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Q&A Spotlight

Fixing a Hot-Water Problem

A GBA reader who just installed two new tankless water heaters wants to know why he has to wait two minutes for the hot water to reach his faucet

Tankless water heaters eliminate some energy losses, but they don't necessarily make the wait for hot water in the kitchen or shower any shorter.
Image Credit: John Hartman - Fine Homebuilding magazine

John Metcalfe’s San Francisco renovation included the installation of two tankless water heaters and a small circulation pump in his four-story, 3,200-sq. ft. home. The water heaters, connected in series, are located on the second floor, which is a more or less central location.

His hot-water problems should be over, right? Except they’re not.

“We now wait for up to two minutes for hot water in the kitchen and upstairs shower,” he writes in a Q&A post at GreenBuildingAdvisor.

The reasons for the long wait, and the best ways to correct this problem, are the topics for this Q&A Spotlight.

Longer waits are the norm

Traditional tank heaters keep water hot all the time, leading to what are called standby losses but also making hot water available at the turn of a tap. Not so with tankless heaters, which begin heating water only when a faucet is opened and water begins to flow.

“Different brands of tankless water heaters perform differently,” writes GBA senior editor Martin Holladay, “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.”

Holladay notes that Metcalfe’s wait for hot water would probably be shorter if he went back to a tank-style heater. “Other ways to decrease your wait include moving the water heaters closer to the bathrooms and kitchen, and decreasing the diameter of the hot water lines,” he says.

Holladay writes that Metcalfe should make sure that his circulation pump operates by a switch located in the kitchen or bathroom. This is a less wasteful option than either a continuously operating pump or a pump on a timer. Switched pumps bring hot water to the point of use without wasting water. Holladay suggests that Metcalfe consult a Fine Homebuilding article on hot water circulation pumps. The article recommends the D’Mand pump manufactured by ACT Metlund.

Jin Kazama agrees that a water heater should be located close to the fixtures it serves. “To [reduce the delay], one needs to install the tankless water heater in close proximity to the usage point,” Kazama writes. “So you should’ve installed small tankless heaters near [the] kitchen sink or a close by embranchment serving many points, and another close to the shower.”

In Kazama’s view, the only reasons for choosing a tankless heater are low water use or reducing the delay for hot water (“which you obviously didn’t implement correctly”).

Why connected in series?

When two hot water heaters are connected in series, it means one follows the other on the same water supply line, and to David Meiland that doesn’t add up.

“I’ve never seen two tankless heaters in series,” he says, “and I’ll got 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.”

When connected in series, he adds, both heaters are trying to supply hot water for the same demand, “however small it might be.” A single unit running at a higher load all by itself would probably be more efficient, he says, and locating the heaters close to each other all but guarantees that piping runs will be longer than necessary.

“What your plumber probably should have done was install at least one tank, if not two, put them farther apart, piped them separately, installed small-diameter home runs to the sink fixtures, and installed circulation pumps to keep wait times down at the farthest fixtures.”

Nor does Richard Patterman understand the plumber’s thinking.

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

Tweaking the circulation system may help

While Holladay likes the D’Mand hot water circulation pump, Dennis Heidner suggests a competing product: the Chillipepper pump.

“Easy solution that I used, Chillipepper on demand circulating pump that stops pumping when water temp at the pump reaches your desired setpoint,” Heidner says. The pump is only activated by a low voltage contact closure switch/relay, he adds, with the relay controlled wirelessly or when a bathroom light is turned on.

“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,” he says. “Not all circulating pumps will trigger a tankless to heat. You need to exceed 1 [gallon per minute]. And you need to check the manufacturer’s warranty. Some will not cover you unless you add some kind of moderating tank.”

Heidner says he had a nearly identical problem to Metcalfe and would typically waste 2 gal. of water before it was hot enough to use.

“Now the water is almost always at temp when you want it,” he says. “Water use is very low, energy use is low (I measure both!).”

Our expert’s opinion

GBA technical director Peter Yost had this to say:

First, regardless of the domestic hot water power plant — tank or tankless — every system suffers or excels based on the efficiency of the distribution system. And just as there are ACCA standards (Manuals S and D) for forced-air, space-conditioning distribution systems, we really need standards for DHW distribution systems as well. Both types of systems deal with efficiently moving fluids around, and efficient piping is as important as efficient ducting.

We do have the structured plumbing approach. For more information on structured plumbing, see this presentation from Gary Klein.

In my view, either efficient piping or structured plumbing is a no-brainer in new homes; get the distribution system right and the need for a pumping or mechanical solution is either eliminated or easily achieved. Get the piping layout/pipe sizing wrong, and use hard 90s rather than soft 90s, and adding a recirculation pump is just brute force trying to overcome a design problem.

