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Community and Q&A

On-Demand Hot Water Re-Circulation Loops

qofmiwok | Posted in Energy Efficiency and Durability on

I haven’t found any recent articles on hot water recirc pumps.  I like the idea of on-demand but not having buttons around the house to push.  Timers won’t work well for us because we don’t have consistent schedules for water use.

Anyone have experience with or suggestions for a good system?

This system works by turning the hot water on and off quickly from any faucet.

This one was mentioned in the comments of a 2009 GBA article.

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  1. Expert Member
    BILL WICHERS | | #1

    Turning on a circulation pump when flow is detected doesn't really accomplish very much. Once you have flow, you're already "warming up" the system, and typical recirculation pumps are low enough flow that they won't do a whole lot to cut down on that warm up time. For an on-demand recirculation system to work, it really needs to start running a little BEFORE you need hot water. Timers can do this if the schedle is predictable, otherwise you need something more fancy. You could put switches in every bathroom tied to the pump, but that's an extra step that many people would probably just not bother to do.

    What you could potentially do is tie the coil of a relay into the light switch in every bathroom. This would mean one relay per bathroom. Use the normally open contacts (which close when the relay is energized, meaning the relay would "turn them on" whenever the bathroom light was turned on) in parallel to power the circulation pump. In this way, any time anyone turned on a light in one of the bathrooms, the circulation pump would run. Since lights probably get turned on at least a little while before the hot water does, this would help speed up the "warm up" time. The downside would be the pump would continue to run until the lights were turned off, which wastes some energy, and if someone wanted to do a quick hand wash (for example) during the day with light coming in through the window, they might not turn the light on, so no circulation pump helping to warm things up faster. You could tie a flow switch into the system along with a time delay relay to automatically shut the pump down after a period of time with the water flowing, but that would greatly complicate the otherwise relatively simple system.

    What I personally do is to use a thermosyphon system, which is just a 1/2" line coming back from the farthest point on the main hot water run. My water heater is in the basement, so this works well. Hot water is always pretty quick at the two bathrooms furthest from the hot water heater, and one of those is the master bath where the problem with slow hot water was. The advantages to this system are that it is very simple (just some extra pipe and a check valve), it has no expensive potable water rated circulation pump to buy or break, and it works well and has for years. The disadvantage is that the system runs all the time, so it acts as a very long, skinny, radiator and adds to the standby losses for the hot water tank, so it does use some energy. I have insulated the entire hot water reciculation loop, BOTH sides (some articles say don't insulate the return pipe, but that's not correct, you should insulate ALL of the piping in the system). Insulating the pipes minimizes the energy loss, but it's still worse than the tank alone. In the winter, that "lost" energy actually helps to heat the house a little, so it's essentially recovered and I get the recirculation system "for free". In the summer, that lost energy adds to the heat the air conditioner must remove, so it does result in a bit more overall energy use during the cooling season.


    1. qofmiwok | | #2

      Your assumption stems from your idea that " typical recirculation pumps are low enough flow that they won't do a whole lot to cut down on that warm up time". In fact the pumps recommended for on-demand systems are up to 35 or more gpm (compared to about 1 coming off a water tank). So they move the water very quickly then turn off.

      1. DC_Contrarian_ | | #3

        Regardless of pipe diameter, flow rate should be limited to 5 feet per second (or 6 depending on your source) to avoid noise, hammering and erosion. So if you have a 100 foot pipe, it's going to take 20 seconds to replace all the water.

        I think the intention of the flow-activated systems is you turn on the hot water for a second and turn it off. Then wait 20 seconds. Whether that is acceptable to you is a personal preference.

        1. Expert Member
          BILL WICHERS | | #4

          35 GPM in a 1/2" pipe is 57.2 ft/sec. Very high. If you keep to around 5 feet/second, that puts you at just over 3 GPM. Trying to force 35 GPM through a 1/2" pipe is going to mean high pressure at the pump too.

          A typical hot water circulation pump for a residential system is going to be something under 10 GPM at most. Taco, a large manufacturer of circulation pumps, has one system targeted at hot water recirculation systems that is adjustable from 0 to 8 GPM.

          While it would be possible to run what would be considered excessively high flow for a short period of time, it's probably not a good idea. Most on demand systems assume that "demand" will have a little warning, such as the timer-based systems. Don't try for instant hot water if you don't want the system to run constantly. Try to shorten the warmup time from a minute or two down to 10-20 seconds and you'll have a much more reasonable system.


          1. qofmiwok | | #8

            I think it's 3/4" pex but your point is taken.

    2. BirchwoodBill | | #16

      There are multiple versions of that controller. It also activates the pump when deltaT on loop is more than 2F.

