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Musings of an Energy Nerd

Solving Hot Water Piping Problems

To avoid long wait times for hot water, cluster your wet rooms

Typical single-family home plans make no attempt to cluster wet rooms. The red line defines the "hot water system rectangle," and in this case, the rectangle is much too large. That's a sign of poor design. [Image credit: Gary Klein]

Gary Klein, a residential energy expert who has been nicknamed “Mr. Hot Water,” has for years been providing advice to builders on ways to reduce the wait time for hot water to arrive at a faucet. (His 2012 article on the topic for GBA was titled “Inefficient Hot Water Piping Layouts Waste Hot Water.”) Among the common problems he’s identified: designers fail to cluster bathrooms near the kitchen, making it hard for a single water heater to serve all of a home’s wet rooms, and hot water pipes are often oversized, lengthening wait times as large volumes of cold water in the oversized pipe flows down the drain.

At a presentation on August 7, 2019, at the Westford Symposium on Building Science — an annual conference in Massachusetts that is sometimes called “summer camp” — Gary Klein shared the results from a recent research project funded by the California Energy Commission (“Code Changes and Implications of Residential Low-Flow Hot Water Fixtures”). The project manager for the research is Amir Ehvai; Gary Klein is one of the team members.

In recent years, the situation has worsened

Wait times for hot water used to be shorter. Low-flow fixtures, oversized pipes, and ballooning home sizes have conspired to lengthen hot water wait times, leading to grumpy homeowners.

“Plumbing fixture flow rates, flush volumes, and appliance fill volumes have gone down every decade year since the 1950s,” Klein said. “Toilets used to require 5 gallons per flush; now they need 1.6 gallon. Water flows for fixtures and appliances are down by 49% to 96%, depending on the fixture, since 1980. But pipe sizing rules have not been revisited since they were first written down in the 1940s. The median square footage of a house is roughly 1.5 times larger than it was in 1970. The result of…

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  1. user-6195425 | | #1

    I understand the preference for dual handles, but single levers are far more accessible for folks with arthritis or otherwise 'differently abled.' Thanks for the informative post!

  2. Expert Member

    Architecture is a bit of a dance. For people with an interest in efficiency that dance can become one of deciding whether the changes necessary to achieve this are justified by the compromises they entail.

    To me the primary generators of the building are how the client imagines going about their lives in the house, and the relationship between it and it's surroundings. If clustering the bathrooms didn't make much difference to the plans then I'd be all for it, but to generate the plans based on that goal is a disproportionate response to a minor problem. I can't imagine having people into my new house and saying: It may seem a bit odd, but hey - the bathrooms are clustered!

    1. GBA Editor
      Martin Holladay | | #3

      I agree. In some cases, it's perfectly OK to accept two water heaters or a circulation system controlled by a switch in the remote bathroom.

      1. Expert Member
        MALCOLM TAYLOR | | #4


        I picked on the one aspect of blog dear to me as a designer - then went to prepare lunch and realized I habitually use the single lever tap in the middle position. Lots of good solid advice in Gary's presentation.

        1. Expert Member
          Peter Engle | | #5

          As a seasoned skinflint, I taught myself (and most of my family) long ago to always use the lever in the cold position. It usually works.

    2. morganparis | | #9

      I hear you Malcom but a little discipline with hot water planning can often be copacetic with efficient and elegant functional planning of the spaces. Look at the awful functional layout and shocking wasted space of the owner's suite area of the example plan.

      1. Expert Member
        MALCOLM TAYLOR | | #10


        I agree. That looks like a track house plan devoid of any architectural ideas governing it's layout. But take the house I designed last spring. It is on a south-facing sloped lot with views of the nearby straits. Although only 1500 sf, because it includes breezeways and covered porches which work so well in our wet PNW climate, it is over 80 feet long. If energy efficiency was the main driver of the design, there is no way it would be that shape or have that much exterior wall surface - and while most of the water using rooms are close together, there is one stray at the far end. Should the plan be re-worked because of it?

