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Community and Q&A

Best fix for hot water waiting time for tankless water heaters?

John Metcalfe | Posted in Energy Efficiency and Durability on

We recently underwent a major remodel of our home (four stories, ~ 3200 sq ft), and in the process installed two tankless water heaters in series with a small recirculation pump. We now wait for up to two minutes for hot water in the kitchen and upstairs shower. What would be the most cost-effective and energy -efficient means of correcting this situation?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Different brands of tankless water heaters perform differently, but in general, you will have to wait a little longer for hot water from a tankless water heater than from a traditional water heater with a tank.

    So, one answer to your problem is to remove your new tankless water heaters and to go back to a good old tank-style heater. But you don't want to hear that.

    If you have a circulation pump, make sure that the pump is demand-controlled -- in other words, controlled by a switch in your bathroom or kitchen. You don't want a continuously operating pump, and you don't want a pump on a timer.

    Other ways to decrease your wait time include: moving the water heaters closer to the bathrooms and kitchen; and decreasing the diameter of the hot water lines.

    All these topics, and more, are discussed in this Fine Homebuilding article: Hot-water circulation loops.

  2. Mike Collignon | | #2

    Are both tankless units located on the bottom floor? Are they centrally located? Reducing the distance the hot water has to travel would be helpful.

    If the units are on the bottom floor, is that conditioned or unconditioned space? if unconditioned, are the pipes insulated? Or are you using PEX?

  3. David Meiland | | #3

    Are you saying that the two tankless heaters are in series with each other?

  4. Jin Kazama | | #4

    Hi, why have you choosed tankless WH ??

    I can only think of so few reasons to justify the tankless :

    - low or periodical water usage

    - reducing hot water delay ( which you obviously didn't implement correctly )

    To achieve the second point, one needs to install the Tankless WH in close proximity of the usage point ...

    So you should've installed small tankless heaters near kitchen sink or a close by embranchmnent serving many points, and another close to the shower.

    For energy efficiency, tankless water heaters need to be used with an energy recuparator on the drain of the most used equipement ( read shower ).

    Where are you located? If you are in a heating dominated climate, using regular tank water heater doesn't impose a very large penalty other than its COP on heat loss during all of the heating season.

    Also interested in learning the reason for a serie of tankless like mr David is.

  5. Nick Welch | | #5

    A home-run hot water line with the minimum necessary diameter seems like a good step no matter what else you do -- there's no sense in having excess hot water sitting in the pipe all the time.

    I've been considering using 1/4" PEX for a vanity hot water line that has a 0.5gpm or 1.0gpm flow rate, but I haven't found any mentions of this anywhere. 3/8" seems like the minimum anyone ever mentions. The 1/4" stuff is often sold for refrigerator ice makers. Can anyone comment on why no one ever mentions 1/4" line? The velocity seems like it would be acceptably low as long as you have a low-flow faucet, and while pressure drop is always cited as a reason for over-sized plumbing, I don't really see how pressure would matter in a vanity.

  6. John Metcalfe | | #6

    Hi All, The tankless units are located centrally - 2nd floor of a four story house in San Francisco. I do not believe the pipes are insulated, and definitely are not PEX. Two units were placed in series with the thought that one might not be enough for five bathrooms and four people (not sure of the validity of this assumption). Of note, it seems that with most new construction or major remodels, tankless water heaters are the default in the Bay Area. I realize that the evidence thus far may not support this practice. Reverting a tank water heater would not make cost or energy sense at this point, so we will likely go with a larger recirculating pump with manual switches at three of the furthest and most used taps. Thanks for your thoughts.

  7. David Meiland | | #7

    I've never seen two tankless heaters in series and I'll go out on a limb and say it makes no sense to me. Each should be large enough to provide for part of the load, and they should be piped separately. So, one heater serves the first two floors, the other one the second two floors... or something like that. The way you have it, it seems likely that both heaters are trying to respond to *any* call for hot water, however small it might be, and both are cycling every time any water is drawn. Short cycles and partial loads are probably less efficient than a single unit running at a higher load by itself. In addition, by locating them together, longer-than-necessary piping runs are all but guaranteed.

    And, in my opinion, recirc and tankless don't make that much sense. Your recirc will have to move enough water to turn the heater on (probably 1/2 gpm or more) instead of moving the very tiny amount of hot water that would be necessary with a tank heater.

    What your plumber probably should have done was install at least one tank, if not two, put them further apart, piped them separately, installed small-diameter home runs to the sink fixtures, and installed recirc pumps as needed to keep wait times down at the furthest fixtures.

  8. Jin Kazama | | #8

    There are very small and cheap tankless WH avaiable now if wait time is killing you at the kitchen,
    which can be installed right under the sink.

