is not wasted even if hot water is not immediate, but the sink is the most used and would much prefer hot water fast).

(1) is recirculation pump the best solution
(2) is 2 “trees” the best layout of should there be direct line to kitchen sink?
(3) is the current layout the best to get hot water to the top floor double sink?

And what is the realistic pipe diameter to reduce waste and NOT suffer “trickling” water supply?
Any other comments?

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  1. Expert Member
    DCcontrarian | | #1

    The thing I have found makes the most difference is a low-flow showerhead. My current favorite is the High Sierra -- -- it's 1.25 GPM and I feel it doesn't negatively affect the shower experience at all. Using less water just makes everything else easier. I'd buy one for your current shower and see if you like it.

    I'm not a big fan of recirculating pumps, I prefer the let-the-water-run-until-it's-warm approach. The biggest drawback of the low-flow showerheads is that it takes them longer to let out the cold water -- although if you have a tub diverter that's not an issue.

    The way to minimize wait time is to have the shortest, thinnest pipe possible between the showerhead and the water heater. The chart here has stats for various sizes of PEX: , copper will be similar. You want to look at gallons/100 feet and GPM. With a 1.25 GPM shower head you could probably get away with 3/8" supply lines to the shower, but 1/2" is code minimum in most places.

  2. joshdurston | | #2

    Was wondering about putting a small 120v under counter hot water tank in series on the hot pipe in distant bathrooms.
    Is that better or worse than recirc/dumping cold water.

    1. Expert Member
      DCcontrarian | | #3

      That's fine for handwashing. For showers, it's not going to be able to heat the slug of cold water sitting in the pipe fast enough. You're still getting a slug of cold water at some point in your shower.

      1. joshdurston | | #4

        I guess the ideal solution is occupancy or switch driven recirc. Was thinking it would be nice if the toilet could selectively do a flush with hot water (to drain the cold slug) since it's often used prior to a shower anyways.

    2. 4khz | | #16

      This is what I did. But as DC stated, I did it for lavoratory sinks only. For the shower, I am using a Kohler remote (activates digital valve) located at the entrance of the bathroom to start the shower. For the tub, it doesn't really matter for me since it takes 20+ minutes to fill.

    3. lbutler | | #19

      I did this and it worked well. I never noticed a cold water slug, and it was great having the hot water immediately, whether for the sink or shower. No doubt a lot depends on how long your run is from the main water heater to the under-sink unit. Mine was relatively short, 20 feet or so, which meant that the incoming cold water was able to mix readily with the hot in the small tank heater. I used a 2 gallon unit, but one could easily go to a 4 gallon tank, which could handle a run of considerable length.

  3. matthew25 | | #5

    Occupancy sensors in the bathrooms (Aeotec Multisensor for example) on a smart network such as Z-Wave could send a command to a smart outlet where the recirc. pump is connected. The pump would also have a temperature sensor to know when the water is already hot enough and not kick on needlessly. Most of the brands already have this feature.

    I don’t think you can beat the response time waiting for hot water or the reduced water waste of a properly designed recirculation system.

    1. pnw_guy | | #13

      Hah, wow, never thought of anything like that. I love smart home stuff and think that's a really cool idea!

    2. user-5946022 | | #20

      This is the way

  4. Expert Member
    DCcontrarian | | #6

    There are two problems with continuously running recirculation systems:

    1. They waste energy. Recirculating the water cools it. You can mitigate the problem somewhat with insulation, but there's a limit to that. If you're heating the house anyway the heat just goes into heating the house too, so it's not a complete waste, but if you're cooling the house you not only lose the heat, the cooling system has to work harder to remove the heat from the house.

    2. It's not compatible with heat pump water heaters.

    1. Deleted | | #7


    2. matthew25 | | #8

      I just realized your response specified “continuously circulating”. I can fully agree that is a bad idea. What I am writing below assumes an occupancy-based trigger for the recirculation system.

      For point #1, whether the hot water is moved through the pipe via a pump or through opening a tap, you will have the same heat loss to the home. I don’t see what the difference is? There is obviously an electrical load in the case of the pump to consider.

      A proper hot water recirculating system will have the “return” water on the outlet of the pump directed back into the water heater, so the cold inlet of the water heater has a tee with both the municipal/well water inlet as well as this second recirculating water line inlet. The pump would preferably be located after the last fixture in the loop, and would have a built-in thermostat and turn off as soon as the water reaches a reasonable set point, at which all the water in the line leading up the pump is hot enough for use without having to wait and dump water down the sink. The load on the water heater would not be any extra than opening a tap at the end of the line, because the total volume of displaced water is the same. If it’s flow rate you are worried about, you can size the pump to mimic a shower head at, say, 2.4 gpm.

      1. Expert Member
        DCcontrarian | | #9

        Having a thermostat is better than running continuously. But let's say it takes 30 minutes for the pipe to cool down. So every 30 minutes, 48 times a day, the pump runs, and the volume of water in the pipe needs to be heated. Let's say the pipe holds one gallon of water, that's 48 gallons of water a day that's being heated.