        1. morganparis | | #12

          Hey Malcom. Sounds like a wonderful site-specific and climate-appropriate design. And I agree the tract house design is irredeemable. As so much these days.

  3. vivian_girard | | #6

    I replumbed multiple bathrooms in a mutifamily a few years ago, and after some experimenting, I found that a single home run of 1/4" pex (not a typo) to each bathroom is sufficient.

    The hot water fixtures are only a sink and a shower (no tub), and with a low flow 1.5 GPM Niagara showerhead, the showerhead flow is adequate.

    The longest hot water pipe run in the building is around 90' long, and with a 1/4" hot water pipe, I only need to run only about 1/2 gallon before the hot water arrives. It would take about twice as much water and time by using a 3/8 pipe, and 4 time as much with a 1/2" pipe.

    I believe that the plumbing code (where efficiency is often an afterthought at best) would allow a 3/8" pipe home run for the shower and the same for sink (one 3/8" home run for each hot water fixture). Even that is way too wasteful in my opinion since under this design, someone using hot water from the shower and then the sink shortly after would still waste a total of 2 gallons of hot water. In a small bathroom (and even a large one), it is quite uncommon to use hot water from the sink and shower at the same time. Even when I tried that on my single 1/4" hot water line, the flow was certainly weaker but still surprisingly usable. Although I never asked the building inspector about his opinion on my little plumbing improvement project, I have had zero complaints from the occupants over the years. My city water pressure is a fairly typical 40 PSI I think.

    For those who want to try this out, I highly encourage it. 1/4" pex pipe and 1/2" to 1/4" Sharkbites adaptors (to transition from 1/2" to 1/4" pipe) are readily available online at As an added bonus, 1/4" pex pipe is very easy to install in a retrofit as it is about the size of 14/2 Romex wiring. It can take fairly sharp turns and be run inside walls with a snake if needed. I didn't bother insulating those small hot water lines because it would have been quite challenging, and I figured the heat loss through a 1/4" pex pipe to be minimal overall.

    1. gary__b | | #7

      Wow, interesting stuff Vivian. I wish I'd read this article and your comment about 2 weeks ago, before I replumbed several fixtures with 3/4 and 1/2 inch pex. Just so I'm clear, what exactly is a home run? I assume that means decdicated lines serving a single fixture, rather than branches?

      1. GBA Editor
        Martin Holladay | | #8

        For more information on home-run plumbing pipes, see this article from the GBA Encyclopedia: "Efficient Plumbing Supply Layouts."

        Like you, I was intrigued by Vivian's post. I've heard of 3/8" supply lines, but never heard of anyone bold enough to experiment with 1/4" lines. It's an interesting approach.

    2. schreib77 | | #22

      best advice in the whole column. I would only add that in some cases installing a continuous DC motor driven circulation pump works best in retrofits; sometimes in may also be best in new homes that want to allow the HOME design take priority over the hot water piping and clustering.

      1. GBA Editor
        Martin Holladay | | #23

        Continuous circulation systems use a tremendous amount of energy. See more details in this article: "Marc Rosenbaum’s Monitoring Results."

        In that article, Rosenbaum said, “The total daily kWh usage for hot water [in a house he monitored] was 3.94 kWh without recirculation. With recirculation, the daily usage was 12.30 kWh — three times as much energy as the system used without recirculation. That’s 3,044 kWh per year for recirculation — enough energy to make 100 gallons per day of domestic hot water. So continuous recirculation is a bad idea.”

        1. schreib77 | | #24

          Yes. An obvious and good correction on your part. . . makes sense for MOST cases. . .

          However, I failed to point out my situation which influenced me to state that: I have all my PEX embedded in concrete floor and have HEATED floors. So, in my MINNESOTA case, all that propane heated recirc. water serves to keep me warm and levels out the floor heat during low demand and between zones. In the summer I still have some minimal floor heating going on so it is, even then, able to "kind of" be rationalized.