In retrofit, adding an on-demand recirc crossover system (cooled hot water returning to the plant or tank by way of the cold water line) can make a lot of sense. There is more than one company employing the ACT Metlund system (and now there is the distinctly separate Chilipepper on-demand recirc system). The keys to these systems are high-speed, efficient pumps, on-demand operation, and thermostatically controlled shut off. The cooled hot water sitting in the pipes is sent back to the tank or plant while “new” hot water arrives at or very near all points of demand.

The ACT Metlund system (including systems from Uponor and Taco, two companies that use the same base components) comes in three sizes, meant to be sized for the total system resistance or pressure drop, primarily the length of the runs. The Chilipepper has some significant differences compared to the Metlund systems, with the most significant perhaps being that Chilipepper has just one size of pump.

Metlund lists which of their models should be employed for various plants (tank or tankless) and run lengths, but does not include resistance factors such as piping diameter changes, or number and severity of elbows or bends. I asked two plumbing engineers about this, and since they both have installed retrofit Metlund systems with good results, they felt that the piping run length guidance is probably all that is needed. But they did admit that it would be helpful to actually test different distribution layouts to determine the impact of resistance factors other than length.

Another plumbing engineer expressed concern with any of the retrofit systems, given this part of the plumbing code:

“604.2 System interconnection. At the points of interconnection between the hot and cold water supply piping systems and the individual fixtures, appliances or devices, provisions shall be made to prevent flow between such piping systems.”

But he admitted that both the Metlund systems and the Chilipepper system claim code compliance on their websites and appear to be successfully installing their retrofit systems regardless.

On the Chilipepper website, they strongly encourage use of a true plumbing professional for any tankless water heating system. Tankless water heating systems are much more complicated than conventional tank systems, and therefore adding an on-demand recirc system requires a stronger, deeper background to achieve a successful installation.

8 Comments

  1. Jin Kazama | | #1

    Marketing and design ...
    Marketing: almost off every website selling Tankless you can read about how much energy you are going to save... this influences greatly homeowner who are trying to be green or efficient and pushes them to purchase something that might not be fit to save a single cent on their current or new insallation .
    We will need to address and educate about the purpose of tankless water heaters,
    and the standby losses value$$ incurred by electrical tank heaters.

    System design for quick hot water is the most influential factor as we have again experienced here.

    Then again ( sorry if i repeat myself alot ) , heating dominated do not loose much by using regular tank water heaters, and cooling dominated should seriously consider HPWheaters inside their building envelope as discussed with Martin in a another thread.
    Now where does it leave a place for tankless ??

    On another note, i have used ( unsuccessfully) a Stievel-Eltron Tempra 36 ( 36KW i believe )
    and i can tell you that they mention instant ( almost ) hot water and it is what it does.
    I've used it while it was still hang on the wall, to fill up hot water pails ( from a 1ft connection distance from the unit ) and the water was hot within 1-2 seconds.

    I do not know if their other smaller units achieve the same result
    ( albeit within their flow range ) .

    Now we still need for someone to get out and tapp in a -25C compressor unit from Fujitsu or Mitsubishi and use it as water heater!! :p

  2. Mark Dickerson | | #2

    steibel and design
    I installed a Steibel eltron 24 (kw) for our passively designed house, and it supplies floor and domestic water heat. We use a water to water heat exchanger for the floors (Massachusetts home), and it supplies plenty of hot water for daily baths, and dish and laundry washing requirements (for 2 person household). The only drawback it has over a gas fired version which I used in another house is that the temperature is maintained by a flow reducer if you ask for too much volume. the 24kw supplies about 3.5 gallons per minute at a sufficient increase for plenty hot water (the gas fired version supplies almost 7 gallons/minute). But you cannot run your tub and laundry at the same time without a large decrease in volume allowed. As for the person who has a 4 story home and waits 2 minutes for hot water, obviously the plumber put the units too far away for quick hot water, and I do not agree that a tank heater would be much quicker. The hot water still has to travel the same length of pipe. With exception of the 1 or 2 seconds it takes to start getting hot water from the instant heater, the length of pipe is the same. 2 heaters in series is odd though too, and if anything, the heaters should have been installed like mentioned (1 for floors 1/2 and 1 for floors 3/4) or in parallel. Since this is a large house we are talking about, each floor probably should have had its own instant heater within maybe 20 feet of pipe from the need at most. The circulator pump is probably a cheaper way to solve a problem which will cost the owner more in the long run since it will run the heaters much more.

    the moral of the story - if you want it done right, do it yourself (or at least monitor the plumber closely).

  3. Kevin Dickson, MSME | | #3

    Recirc Pumps and Tankless Heaters
    I've experimented with this combination three different times. I found it to be successful only once. The most common problem is not enough pump flow to turn on the heater, but it's not the only issue. The controls and operational modes of these heaters are unpredictable enough for me to say that don't expect either the Metlund or Chilipepper to work correctly. I've even added buffer tanks without reliable success.