  2. Expert Member
    Akos | | #5

    I've tried pretty much every configuration at home and narrowed it down to two that work the best.

    The easiest setup for an existing system is timer with a temperature cutoff at the return. The temperature cutoff makes a big difference on run time, if you set it to be just lukewarm, the recirc pump will only run when the loop cools down. The timer setup works great until the timer gets out of sync or daylight savings kicks in, throw in a power outage here and there and you are bound to have to fuss with it a couple of times a year.

    The simplest and also the best by far is motion sensor. Perfect on demand, walk into the bathroom, by the time you turn on the faucet, the hot water is there. Nothing to set up, nothing to adjust, it just works.

    The one that sounds like it should work and doesn't is the flow trigged setup. I ran this for a while and removed it. Common misconception is that when you move hot water through a pipe, it pushes all the cold water out as one cold water slug. This can happen with a silly high flow rate but with any reasonable pump size what you get is water mixing with the cold and it slowly warms up at the faucet. The issue is that even with a pump on max this mixing and time to fully hot takes a fair bit of time. Even with a shortish run ~15' (mostly 3/4 pex) from tank to tap, this took about 15s. The whole purpose of recirc is instant hot water, which this won't deliver. Yes you can pulse the tap and wait, ok for a month or so but gets annoying quick especially for a shower.

    1. qofmiwok | | #7

      I like this idea of the temperature cutoff. Can you tell me a little more about how that is implemented? Is it a integrated into the timer? Which one are you using? Thanks

      1. Expert Member
        Akos | | #15

        I used one of those plug in thermostats with a remote sensor sold on your favorite on-line marketplace. These are normally used for programmable control of a plug in space heater but works great with a pump. My recirc pump is a plug in model so it can plug right into the thermostat. The sensor was mounted to the recirc return pipe and covered with pipe insulation.

        When I wanted the recirc to run, I programmed the thermostat to run in heat mode with the target set for 30 °C, the rest of the time the target was set for 10 °C.

        Do check the reviews of the thermostat as the one I got was extremely frustrating to program.

        1. qofmiwok | | #19

          So you're measuring pipe temp, not water temp. Do you have copper or pex? (Mine is pex but should still be measurable.)
          And are you saying that if the water was below 30C you let the recirc pump run but if it was above 30C you stopped it?

  3. 2blackbelt | | #6

    I use a Grundfos circulation pump at the water heater and a motion detector in the bathroom. Always hot water and it is about a 30' run that is looped. I put the motion detector in the toe kick of the bathroom vanity. I forget that its there.

  4. joshdurston | | #9

    I'm considering using a combination of the lutron caseta system (already installed on the lights) and apple homekit automations. I'll probably add a wireless lutron motion sensor to each bathroom, and maybe a push button remote by the kitchen sink. The pump will be on a lutron controlled plug so I can schedule and automate it based on the lights and motion sensors via homekit.

    1. Expert Member
      BILL WICHERS | | #11

      Make sure the Lutron controlled plug can handle a pump -- most of those are mainly for lights, so motor loads can be a problem. An email to Lutron would probably get you an answer pretty quickly. If their device can't run the pump directly, you should be able to use the Lutron device to control a relay, and then use the relay to control the pump. Grainger is a good source for basic relays -- I usually use an octal base relay (they are easy to replace in the future if needed), along with a suitable relay socket. Just get a relay with a 120v AC coil and wire that into the Lutron device. The normally open contacts on the relay can then control power to the circulation pump.


  5. walta100 | | #10

    If the Smart Recirculation system was available 6 years ago I failed to find it when I looked for a prebuilt option. I built my own it functions in a very similar way in that you turn on the hot water for a few second then back off and do something else for a minute or two while the loop is getting warmed up.

    I think timer that would that would heat the water when you think you are going to get out of bed would be a bad fit for my lifestyle one day it is 5 AM the next itis 10 AM.

    I think motion detectors in every bath room would be a lot of wires to run and turn on the pump 5 or 10 runs for the one run I wanted. Do I really want or need hot water every time I urinate.

    I think using the light switch to start the loop would be a bad fit for my lifestyle as I often shower during daylight hours and would be unlikely to turn on the light.

    This screen shot shows the power usage from my tankless electric water heater.


    1. qofmiwok | | #12

      Thanks. I agree with everything you said. So you're pretty happy with the activate and wait? It's not insanely irritating?

    2. joshdurston | | #13

      I guess it comes down to what's more wasteful. Running water down the drain cause it's cold, or wasting some energy running the recirc when warm water isn't actually needed? Seems to me that more often than not, if you're in the bathroom your likely to want warm water within the next couple of minutes. The kitchen is a tougher one, maybe a kick plate button.
      Another option is having some small distributed 120v hot water tanks, just big enough to buffer out the cold water. I have a gas main tank, and have considered putting an small inline electric tank in my furthest bathroom since it's the only one with a a real wait time.
      There are wireless motion sensors with multiyear battery life, this is the one I'm considering.