  4. gustave_stroes | | #11

    Great informative discussion. We recently designed a house that we plan to build. My wife hates to wait for hot water. She never stops complaining about it, especially in the kitchen. So I gave in and plan to install an electric heater under the kitchen sink. Then two individual tankless heaters for other portions of the house. I've attached my DHW plan. Feel free to critique. Note the red lines are not meant to be water lines, they just indicate which areas are served by which heaters. I had yet to consider pipe sizing and home run vs. branch. Now I have a lot to think about!

    1. morganparis | | #13

      The under sink unit looks too puny to also serve three remote locations. I'd suggest linking the laundry to the left side tankless - the washer won't care very much about slow feed times and you may be using mostly cold water wash anyway - and having small individual under sink units for the powder room and the bar. I suspect the hot water draws at those locations will be small and infrequent with minimal energy use.

      1. gustave_stroes | | #14

        Thanks James. Great ideas. Yes, the bar and powder room hot water demand will be rare and low. Do modern washers even need a hot water supply? My wife thinks they make their own hot water, and the hot water supply is redundant. But I have never investigated this.

  5. Jaero | | #15

    Thanks for this article. It comes at just the right time for me as I continue with plans for my new house.

    This statement has me a little confused...

    "Many proposed solutions — including installing typical (inefficient) recirculation systems, or installing an electric-resistance water heater under every sink — increase rather than decrease energy use."

    I understand the inefficiencies of recirculation systems, but how do under-sink electric-resistance water heaters increase energy use?

    In an all-electric home, installing small electric-resistance water heating units located near fixtures seems a more energy efficient solution than running hot water from a distance. To minimize losses, the goal should be to generate the hot water as close to the point of use as possible. Small units under each sink seem ideal from an energy efficiency standpoint; it is the initial cost of the units and the supporting electrical installation that makes this prohibitive.

    1. GBA Editor
      Martin Holladay | | #16

      All-electric homes often have a heat-pump water heater. Heat-pump water heaters are 2 to 3 times more efficient than electric-resistance water heaters. But a heat-pump water heater won't fit under your sink.

      That said, some homeowners can't install a heat-pump water heater -- for one of several valid reasons -- and decide to heat their domestic hot water with electric resistance. (The added load is sometimes balanced, at least according to the paper accounting method used for net metering calculations, by extra PV modules.) If you're in that category, a small under-the-sink electric resistance water heater might make sense.

  6. gblakev | | #17

    I wanted to install multiple lever faucets in my new bathroom but they are explicitly illegal in my state (Massachusetts) for showers. A plumber could lose his license if he installed one.

  7. maine_tyler | | #18

    "A hot shower is not 100% hot water — it’s mixed. So I don’t need full hot water flow equal to the rated flow of the shower. The flow can’t ever be that big. A legal shower only has 1 gallon per minute of flow on the hot side. "

    I never knew about this. Which code governs this?
    Wouldn't setting water heater temp lower and using (closer) to 100% hot water be more energy efficient than a higher temp setting and mixing?

    1. GBA Editor
      Martin Holladay | | #19

      Some homeowners like to set a high water temperature for their dishwashers. In all cases, it's worth considering risks associated with Legionnaires' Disease -- higher water heater temperatures lower the risk of Legionnaires' Disease, but this is a somewhat controversial topic -- you'll hear many arguments in favor of higher water heater temperatures, and some in favor of lower temperatures. In almost all cases, though, you'll want your water heater temperature to be significantly higher than a comfortable shower temperature.

      1. morganparis | | #20

        Most dishwashers sold in the US today have built in heaters designed to raise the feed water temperature from ~ 120° to 150° or so, which I understand is the temperature required at final rinse for NSF (National Sanitation Foundation) certification.

  8. Dan_Madden | | #21

    A few months back I was considering running 1/4" and/or 3/8" pex home runs for a refrigerator icemaker and a toilet (not in the same room). Seems I couldn't find readily available plumbing adapters to accomplish it. Still, haven't got to those projects but may need to check when I'm ready.

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