        Alternately, let's say you just run the water until it's hot. Each time you do that you leave the water in the pipe hot, and when it cools that heat is wasted. You do that twice a day, once in the morning and once in the evening. That's two gallons a day.

        If you have a push-button to call for hot water, then yes, it uses no more heat and saves water. But then you have to wait for water.

        1. Expert Member
          DCcontrarian | | #10

          I think you edited your reply while I was responding.

          1. matthew25 | | #11

            Yes I did, sorry. I did not catch the continuous running assumption you stated in your post.

            I view an occupancy sensor as a good option between a manual push button and a timer or continuously running pump. It will sometimes turn on even when the occupant does not need hot water, but it will not be running continuously.

            Based on the OP’s situation I am not sure why would have to limit his piping design to only a continually running pump, which we both can agree is a bad option. An occupancy-based method can be very efficient.

      2. Expert Member
        BILL WICHERS | | #14

        >"A proper hot water recirculating system will have the “return” water on the outlet of the pump directed back into the water heater, so the cold inlet of the water heater has a tee with both the municipal/well water inlet as well as this second recirculating water line inlet."

        It's actually better to return the "cold" water from the recirculating line into the bottom drain of the water heater tank. That's how I have my thermosyphon system setup. I would use the cold water inlet to the tank as a second choice.

        The downside to any recirculating system is always going to be increased energy loss due to the recirculating loop acting like a very long, skinny radiator. Using a pump that only runs when needed helps with this. If you constantly cycle the pump on and off with a thermostat though, I think you're going to negate a lot of the energy savings by using power to run the pump while also keeping the losses from the loop about the same as a continuously running system. You'd be better off with a demand system that only runs the pump to "prime" the system before it's needed.

        Some ideas about a demand system would be a flow switch on the toilet, so that a flush of the toilet triggers the pump to run for a set period of time. You could also use the bathroom light switch to trigger the pump to run for a set period of time. There are multiple ways to get the pump to turn on BEFORE it's likely to be needed. You would probably want a timer too to keep the pump from bing triggered at times of the day when the hot water might not be needed. The logic of that needs to work for however you usually use the system.

        You either lose energy from heat in the recirculating loop (which is what happens with a thermosyphon system like mine), or energy running the pump, or both. It's always going to be a tradeoff with any kind of recirculating system, so the best you can do is to limit the losses. With my system, the hot water supply side of the loop is 1" copper (that's what was already there), but I only added a 1/2" line for the return side of the loop, since that's all that is needed for the very low flow and the 1/2" line loses less heat compared to what a 1" line would. I insulated BOTH lines completely, and I TAPED the return line to the supply line and immediately beneath it. This also helps to keep losses to a minimum. While I haven't every tried to measure those losses, I know they're there, and during the cooling season it's just wasted money. During the heating season, the "losses" help to heat the house, so it's not really "wasted" during those times.

        Keep in mind that those recirculating pumps are expensive, since you have to use units rated for use with potable water systems. The pumps are also a maintenance item that will eventually fail have have to be replaced. I would, for that reason, be sure to put some ball valves into the system to allow you to isolate the pump for future service work.


    3. jadziedzic | | #12

      Re point 2, Stiebel has a document detailing how to connect a recirculation pump to their "Accelera E" model heat pump water heaters so the water heater will work properly. I don't see a link on their Web site for the document, but it is available from their tech support folks.

      In a nutshell, pipe the return to a tee fitting at the temperature & pressure relief valve, set the flow rate to 1 GPM or less, and have an aquastat (or pump with built-in temperature control) so the pump is not always recirculating. Laing / Gould 60A0G3001 "ecocirc e1" pump with an optional 60AAGT001 timer is one option.

    4. sleaton | | #17

      I had a house with a continuous recirculation pump. I put it on a simple wifi outlet. I programmed it to stop after 5 minutes. It was very easy to just start it a couple minutes before the shower. much less energy and waste, and you can still put it on a schedule for those that insist on no button.

      1. matthew25 | | #21

        This is really not necessary for any of the modern pump options with built-in thermostats. You shouldn't have to tell it to turn off after a set amount of time, it will know based on the water temperature. Continuous recirculation is pretty archaic, we should be able to do much better. Schedule-based is just as archaic. And the water exiting the pump should go to the inlet of the water heater, not down the drain.

        Occupancy sensor + thermostat is definitely the most efficient if you want it to be completely hands-free. If you're okay with a demand button then button + thermostat is a little more efficient since it won't waste time turning on if you are not in the bathroom to use hot water like it might with an occupancy sensor. But for the sake of your guests who aren't trained in the button my vote is still occupancy sensor.

  5. jollygreenshortguy | | #18

    For what it's worth, here's what I did. I'm an American, living in France. When I remodeled my old house here I ended up with 2 bathrooms at opposite ends, with multiple stone walls in between.

    So I ran cold water lines to each bathroom only, and in each bathroom I installed on demand water heaters sized to suffice for a shower. The heaters are wall mounted right in each bathroom, so the length of HW pipe is minimized. One of the heaters also serves the kitchen sink, which is adjacent.

    I don't know about the economics of this approach in the USA. But as far as practicality, I like it.

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