  9. Richard Patterman | | #9

    The logic of two tankless heaters in series completely escapes me. The first heater heats water from 50 degrees to 120 degrees. What does the second heater do? And a recirc pump on a tankless defeats the purpose of a tankless, the heater should only come on when you turn on faucet.
    I think your plumber owes you some serious repairs!

  10. Aj Builder, Upstate NY Zone 6a | | #10

    I would go back to the installer.

    As to some of the advice, here, not the best. IE properly installed tankless units that are in series are actually designed for such an installation. Their electronics incorporate what is needed to turn on one or both units depending on demand.

    Get back with your installer with your issues. If not satisfied, get another more competent contractor. Most companies (your tankless company) have reps that love to resolve issues. I turn to them often with satisfaction most of the time.

  11. David Meiland | | #11

    AJ, please provide a link to a tankless manufacturer's instruction manual that details a series installation. Then I'll believe it.

  12. Hein Bloed | | #12

    As previous posters suggested: the existing system seems to be absurd.
    What is the plumber saying about it?
    Circulation pumps are installed to avoid waiting times, b.t.w..

  13. David Seitz | | #13

    Virtually all tankless are designed to be installed to a cold water line and can have serious performance issues dealing with preheated water. Reason is flow switch activates heater just as soon as there is flow.. In your design you either have units with very low kW or they can be working against one another.
    Although there are tankless products that are designed to work with multiple units, they typically deliver water to a manifold and have separate cold water supply. So to solve your issues, First determine whether your heaters are designed for use with preheated water and if not replumb them in a manifold. Second determine if your heaters have sufficient power even with the two combined, to raise the water temp to your desired set point. I am assuming as I believe these are electric but if not the same applies..Thirdly determine whether or not the temp setting is high enough to deliver recirculated hot water not luke warm to your fixtures. Remember if the water temp is not high enough to satisfy you, then you will have the same wait while the heater is adding additional heat. In your case with uninsulated lines it is highly unlikely that you are getting hot water and you are probably just keeping the pipes warm. This is extremely inefficient and very costly. If you check out these items and have sufficient power to heat your water then convert your recirculating system, as was previously suggested, to the "On Demand". By the way "" identifies products that can be used well with pre-heated water but only when one is not sufficient and not in a design as you suggest.

  14. David Meiland | | #14

    HIghly likely that someone in San Francisco is using NG and not electric for water heat.

  15. User avatar
    James Morgan | | #15

    Not saying that this is the case here (though it certainly seems an odd installation) but it seems not uncommon for installers to fail to correct the impression (I'm putting it charitably here) that the 'instant' hot water generated by a tankless heater will somehow miraculously result in equally 'instant' delivery to a remote fixture. Could it be that there are around two thousand or so little $$$ reasons for this failure to communicate?

  16. Jin Kazama | | #16

    David Seitz: thinking of it, i don't see how using multiple units in a serie water connection
    would have any negative impact, most units have output temperature settings and the flow monitoring isn't dependant on temperature.
    So 2 units inline might result in getting a much larger possible heated water volume at the same temp.
    Probably not double, but maybe close to it.

    Also, if the incoming water temp is too low, some smaller tankless WH can have difficulty to maintain desired temperature to a shower head, or multiple outputs at the same time.

    Stil, if you want instant hot water, install the heater next to the output.
    The recirc method used here completely defeats the purpose of tankless heaters,
    and probably cost a lot more .

  17. Dennis Heidner | | #17

    Easy solution that I used, Chillipepper on demand recirculating pump that stops pumping when water temp at the pump reaches your desired setpoint. The pump is only activated by a low voltage contact closure - switch/relay. The relay is controlled wirelessly or via PLC a bathroom light is turned on OR if a wireless button by selected sinks are pushed.

    By triggering the circulation pump to run when the bathroom light is turned on - the water is hot and ready at the sink when you want it.

    The recirc pump is located close to the bathrooms. You can force the water back through the cold water supply line -- or send it back to the tankless with its own return line.

    Insulate the all the hot water pipes to the max!. And tape the seams, elbows and joints.

    I also long ago switched to thermostatic shower valves -

    Not all recicrulating pumps will trigger a tankless to heat - you need to exceed 1gpm. And you need to check the manufacturers warranty! Some will not cover you unless you add some kind of moderating tank...

    For wireless function you can use z-wave like switches, insteon, if you have an home automation system system installed - you may be able to trigger off motion sensors or light switches that are already installed.

    This system works well - I had a nearly identical problem - and would spill two gallons of water down the drain before water was warm. Now the water is almost always at temp when you want it -- water use is very low, energy use is low ( I measure both !!!!)