  4. Greg Winger | | #4

    @ Jin
    Someone is making Fujitsu (?) based mini split water heaters in Australia. Just need to get them to the U.S. Seems too good to be true.

  5. Roger Brisson | | #5

    totally disagree with this article
    I couldn't disagree with this article more, from my experience it's totally bogus. I live in two houses, one with a hot water tank, the other with a Paloma tankless water heater. With the tank, I have to let water run for at least two or three minutes to get hot water, wasting several gallons of water each time I need hot water. With my tankless water heater I have to wait only a few seconds. Also, the tankless water heater is much less expensive to operate than the wasteful energy consumption of always on. Over the period of a single year the difference in water and energy efficiency is dramatic. Multiply this by several million New England homes and you gain a sense of how unbelievably irresponsible American building standards are in comparison to more enlightened countries in Europe.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Roger Brisson
    Roger,
    You are correct that a tankless gas water heater will save energy compared to a tank-type gas water heater.

    However, there is only one reason that explains why you have to wait so long for hot water from your tank-type heater: obviously, that heater has a longer pipe run (or a larger diameter pipe) than your tankless heater.

    The most important things you can do to reduce your wait time for hot water is (a) to install the water heater as close as possible to the point of use, and (b) to use the smallest feasible pipe diameter.

    Many brands of tankless water heaters have a slight ignition delay that irritates users. For tank-type heaters, however, ignition delay is irrelevant to the topic at hand, because the tank stores water that is (almost) always hot.

  7. Robert McClellan | | #7

    There are few simple answers to complex problems
    or as H.L. Mencken said, "For every complex problem there is an answer that is clear, simple, and wrong."

    I have 2 experiences with DHW. For the last 22 years, my 125 BTU/hr Aquastar (pre Bosch) has provided wonderful hot water...as long as I was alone and no guest tried to do dishes while I was taking a shower. I've put in a rebuild kit, a few thermocouples and flushed it with vinegar as the water I was using was quite hard. But there is only 25 feet of copper pipe in the whole hot water system, so 12 or so in the DHW.

    I now have a large house with DHW provided by the sidearm on my Buderus gas boiler. In the winter, the water is preheated from the heat storage tank that my wood boiler uses. I made the most of home run pex plumbing and as a result, have a long wait for hot water, particularly at the kitchen sink. To get around this problem, last month I installed an EcoSmart (don't you love marketing!) 11 kW demand heater under the sink. It's fed by the tank from the boiler so it only preheats the water and it solved the problem. I still lay awake at night wondering how much heat is being lost from the hot water cooling in the long pipe, but that's just an indication I have too much time on my hands.

    But it does make me think that a recirculating system must waste a LOT of heat. Even with insulated pipe, the R-value is quite low, the delta T high and the area of heat loss considerable. So instead of just losing the heat from hot water as it cools, the heat is lost continuously. Fine in the winter, we use that heat, so so fine in the summer when we are either praying for less heat or actively moving that heat back outside.

  8. Bradley Potts | | #8

    Hello - Insulation!!
    @Robert, thank you for being the first to bring up what should be one of the first considerations in this discussion, that of insulation and piping material. Needless to say, too bad it NEEDS to be said in this case, pipe insulation is a critical component in a successful tankless installation. What's more, absent from the conversation is that a tank-style WH operates on the premise of maintaining a temp setpoint for a large reservoir of water that includes the entire column of water that runs to each and every HW fixture and the pipe is part of the heat transfer equation. DHW offers the significant energy savings that it does largely because of the fact that the distribution acts as a heat-sink that is being continuously battled by the thermal mass of the water and containment of the tank-style WH -- DHW chooses not to fight the loss of heat until the water is required. Assiduous care in how well and how much the piping is insulated will yield much better system performance, duh. We are talking about a hot water system, so we must approach it holistically.

    My brother's DHW system had the on-demand wait problem because his DHW is ~40' from his bathroom, so he called his plumber back, and the plumber installed a small pump + switch for a crossover solution at the terminal point of use, and that solved the problem. They simply press the button for about 2 seconds, and the water is warm enough to start using without any water loss. We live in California, so water conservation is a big, big deal.

    In my 125KBtu DHW system, our 2-bath house is over 100 years old and there are pipes that I cannot access without tearing walls open, so we are still suffering the fact that our galvy pipes are giving up our heat. Our system is located as centrally as possible and our longest run is about 25'. We have 2 big problems: 1) We experience the "thermal sandwiching" in the 1st minute after acceptable running temp is achieved, 2) our Thermidor/Bosch dishwasher has only a 1/4" feed line, which limits the HW flow enough to keep the DHW kicked on and that leaves our energy efficient dishwasher to electrically heat its own water, not good.

    The correction plan is to add the light switch initiated recirc pump plus flow meter activation, and ongoing efforts to improve distribution insulation. To the end of improving insulation, I'll probably include PEX.

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