      1. Expert Member
        BILL WICHERS | | #21

        >"I guess it comes down to what's more wasteful. Running water down the drain cause it's cold, or wasting some energy running the recirc when warm water isn't actually needed?"

        I would make that decision based on the conditions in your local area. If you're in an area like Southern California, where there is a multiyear drought and severe water shortages, I would prioritize "wasting energy" over "wasting water", because water would be more valuable there.

        If you're in an area like I am, in the great lakes region where we sometimes think we have too much water (shore erosion problems), I would prioritize "wasting water" over "wasting energy", because the water is practically free.

        All things considered, in those areas with severe water shortages, it probably makes sense to minimize your overall water use, so any system that helps eliminate water waste is a Good Thing. In all other areas, there probably isn't all that much difference either way, so I'd proritize whatever makes you more comfortable in your home.


  6. walta100 | | #14

    “Most of the baths (especially the master) are fairly close to the hot water tank, but the kitchen is further away.”

    If the only problem area is the kitchen sink consider adding a 2–5-gallon heater next to the sink.
    Modern faucets are flow restricted to the point that 2 gallons is more than most people will use and is more than enough for a dishwasher.

    No, I do not find the delay to be a problem. I just plan a head and start the cycle early. At the kitchen sink I start the hot water and then load the dishwasher while the pump is running. If I am showering, I start the pump before getting undressed by the time the dirty clothes are in the hamper the water is hot.


  7. benwolk | | #17

    The motion detector suggestions above are great. I would see if you can pair it with a smart plug or smart device for scheduling purposes and manual control if installing a motion sensor in the kitchen isn't feasible. I know that with my open floor plan, my dog would end up causing the motion sensor in the kitchen to constantly get tripped.

    Putting a motion sensor in the bathrooms with a manual button near the kitchen sink seems like the best combo to go with.

    I do like having a smart plug for my recirc pump as I can tell google to turn on the recirc pump whenever I want since I don't have the benefit of motion sensors. I have it set to only run for a set period of time once activated so that I get hot water without having the pump run excessively.

    1. qofmiwok | | #18

      I don't really see how the motion sensor helps much if it takes 2 seconds to get from the door to the sink and 30+ seconds for the hot water to come. Plus it's a lot of wiring of sensors. Same thing with a manual switch.

      1. joshdurston | | #20

        I'm not sure of a typical recirc flow rate, but the recirc pump doesn't have the facet's flow restrictor so it may be able to purge the line much quicker than just opening the tap.

      2. Expert Member
        BILL WICHERS | | #22

        The pump will help, and the more advance notice you can give the recirc system, the better off you are. The motion sensor setup is clever. I would use motion sensors here that are made for card access systems, known as "request to exit PIR sensors". These have settable delays (how long they stay activated after first detecting motion) that are rather long, so they would let you drive a relay to control the pump with little extra control circuitry involved.

        If you really want "instant" hot water, you're only real option is a continually operating sytem. If that's the way you want to go, and you have a hot water heater on a lower level, I'd use a thermosyphon system since it's cheapest, and probably uses the least overall energy too since there is no pump to run -- only thermal losses in the very slow convective flow in the recirculation line. I have such a system, and it's always worked well for me.


        1. qofmiwok | | #23

          Is your home one or two story? Would it work with a 2 story with water heater in the conditioned crawl?

          1. Expert Member
            BILL WICHERS | | #24

            A thermosyphon will work anywhere the water heater is BELOW most of the hot water using fixtures. In my case, the water heater is in the basement. The house is two stories. The thermosyphon loop is setup to make a 'loop' out of the longest section of hot water line. That means some things get hot water very fast, if they are connected to the loop by a short pipe. Other areas are slower. Everything is better than it would be without the system in place though. The system completely solved the issue with very slow hot water in the master bath, which happens to be at the far end of the longest run of hot water line, and is where the return pipe connects to form the loop.

            If you water heater is in a crawl space, you should be able to setup a thermosyphon system. What you need to do is connect a small (1/2") pipe to the farthest point on your hot water system (and note that you really need a trunk type system for this to work well, with a manifold you can only really help ONE of the fixtures, assuming the manifold is centrally located near the water heater), and bring it back and connect it to the lower drain port of the water heater. If you have anti thermosyphon features on your water heater (usually a plastic piece in the fittings on top that act as valves to shut off very low water flows), you'll need to remove them. The return line needs to be located lower than the main line. In my system, I insulated the main line, then insulated the return line, and taped them together with good quality Scotch 88 electrical tape, with the return line directly beneath the trunk. This is the easiest way to do it, and results in slightly lower thermal losses compared to spacing the lines apart.