  18. Richard Patterman | | #18

    Dennis, so every time you go into the bathroom your tankless heater fires up, whether you need hot water or not? Defeats the purpose of ON DEMAND hot water.

  19. Dennis Heidner | | #19

    Richard, I have the automation so the recirc pump is only triggered if it had not been triggered in the last 20 minutes by ANY of the bathroom switches. And the bathroom fixtures are within about seven feet of each other, the chillipepper is next to the last in line bathroom sinks.

    Before I installed it, I actually measured all the water flows and logged our typically use of the sinks and hotwater. > 90% of time we used hot water tap for something after entering the bathroom.

    Triggering only if it has been at least 20 minutes brought that up to > 95% of cases. After doing this the total house water and total therm use BOTH dropped to by more than half of what it was with standard tanked heaters.

    I also used doubled up the pipe insulation. A second larger diameter pipe insulation over the K-Flex insulation (seams turned 180 degrees) I wanted low loss from the hot water lines so the 20 minutes would work.

  20. John Metcalfe | | #20

    Hi All, Just to give some closure on our situation: we installed a D'mand recirc pump (sounds very similar to what Dennis Heidner describes) triggered by manual wireless switches at key locations (as advised in Martin Holladay's initial post) and time to hot water flow is much reduced - even better than when we had a tank water heater. To clarify, yes we use natural gas and have a dedicated hot water return line; and no, hot water pipes are seldom insulated in our neck of the woods due to mild climate. Kudos to Dennis doing some before/after measurements. Anecdotally, I feel we now have the "best of both worlds" but agree more data is likely needed incorporating some of the variables we've discussed here. Thanks.

  21. Robin Whiteside | | #21

    Everybody writing these posts here, are really missing the point. Tankless units are great, because you never run out of hot water. Period. End of story. That alone makes them worth it. And two in series is smart too because you can run many fixtures at once.

    The problem from the original post is a common one: It is the waste of water down the drain waiting for the hot to arrive. I am on the third floor of an old Victorian. It takes about 30-40 seconds. Water costs a fortune in San Francisco. Well I solved it.

    This can be solved one of two easy ways.

    First, you can add that pump and return gizmo, (D'Mand, Gundfos, etc.) but that still wastes energy, and it’s a pain because when you turn on the cold, not the hot, but if you turn on the cold you have to wait for the warm water to flush out for the cold to arrive.

    Second solution, and a far better solution in my opinion, is this: Hook up an in-line small hot water tank under the sink. All hot water flows through that tank to get to your hot faucet. I have mine under the kitchen sink. That gives you instant hot water on the third floor for washing hands, dishes, quick face wash, etc., even a load in the dishwasher. Those are the scenarios when you really need hot water fast. For a shower, it allows the incoming cold water a chance to blend with the already hot tank, until the real force of hot from the tankless arrives.

    And you aren't paying to heat cold water in the tank. The tank always has an influx of hot water from the tankless several floors below.

    And in any event, I always turn on the hot shower while I'm undressing so it will be perfectly steamy when I get in. That's when a tankless delay isn't all that big a deal. I am always in awe of the IQ scores of people who get in a cold shower stall, close the door, stand naked and THEN turn on the spigots. Who does that?? Really, stop already.

    Doing this solved my issues. Will you spend a few bucks a month on electricity for the in-line hot tank? Sure. But you don’t waste water, and you don’t spend $500 on a recirc pump.

    See? Easy solution.

    Thank you for listening to my Ted Talk.

    1. User avatar GBA Editor
      Martin Holladay | | #23

      This thread is five years old, so I doubt whether the original poster will read your suggestion.

  22. User avatar
    Dana Dorsett | | #22

    > And two in series is smart too because you can run many fixtures at once.

    Two in series the OPPOSITE of "smart".

    Series isn't nearly as useful as two in parallel. Water tube heat exchangers in tankless water heaters have a significant pressure drop with higher flow, and the output of the first one can easily exceed the maximum input temperature of the second if plumbed in series. You'll never get anywhere near the maximum BTUs out of both burners if plumbed in series.

    To run a pair of tankless units (series or parallel) usually exceeds the capacity of a standard residential gas meter, and the installed cost of a pair is usually more than twice the cost of installing just one.

    The local small tank solution has merit.

    An indirect water heater running off a hydronic space heating boiler doesn't run out of hot water either, and doesn't have the tedious tub-filling speed or peak flow limitations of a single tankless or any of the other quirks of tankless water heaters.

    Electric tankless water heaters are a wretched "solution" in any climate, but particularly so in cold water locations, drawing significantly more peak power than an level-II electric car charger.

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