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Community and Q&A

how to reduce the amount of water wasted from a tankless heater?

BTC | Posted in Mechanicals on

We have a new tankless water heater serving two bathrooms. The shortest run is about 14 ft. to the nearest shower and about 25 ft. to the further one. We live in the San Diego area, and are in major drought. We save gas, yes, but we waste gallons of water waiting for the water heated at the heater to reach us. The only circulating pump authorized by the manufacturer only works when the water is running, so I can’t see how that saves water. We are considering point of service heaters for the sink faucets (we never run them long enough to get hot water in at all) but what can we do about the showers? Five gallon buckets are a last resort!


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    I'm assuming that the circulating pump authorized by your manufacturer is the D'mand pump by Metlund. This is a high velocity pump that is off most of the time but turns on when triggered by a button in the master bedroom or kitchen (or a motion sensor in the powder room if desired) and rapidly moves hot water from the heater to the furthest faucet in the line. It gets rid of the cold water by pumping it backwards into the cold water lines back to the intake of the water heater rather than just letting it flow down the drain. As soon as the water in the hot water pipes warms to above room temperature the pump shuts down.

    The result is that when you go to get hot water to wash with most of the cold water has already been flushed out of the pipes so your wait is minimized. If it is cold water you are looking for that water is no warmer than room temperature so you don't get a nice glass of warm water. the use of push buttons rather than motion detectors in all bathrooms means that the occupant has control over the activation of the system. If you don't need hot water, just don't press the button. It's what we call a "demand activated recirculation system."

    You can further improve the performance of any hot water system by simply removing elbows from the line to cut down on turbulence and mixing of hot and cold water inside the pipes. Rather than having the hot water leave the heater and immediately hit an elbow you can buy long sweep water heater elbows that are basically a pre-bent piece of 3/4" copper pipe that eliminates that critical first elbow at the top of the water heater. It cuts down on turbulent "laminar flow" inside the pipes and promotes "plug flow" by which a "plug" of hot water pushes the cold water ahead of it on it's way to your faucet rather than mixing with the cold water.

    If you have ever held your hand under a faucet and felt the water gradually get hotter and hotter as it transitions from cold to hot you have experienced the result of hot water mixing inside the pipes. All the water leaves the water heater at 125 degrees or so. It becomes lukewarm on its way to you by mixing with the cold water that is left in the pipe rather than pushing that cold water ahead of it. The result is that it takes many more gallons of hot water to push the lukewarm water out of the pipes caused by this mixing. Eliminating elbows and switching to a flexible pipe such as soft copper or PEX can help reduce this mixing effect and improve hot water delivery.

    Gary Klein is the guy who did the research that brought this mixing phenomenon to light. He is now working with a group called "Green Plumbers" which is training plumbers in more efficient plumbing systems. If you have a plumber who you work with regularly you might want to encourage that he or she take one of these courses.

  2. GBA Editor
    Martin Holladay | | #2

    Michael has provided some excellent advice. Here's another possible solution: If your existing water lines are 1/2 inch or 3/4 inch, you could consider changing the hot-water supply lines to 3/8 inch PEX. If you go this route, each 3/8 inch line can only serve a single fixture. (However, a bathtub will fill slowly with a 3/8 inch line, so the bathtub may need 1/2 inch.) The 3/8 lines should follow as direct a route as possible between the water heater to the fixture.

    By reducing the size of the water line, you are reducing the volume of water that must be purged before hot water arrives.

  3. homedesign | | #3

    Martin, That is a very interesting suggestion...I have never heard of that strategy. I understood the homerun advantage but never considered REDUCING the diameter of the line.
    This sounds like a good solution for new construction.
    Can I assume that a 3/8" pex line will deliver more than enough pressure for a reasonable code compliant shower head?
    Is there a downside?

  4. GBA Editor
    Martin Holladay | | #4

    The delivery rate of water through a 3/8 inch line depends on the pressure (which could be 20 psi in the country, or 80 psi in the city), the length of the pipe, and the fixture dispensing the water. I have heard homeowners say they are happy with 3/8 inch pipe, but I've never done it. Anyone with experience want to chime in here?

  5. Riversong | | #5

    Your situation exemplifies the disadvantage of a centrally-located demand heater. The European approach to small point-of-use heaters is far more effective.

    Given the size of American houses, the number of fixtures and the length of plumbing runs, a tankless demand heater can be less (or very little more) energy-efficient than a quality, well-insulated storage tank heater. In addition, the size of gas burners on central demand heaters requires larger gas piping and larger vent diameters than equivalent tank-type heaters, making installation sometimes extremely costly.

    Given that they are much more expensive to purchase and can also require more maintenance (particulary with hard water), the life-cycle costs often don't justify them. If they have a standing pilot, then the efficiency is reduced. If water is costly or in limited supply, then the water waste needs to be factored in to the life-cycle analysis.

    In addition to your longish plumbing runs, demand heaters don't turn on until there is a minimum flow rate and then heat up gradually, unlike tank heaters which have hot water at the ready. Ironically, demand heaters tend to discourage use of low-flow shower heads, since they only increase the time for hot water.

    Sorry that I can't offer a good solution to your problem beyond what's been proposed, but if we can learn from your experience then it has value.

  6. Brent_Eubanks | | #6

    Let me also plug the D'mand pump by Metlund. We have one at home, and we love it. We have a conventional tank-type heater, but as far as I can tell, there is no reason why it would not work equally well with a tankless heater. And as the previous poster points out, the recirculated water is simply pushed back to the cold side, so nothing is wasted.

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