            You need to put a check valve in the line near the lower return, and it can't be a spring check valve, it has to be a 'flapper' check valve that opens with very little force. These type check valves need to be installed horizontally in this application to work well. I recommend you also put a ball valve ahead of the check valve so that you can shut off the loop if you want to service things, or just stop the flow in the recirculating loop.

            You MUST insulate ALL of the hot water piping! If you don't, your losses will be MUCH higher!

            The way this system works is that cooler water will drop down into the return line and flow back to the bottom of the water heater, with hot water rising up into the trunk line and circulating out to the return line. You end up with a very slow convective water flow which acts to keep the lines warm all the time. You will increase the standby losses for the water heater, but I've found it's not too bad, and I didn't have to buy a pump -- or pay for electricity to run the pump (although I do pay for a little more gas to deal with the increased standby losses). As recirculation systems go, this is about as simple as it gets, and it works just fine.


        2. DC_Contrarian_ | | #26

          The other way to do "instant" is a small water heater at the point of use.

          1. Expert Member
            BILL WICHERS | | #28

            True, but that's so much less fun ;-)

            I agree though, small water heaters are great for distant sinks for hand washing. I'm no sure they'd work as well for a shower, but I've never tried one for that, so can't say for certain.


          2. DC_Contrarian_ | | #29

            Let's say the shower is 2.5 GPM, the tank is 5 gallons, and it takes 30 second for the supply line to clear (ie the supply line holds 1.25 gallons).

            Normally hot water tanks are designed to stratify, so the incoming water doesn't mix with the water already in the tank. With perfect stratification you'd get two minutes of blissful warm water while the tank empties (or more if your shower temperature is lower than the water temperature) then 30 seconds of room temperature water followed by a return to bliss.

            So you want the tank to allow mixing, which should be as simple as taking out the dip tube (I don't even know if 5 gallon tanks have a dip tube). With mixing, after 30 seconds all of the cold water is in the tank and hot water is starting to arrive. At that moment you've got 3.75 gallons of hot water and 1.25 gallons of cold water in the tank. If the cold water is at 72F and the hot is at 110F your tank would be at 100.5F. From there it just gets hotter. In reality it wouldn't get quite that cold because once the tank starts mixing not all of the water being taken out is hot, some is cold, so there probably never would be all of the cold water in the tank at the same time.

            In any case, with a thermostatic valve you'd probably never notice.

  8. greenright | | #25

    I played with all of them- timers, smart, etc… they all waste too much hot water. My current setup is a pump connected to a Alexa- aware outlet. Have created a simple routine to turn on the outlet for 5 mins when I say “Alexa- hot water”. Works like a charm and is the most economical on demand setup. Keeping it simple

  9. frasca | | #27

    At my last house I installed a Taco pump at my furthest bathroom faucet from the water heater, and had two doorbell-style push buttons in wall plates; one at that bathroom (my children's), and one at the kitchen faucet. My wife's and my bathroom was right next to the water heater and got hot water almost immediately without any need for recirc.

    The doorbell-buttons weren't really a problem at all; the house was in Seattle and my wife knew how high our water bills were so she enjoyed pushing the button and waiting a minute or however long it took before drawing the bath, filling the kettle for coffee brewing, or whatever. She saw what Seattle Public Utilities charged for water every other month and was glad to do other things for a minute to avoid putting that liquid gold down the drain.

    When you were in the children's bathroom you could hear the pump running and you knew when it stopped that the lines had hot water. In the kitchen you didn't have that audio feedback but we got to know about what other stuff we had time to do to kill 30seconds.

    I would imagine the children's use of the button when they wanted hot water themselves was less than 100%, but that wasn't often... they're still not at the showering-or-bathing-themselves age yet and I doubt they were brushing their teeth with hot water.

  10. kwoolfsm | | #30

    To the OP, we just took possession of a new build home, we had little influence in the plumbing/HVAC side of the build. But the builder installed a small recirc line from the DHW tank to the various stops along the way. The furthest point from the tank is the master bathroom sink and shower. On the recirc line is a small (the size of a D-size flashlight battery) circ pump with a delta-T sensor on it. It's not precise, but it works. As it senses the D-T grow from supply-return, it runs for about 20 seconds. I have yet to adjust it, but will over the coming weeks. The hot water is at temperature within a second or two of turning the tap on. No issues, yes, there is likely stand-by losses like those associated with a tank-style, and running hot water through the day when we aren't home. Its pretty nice, as we've had on-demand systems in our last homes, and this wastes less water. Lots of goodoptions provided above, this one, requires no training for my children nor spouse, nor tinkering from me.... yet...

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